Science Case for the Chinese Participation of TMT

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Science Case for the Chinese TMT Project
(http://tmt.bao.ac.cn/)
DRAFT (Jan. 18, 2010)
1
Executive Summary
The Thirty-Meter-Telescope (TMT) is a proposed world-leading 30m optical-infrared telescope to
be built in Mauna Kea, Hawaii. The international TMT partnership currently consists of California
Institute of Technology (Caltech), University of California (UC) and Canada, with Japan being a
participant (the second of three stages in becoming a partner). Caltech/UC built the first 10m
telescope (KECK) on time and within budget. The expertise and technology accumulated in
building KECK is a key to reduce the cost and risks involved in this billion-dollar project. TMT’s
large aperture, state-of-the-art instrumentations combined with excellent observing site and
advanced adaptive optics will revolutionise many fields in astronomy. TMT will directly image and
probe the atmospheres of extrasolar planets; it will provide key insights into the nature of dark
matter and dark energy, and shed light on how galaxies, stars, black holes and planets form and
evolve.
Chinese astronomy has made considerable progress in the last decade. The near completion of
LAMOST, with its scientific potential, has attracted worldwide attention. Large ground-base
facilities and space missions either currently under construction, or are in the planning stage open
unprecedented windows of opportunity for Chinese astronomers to make globally competitive
contributions. Several high-profile centers of excellence are being established as international
forums for intellectual exchanges and stimulating incubators for conceptual innovation. These
developments are paving the path for future generations of Chinese astronomers to enter the center
stage of world scientific community in the foreseeable future.
The Chinese Astrophysics Strategy Committee (天体物理发展战略专家委员会), after careful
deliberations, recommended the participation in the TMT as the highest priority for the future
development of Chinese astronomy. TMT will thrust China into the forefront of astronomy for
many decades to come; it will bring China transformational benefits in terms of science,
management, technology and industry through international collaborations. It will also maximize
the scientific potentials of China’s other large astronomy projects, such as LAMOST and FAST.
This document sets out the key science goals and sample observational programmes. We also
survey the current astrophysics expertise within China and identify areas that need to be
strengthened in the next decade in preparation for the first light of TMT in 2018.
2
1
INTRODUCTION...................................................................................................................................................... 4
2
JUSTIFICATION AND BENEFITS IN JOINING TMT....................................................................................... 6
3
OVERVIEW OF TMT OBSERVATORY AND INSTRUMENTS ....................................................................... 8
3.1
3.2
4
TMT OBSERVATORY ............................................................................................................................................ 8
TMT INSTRUMENTS .............................................................................................................................................. 9
TMT SCIENCE ........................................................................................................................................................ 11
4.1 SEARCH FOR AND CHARACTERIZATION OF EXTRASOLAR PLANETS .................................................................... 13
4.1.1 Radial velocity detections of near planets .................................................................................................. 13
4.1.2 Direct imaging of extrasolar planets .......................................................................................................... 16
4.1.3 Planetary atmospheres from absorption line studies ................................................................................. 17
4.1.4 Protoplanetary disks ................................................................................................................................... 18
4.2 FUNDAMENTAL COSMOLOGY .............................................................................................................................. 21
4.2.1 Nature of dark matter ................................................................................................................................. 21
4.2.2 Nature of dark energy ................................................................................................................................. 22
4.2.3 Constancy of fundamental constants .......................................................................................................... 24
4.3 THE FORMATION AND GROWTH OF BLACK HOLES ............................................................................................... 26
4.3.1 Massive black holes at the center of external galaxies ............................................................................... 26
4.3.2 Black hole at the Galactic center ............................................................................................................... 29
4.3.3 Black holes at higher redshift ..................................................................................................................... 31
4.3.4 Physics of active galactic nuclei ................................................................................................................. 34
4.3.5 Stellar mass and intermediate mass black holes ........................................................................................ 36
4.4 STAR FORMATION IN THE LOCAL UNIVERSE AND AT HIGH-REDSHIFT .................................................................. 40
4.4.1 Star formation in the local universe ........................................................................................................... 40
4.4.2 Star formation at high redshift ................................................................................................................... 42
4.4.3 Near-Infrared Emission Line Studies of ULIRGs ....................................................................................... 45
4.5 GALAXY FORMATION AND EVOLUTION ............................................................................................................... 49
4.5.1 Evolution of galaxy luminosity/mass function ............................................................................................ 51
4.5.2 Probing the evolution of galaxies with kinematic studies ........................................................................... 53
4.5.3 Chemical evolution of galaxies................................................................................................................... 56
4.5.4 The Intergalactic Medium (IGM) ............................................................................................................... 59
4.5.5 Strong gravitational lensing ....................................................................................................................... 62
4.6 NEAR FIELD COSMOLOGY AND STELLAR ASTROPHYSICS ................................................................................... 65
4.6.1 Resolved stellar populations and kinematics in nearby galaxies ............................................................... 65
4.6.2 Searching for first stars and cosmic stellar relics in the Galaxy ................................................................ 68
4.6.3 The merging history of the Galaxy and stellar abundances of Galactic globular clusters ........................ 70
4.6.4 Isotope abundances .................................................................................................................................... 72
4.6.5 Chemical abundances of resolved giant stars and HII regions in Local Group dwarf galaxies ................ 74
4.6.6 Mass distributions of the Milky Way and local group ................................................................................ 78
4.6.7 Spectroscopic study of low-mass stars, brown dwarfs and planets ............................................................ 79
4.6.8 Observational Studies of Neutron Star Systems ......................................................................................... 82
4.7 EARLY LIGHT HOUSES AND COSMIC REIONIZATION ............................................................................................. 85
APPENDIX A: ACRONYMS AND ABBREVIATIONS ........................................................................................................... 88
APPENDIX B: DWARF GALAXIES OBSERVABLE BY TMT ............................................................................................... 89
3
1 Introduction
Astronomy is the oldest science where curiosities have driven its advancement since the beginning
of the mankind. Every man has asked the question: How did the universe begin? Are we alone in
the universe? Astonishingly, astronomers are now on the verge of answering these fundamental
questions with the next generation extremely large class telescopes such as the Thirty Meter
Telescope (TMT). The Chinese participation in the TMT will place China in the forefront of
astronomy for many decades to come; it can be a transformational experience for Chinese
astronomy in terms of science, management and technology through international collaborations.
The TMT partnership will form in the next two years, which offers a once in a lifetime opportunity
that China simply cannot miss.
China has a rich and proud history in astronomy; Chinese civilisation kept the best record for
comets, Sun spots, and supernovae in the world. For example, the supernovae Chinese record in the
Song dynasty provides the best, accurate dating (in the year of 1054) of the supernovae explosion in
the Crab nebulae. Only in the last few hundred years, China lagged behind the western world. This
largely coincided with the invention of the first modern telescope 400 years ago by Galileo Galilei
that opened new horizons for astronomical observations.
Astronomy has been at the forefront of scientific revolution, starting with the Copernicus's view of
the Solar system. With increasingly large telescopes covering virtually all the wavelengths ranging
from the radio to the gamma-ray, modern astronomy reveals a rich and beautiful Universe,
revealing surprising unity between the smallest to large scales. In this regard, astronomy has
contributed to our fundamental understanding of the physical world. Recent astronomical surveys
reveal that the Universe is dominated by dark matter and dark energy. The nature of these two dark
components is the most fundamental question in (astro-)physics today. The discovery of more than
400 extrasolar planet systems indicates that our solar system may be the exception rather than the
norm; we have, with TMT, the exciting possibility to answer whether we are alone in the Universe.
Chinese astronomy has made considerable progress in the last decade. The near completion of
LAMOST, with its scientific potential, has attracted worldwide attention. Large ground-base
facilities and space missions either currently under construction, such as FAST and SVOM, or are
in the planning stage, such as the Dome A project, open unprecedented windows of opportunity for
Chinese astronomers to made globally competitive contributions in fronts covering a wide range of
wavelengths across the electromagnetic spectrum. Several high-profile centers of excellence are
being established as international forums for intellectual exchanges and stimulating incubators for
conceptual innovation. These developments are paving the path for future generations of Chinese
astronomers and astrophysicists to enter the center stage of world scientific community in the
foreseeable future.
The US Astronomy and Astrophysics Decadal Survey in 2000 listed a 30-m class giant segmented
mirror telescope as the highest priority instrument, to be funded through a 50:50 public and private
partnership. Such a telescope will provide a factor of 10 improvement in the light gathering power
over the current generation of 8-10m class telescopes, enabling numerous new, exciting science to
be done in the future. Since the Decadal Survey, three international collaborations have now
emerged. In the US, the Giant Magellan Telescope (GMT) and the Thirty Meter Telescope (TMT)
are the two forefront runners; in Europe, a 42m Extremely Large Telescope (E-ELT) is currently
under intense discussion.
TMT is currently leading the race among the three in terms of technical design and fund raising.
TMT involves California Institute of Technology (Caltech), University of California (UC) in the
US, and a consortium of Canadian institutions; participation by the Japanese is expected. India and
4
Brazil are also considering joining TMT. Caltech and UC are the two institutes that built the first
10m telescope (KECK) in the world on time and within budget. The same key technologies and
expertise will be available for the construction of TMT. This substantially reduces the cost and risks
involved for the participating partners. The TMT organisation has already raised substantial amount
of funding (300 million US dollars out of the one billion required) with the Moore Foundation
providing about 200 million funding plus the 60 million for the initial (high-risk) feasibility studies
and construction plans.
Clearly the 30m class telescopes will become the standard in the next decade for optical-infrared
observations. Currently the biggest general-purpose telescope in China is 2.4m, already severely
behind state-of-the-art 8-10m class telescopes in terms of light-gathering power and
instrumentations. Without access to a 30m class telescope in the next decade, Chinese observational
astronomy will fall behind even further. The recent successful visit (in December 2009) by the
TMT management team highlights China has the technological and industrial capabilities to play
leading and major roles in a number of high-technology TMT instruments. China simply cannot
miss the golden opportunity to join TMT for us to become a globally competitive force in
astronomy.
5
2 Justification and benefits in joining TMT
China is developing rapidly economically; we are playing an increasingly crucial role on the world
scene. As a nation, China is proud of its technical and cultural heritages in the past. To further
improve our international visibility, we must invest in science and technology, in particular basic
science, which provides the fertile soil for applied technologies to prosper not only at present but
also in the future.
As mentioned before, the Chinese astronomical community has made much progress in the last
decade with rapid advances in observing facilities such as LAMOST. The research environment (in
terms of hardware) is also reaching the western standard. Chinese astronomers are engaged in
diverse research areas with a potential pool of excellent students and invaluable resources of
oversea Chinese astronomers both in observational and theoretical astrophysics. However, a world
class, general-purpose optical-infrared telescope is still lacking: our largest optical telescope in
Lijiang, Yunnan has a diameter of 2.4m; this is in sharp contrast with many state-of-the-art 8-10m
telescopes throughout the world. Even a third-world country such as South Africa has built their
own 10m telescopes. This lack of access to modern telescopes severely hampers our ability to
maximize the science returns of China's own large astronomy projects, such as LAMOST and
FAST etc.
In this context, the Chinese Astrophysics Strategy Committee listed the participation in the TMT as
the highest priority item for the future development of Chinese astronomy. China has gained
invaluable experience in the large scientific engineering project LAMOST that has an aperture of
about 4m. However, to make the next quantum leap in terms of aperture to 30m, China cannot go
alone for several reasons. First, the 30m telescope science requires the best observing site in terms
of seeing, sky background, photometric nights etc., in the world, so far no such site has been
identified within China. Second, much of the science of 30m class telescopes is in infrared (e.g., for
studying the high-redshift universe and the detection of extrasolar planets), and China has no access
to infrared CCD technology. Third, the TMT project requires a multitude of expertise in science,
technology, engineering and management; such teams are not yet mature within in China. Lastly,
the cost involved is prohibitive for any single country. All the proposed 30m class telescopes (TMT,
GMT, and E-ELT) involve large international collaborations. The Chinese participation in TMT
will bring a number of scientific, managerial, technological and industrial benefits to China:

It will thrust China into the forefront of astronomy in the extremely large telescope era; it will
establish a platform for China to collaborate and compete with world astronomers.

It will also enable other large Chinese science projects such as LAMOST and FAST to realise
their fullest science capabilities and potentials.

It will enable China to learn key technologies in advanced optics, mechanics, electronics and
automation, including technologies in IR CCDs that are inaccessible currently.

Chinese industry can participate in large-scale, high-technology manufacturing, and improve
international visibilities.

State-of-the-art facilities TMT will prove to be a magnet to attract high-level Chinese
astronomers to collaborate with Chinese astronomers (and possibly return to China). This will
further expand the talent pool that will be crucial for the future success of Chinese astronomy.

TMT will also provide an important platform for Chinese astronomers to collaborate with
world astronomers to access other state-of-the-art facilities in other wavelengths, such as SKA
(in the radio), ALMA (in the sub-mm), and JWST (optical and infrared, but in space). Such
6
multi-wavelength observational approaches are becoming increasingly important in
astrophysics.
It is important to stress that China will not only be able to build specific instruments as in-kind
contributions, but also send delegates to learn key technologies in virtually all areas.
The final TMT design/construction readiness review will be in June 2010. It will have its first light
with full primary mirror in October 2017, and the first science will be performed in June 2018. The full
TMT partnership will be formed in about two years (from December 2009). The later we join, the less
China will be able to influence and define our own technical and scientific contributions to the TMT
project. It is crucial that our technological capabilities are matched by our success in scientific
programs. This can only be ensued if a strong and internationally competitive science team is built up.
The rapid emergence of Chinese astronomy on the world scene in the last decade lends confidence that,
with appropriate planning and a united effort from the Chinese astronomy community, this can be
achieved in the next decade.
7
3 Overview of TMT Observatory and Instruments
3.1 TMT Observatory
The core of the TMT Observatory (Figure 1) will be a wide-field, alt-az Ritchey-Chretien telescope
with a 492 segment, 30m-diameter primary mirror, a fully active convex secondary mirror and an
articulated flat tertiary mirror. The optical beam of this telescope will feed a constellation of
adaptive optics (AO) systems and science instruments mounted on large Nasmyth platforms
surrounding the telescope azimuth structure. These platforms will be large enough to support at
least eight different AO/instrument combinations covering a broad range of spatial and spectral
resolution.
Figure 1 The telescope design (left), and the entire observatory system (right). Taken from the
TMT web site (tmt.org).
The TMT aperture size (30 meters) occupies an attractive and achievable scientific “sweet spot” at
near-infrared wavelengths. It is important to stress that the cost for TMT has been reduced from the
usual volumetric scaling with aperture of D2.7 to D1.15 due to the past experience accumulated from
the successful construction of KECK.
TMT will be the first ground-based astronomy telescope designed with adaptive optics (AO) as an
integral system element 1 . AO is a term that covers systems designed to sense atmospheric
turbulence in real-time, correct the optical beam of the telescope to remove its affect, and enable
true diffraction-limited imaging on the ground. TMT adaptive optics design builds on the
technological and operational heritage of (among others) the Gemini, Keck, and Very Large
Telescope observatories. This is an area of rapid advancement and the TMT project has direct
access to world leaders in this area (e.g. the Center for Adaptive Optics at the University of
California, Santa Cruz). For point sources, the adaptive optics improves the gain in the observing
time from D2 to D4 (due to the decrease in the footprint of the Point Spread Function [PSF]) –
implying a reduction in observing time for a factor of ~100 rather than a factor of 10 compared with
1
Max, C., www.ucolick.org/~max/289C/Lectures/Lecture%2016/Lecture16.2008.pdf; Mountain M., van der Marel R.,
Summer R. et al. 2009, arXiv:0909.4503v1
8
KECK. This vast improvement in efficiency will enable exciting science to be performed on the
nearby and distant Universe inaccessible by any other observatory, on the ground or in space.
3.2 TMT instruments
Current proposed TMT observatory contains 8 instruments, with 3 instruments available at first
light, and five available in the first decade (see Table 1). At first light, TMT will employ a laser
guide star supported, multi-conjugate adaptive optics system called NFIRAOS (Narrow Field
Infrared Adpative Optics System), which will provide diffraction-limited resolution and high Strehl
ratios over a 30 arcsec field-of-view. The resolution at 1 micron is about 7 milli-arsecond, roughly a
factor of ten higher than that of HST.
The three early light instruments are



WFOS: a wide-field, multi-object spectrograph working at optical wavelengths (0.31 to 1.1
microns). It has impressive multiplex capabilities. Using separate masks, it is possible to
observe up to 1500 objects over a 40.5 square arc-minutes with spectral resolution from 3007500.
IRIS: an integral-field unit (IFU) spectrometer with imaging capability working at nearinfrared wavelengths (0.8-2.5 microns). The imaging mode will provide a 10x10 arcsec field
of view with 0.004 arcsec/pixel sampling. The IFU will provide spectral resolving power of
4000 over the J, H and K bands. This instrument will be ideal to study the kinematics of
galaxies in great detail.
IRMS: a multi-slit, near-infrared (0.8-2.5 microns) spectrometer with imaging capability. It
will provide 46 movable, continuously variable width cryogenic slits. The two-pixel resolving
power is 4660 covering Y, J, H and K bands. In the imaging mode, IRMS covers the entire
NFIRAOS field of view with 0.06 arcsec sampling.
The latter two instruments will be fed by NFIRAOS to achieve full diffraction-limited sensitivities
and spatial resolutions in the near-infrared. These three instruments will be capable of exploring a
wide astronomical terrain: from the first stars in the Universe to the planets orbiting nearby stars.
The proposed five first-decade instruments will be developed and deployed on a schedule
depending on a combination of technological readiness and available financial resources. The basic
technical information about the three early light instruments and five first decade instruments are
listed in Table 1. Notice that the TMT has a total field of view of 20, with 15 un-vignetted. The
instruments in Table 1 use only part of this FOV.
9
Instrument Adaptive Mode
FOV
Wavelength
R
optics req.
(arcsec)
(m)
IRIS
NFIRAOS Diffraction- 10x10
0.8-2.5
2-50
MCAO limited
imaging
(imaging)
0.7, 1.6 or
~4000
4.5 (IFU)
92.4 arcmin2 0.34-1.0
/1300
Assembly of galaxies at large redshift
Black holes/AGN/Galactic Center
Resolved stellar populations in crowded fields
Astrometry


IGM structure and composition 2<z<6
High-quality spectra of z>1.5 galaxies suitable for
measuring stellar pops, chemistry, energetics
Near-field cosmology
Near-IR spectroscopic diagnostics of the faintest
objects
JWST follow-up
NONE
IRMS
NFIRAOS Near5
patrol 0.8-2.5
MCAO diffraction- field
limited
2 per IFU
Mid-IR Near4.5-25
3x3
AO
diffractionlimited
20005000
PFI
ExAO
1.1-2.4
50-300

HROS
NONE
0.34-1.0
3000050000

Diffraction- 2.2x2.2
limited
Seeing20x20
limited
10005000




WFOS
MIRES
Seeinglimited
Science Case



5000100000



WIRC
NIRES
NFIRAOS Diffraction- 30x30
MCAO limited
NFIRAOS Diffraction- 2x2
MCAO limited
0.8-5
5 - 100
1-5
500030000





Physical structure and kinematics of protostellar
envelopes
Physical diagnostics of circumstellar/protoplanetary
disks: where and when planets form during the
accretion phase
Direct detection and spectroscopic characterization
of extra-solar planets
Stellar abundance studies throughout the Local
Group
ISM abundances/kinematics, IGM characterization to
z~6
Extra-solar planets!
Precision astrometry
Stellar populations to 10Mpc
Precision radial velocities of M-stars and detection of
low-mass planets
IGM characterizations for z>5.5
Table 1: instruments and sciences of TMT. The top three instruments (light shaded) are the
early light instruments, while the five first-decade instruments are shown below. The
acronyms are listed in Appendix A. For diffraction-limited observations, the resolution is
≈7mas (/m). Taken from the TMT documents.
10
4 TMT science
The powerful combination of the light gathering power and diffraction-limited adaptive optics of
TMT will probe the universe in unknown regimes, from extrasolar planets to the largest-scale
structure of the universe. The collecting area of TMT will be larger than the current generation of
telescope by a factor of 10, and the resolution will be 12 times sharper than what is achieved by the
Hubble Space Telescope. Depending on the observation, TMT will see further and see more clearly
than previous telescopes by a factor of 10 to 100.
TMT will provide new observational opportunities in essentially every field of astronomy and
astrophysics. Because of the decades-long lifetime of TMT and the often-rapid advancement of
astronomy into new areas, in this science case we only broadly outline the key science drivers that
can be envisioned today. China already has many active research groups in these areas, and joining
the TMT will allow these groups to gather first-hand, state-of-the-art observational data and thus
enhance their research activities substantially.

We emphasize that, as a general-purpose telescope, the legacy of TMT may be on the
unknowns, in areas not yet anticipated. It is useful to recall that the major discoveries of HST
and the Sloan Digital Sky Survey were not originally outlined in their science proposals.

TMT will explore formation processes and the physical properties of extra-solar planets. The
discovery of more than 400 extrasolar planets using a variety of methods (radial velocity,
transits, and microlensing) reveals that our solar system may be an exception rather than the
norm. TMT will allow us to make fundamental discoveries in this area. Radial velocity
methods can be pushed into the discovery of terrestrial-planets and planets around types of
fainter stars other than the Sun; TMT can also be used to study the kinematics of protoplanetary disks, and enable spectroscopic detection and analysis of extra-solar planet
atmospheres through absorption and the direct imaging of extra-solar planets in reflected and
emitted light.

TMT will provide important constraints on the properties of dark matter and dark energy, and
whether fundamental constants of nature changes as a function of cosmic time. The properties
of dark matter will be probed with a variety of methods such as gravitational lensing,
kinematics of dwarf galaxies, and small-scale structures of Lyman-alpha clouds, while the
dark energy can be studied using Lyman-alpha clouds and deep surveys of high-redshift
supernovae.

TMT will allow investigations of massive black holes throughout cosmic time. TMT can
probe central massive black holes to greater distances and to smaller black holes. It will, for
the first time, gather a large statistical sample of black holes to study how they correlate with
galaxy masses, kinematics, morphologies, and how they evolve out to redshift ~0.4 and, when
combined with other methods, beyond. TMT can also study the central black hole in the
Milky Way in more fundamental ways using many more stars, with the possibility of
measuring general relativistic effects for stars on highly eccentric orbits.

TMT will make important contributions to the detection of first light, an outstanding question
in astrophysics. The Universe entered a period of “dark ages” when it re-combined at redshift
z ~1000 at an age of about 300,000 years old. The “dark ages” were ended when the first light
sources turned on. TMT, in synergy of the James Webb Space Telescope (JWST, to be
launched in 2014 with a 5-10 year lifetime), will provide spectroscopic observations of the
first lighthouses, providing detailed information about the physical properties of these objects,
and also their chemical enrichments.

The tremendous light gathering power of TMT will allow galaxies, rather than just quasars, to
11
act as background sources to probe the structure along the line of sight. The much higher
source densities can be used to perform 3D tomography of the matter distribution from the
large-scale structures to small scales. Such studies will also probe the nature of dark matter,
dark energy, the constancy of physical constants and chemical enrichment and feedback
processes via structure formation.

TMT will probe how structures form on different scales in the universe, ranging from planets
(as discussed above), stars and galaxies. It will address how galaxies form and evolve as a
function of environment and feedback processes from star formation and active galactic
nuclei.
The potential science areas to which TMT can contribute are remarkably diverse. This document
builds on the science case science cases from the three extremely large telescopes 2; we further
develop the cases with special emphases on the Chinese astronomical community. The document is
divided into a number of related research themes. Each theme follows roughly the format




What are the key science questions in the next two decades?
Overview of current research activities.
Detailed discussion of key science areas. Why is TMT essential for the research?
Strengths and weakness of Chinese astronomy in the theme:
i. Existing research teams within China.
ii. Identify areas that need to be strengthened in order to improve the competitiveness of
China in terms of student training, and team building.

Possible TMT observational programs:
i. Complementarities with other Chinese projects: SVOM, FAST, DOME-A, HXMT?
ii. What can be done in the immediate term using 8-10m class telescopes?
2
Giant Magellan Telescope Science Case (July 2006); TMT Science Advisory Committee, Thirty Meter Telescope
Detailed Science Case (2007); http://www.eso.org/sci/facilities/eelt/science/index.html
12
4.1 Search for and Characterization of Extrasolar Planets
Jilin Zhou (Nanjing), Doug Lin (KIAA/Santa Cruz), Jian Ge (Florida), Liang Wang (NAOC),
Yujuan Liu (NAOC), Gang Zhao (NAOC)
Key questions:
1. What types of planets exist around other stars?
2. How did Earths and other planets form and evolve?
3. What fates await planetary systems?
4. Is there any sign of life on planets other than the Earth?
5. How did life emerge and proliferate?
Planetary detection and characterization are the most rapidly advancing branches of
astronomy and astrophysics. Space exploration has unveiled that worlds of our solar system come
in a tremendous variety. Even they hardly prepared us for the sheer diversity of extra solar planets.
Barely a decade ago those who study how planets form had to base their theory on a single
example—our solar system. Now they have dozens of mature systems and dozens more in birth
throes. No two are alike. The basic idea behind the leading theory of planetary formation—tiny
grains stick together and swoop up gas—conceals many levels of intricacy. A chaotic interplay
among competing astronomical, geologic, chemical, and biological processes leads to a huge
diversity of outcomes.
Time has arrived for the emergence of a new discipline, astrobiology, which confronts the
fundamental age-old conundrum ``are we alone in this universe?’’ A logical and systematic
approach to estimate the population of extra terrestrial intelligence is to quantitatively determine
each entry in the Drake equation. The advent of TMT will bring to reality the long-soughtafter
census on 1) the fraction of nearby stars with planets, 2) the fraction of these planets which are
potentially habitable, and 3) the fraction of these supporting platforms show any signs of past or
ongoing biological activities. The outcome of these surveys will provide scientific confirmation or
quantitative constraints on conceptual conjectures and philosophical speculations on the origin and
proliferation of life.
Along the paths of these discoveries, we will be able to extract information on 1) the sequence of
planetary assemblage, 2) the dominant mechanisms that led to their diversity, and 3) the selection
processes that channelled their destiny. Some of these issues are also relevant, albeit on different
special and time scales, in the context of star formation, galaxy evolution, and active galactic nuclei.
We list below some important technological advancement anticipated in the search and
characterization of extra solar planets that will be brought about by TMT.
4.1.1
Radial velocity detections of near planets
Key questions:
1. What are the mass, size, and semi major axis distributions of planets around nearby stars?
2. Did planets acquire their structural and kinematic diversity at birth or during the main
sequence life span of their host stars? How did these properties evolve as their host stars
age?
3. How do frequencies, masses, and orbits of planets depend on the mass, metallicity, age,
binarity, and Galactic environment of their host stars?
4. How does extra architecture of multiple planetary systems around nearby stars compare
with that of the Solar System?
5. What fraction of nearby stars bear earth-like planets in habitable zones
13
Introduction:
A vast majority of the known extrasolar planets were discovered indirectly by measuring the reflex
motions of their parent stars through radial velocity (RV) measurements[i]. Most of these planets
are Jovians orbiting G and K dwarf stars where the planet/star mass ratio was large enough to
produce measurable reflex motions. TMT with HROS will expand the number of host stars
accessible to Doppler spectroscopy by a factor of 30 by allowing a greater volume of space to be
explored[ii]. In addition, the higher sensitivity will allow lower-mass stars, such as M stars – the
most common stars in the galaxy, to be observed. These stars are more strongly affected by
gravitational perturbations so lower-mass planets can be detected. In fact, TMT will be able to
detect Earth-mass planets orbiting in the habitable zone of M stars (the habitable zone is the region
surrounding the star where a planet would have a temperature conducive to the formation of life).
Main science areas:
1. Detection of More Planets around Different Type of Stars
Over the next 15 years, a major goal will be to extend these RV surveys to M and L dwarf stars for
several reasons. First, many more M and L stars exist than G and K stars – it is likely that the
majority of planets in the local solar neighborhood revolve around such stars. Second, since M and
L stars have lower mass, rocky, Earth-like planets can produce measurable reflex motions. Third,
planets revolving in M and L dwarf habitable zones will have periods of 30 – 100 days or less,
making it straightforward to characterize such orbits in less than a year.
The intrinsically more efficient HROS design coupled with the 10-fold increase in TMT collecting
area relative to Keck will enable the required RV precision at V=11, 12, and 13 in approximately
1.5, 3.5, and 8.5 minutes, respectively. These shorter exposure times allow for the characterization
of an entire orbital period (30 – 100 days) for tens to hundreds of candidate planets around early M
dwarf stars in one year. Only the vagaries of time allocation limit the potential sample size.
2. Characterization of multiple-planet systems
The probability of finding additional planets around solar type stars with known gas giants is much
higher than that of finding the first gas giants. This tendency may either signify that the formation
of gas giants is prolific above a threshold condition or it can promote the production of secondgeneration planets. Gravitational perturbation among multiple planets also induces secular evolution
and dynamical instabilities lead to the excitation of planets’ eccentricity, inclination, and their
orbital migration. The dynamical ``porosity’’ of multiple planetary systems provides important
clues and constraints to theories of planet formation and celestial mechanics.
The most straightforward approach is to conduct follow-up radial velocity surveys of stars with
known planets. Planets’ mass distribution within multiple systems will show whether rocky planets
and gas giants are specially ordered and segregated around other stars as in the solar system. Such
results will have profound implications on sustainability of habitable planets.
3. Origin of dynamical diversity
With a much larger sample, we can determine the fraction of multiple systems are in well separated
circular orbits (similar to the solar system), mean motion resonances (similar to the Galilean moons),
or highly eccentric orbits. It is possible that the preservation of systems with orderly orbits (like the
solar system) requires planets with modest mass and long periods that can eventually be detected by
14
second-generation instruments on the TMT.
Radial velocity measurement of transiting planets (see below) will provide informative statistics on
the inclination between the axes of stellar spin and planets’ orbits. Highly inclined planetary orbits
suggest that some planets may be strongly scattered whereas spin alignment is indicative of gentle
and gradual dynamical evolution. Follow-up observation of stars with directly imaged long-period
gas giant planets can also place constraints on whether these planets formed through gravitational
instability or through core accretion.
4. Evolution of planetary systems around relatively young stars
Dynamical evolution of planets’ orbits proceeds rapidly around relatively young stars. Due to
changes in the gravitational potential, the depletion of gaseous and debris can lead to both
eccentricity excitation and orbital migration as well as frequent giant impacts. In addition, young
stars reside in crowded neighborhoods where perturbations of their neighbors are likely to
destabilize their orbits. For planets in the proximity of their host stars, intense tidal interaction may
also lead to infant mortality.
With TMT’s NIRES, it is possible to carry out radial velocity surveys around young stars. Direct
comparison with planetary census around mature stars will provide clues on the evolution of
planetary systems.
5. Detections of Earth mass Planets in Habitable Zones
Detecting rocky, Earth-like planets in the habitable zones of cool dwarf stars requires radial velocity
measurements with 1 m/s precision. The California-Carnegie Planet Finder team (PIs: Butler, Fisher,
Marcy, Vogt) has demonstrated that such precision is possible on Keck/HIRES using an iodine cell
near 0.5
μm. However, to reach interesting apparent flux limits for M stars would require 1 – 3
hours per RV measurement with Keck, severely constraining the number of stars that can be
surveyed. Given the diversity of planetary systems discovered so far, it is clearly necessary to
survey tens or hundreds of stars, not just a handful, to truly characterize the potential population of
nearby rocky planets.
6. Followup observations of extrasolar planets discovered by Kepler and CoRoT
RV survey results alone contain ambiguities about inclination angles and hence planetary masses. It
may be possible to remove such ambiguity in systems with multiple stars using high spatial
resolution, precision astrometric observations with TMT/IRIS (see, e.g. Neuhäuser et al. 2006).
Transit surveys like Kepler and Corot have a different challenge. They may detect dozens to
hundreds of nearby rocky planet candidates with known inclinations. But followup RV
measurements will be necessary to constrain the masses of these candidates and separate “false
positive” icy planets on long orbits from rocky planets on short orbits. At V = 14, TMT/HROS can
reach 1 m/s in less than 30 minutes per point. Hence, HROS (and perhaps NIRES) will be ideal
instruments for Kepler and Corot follow up investigations.
Possible TMT programs:
1) A systematic and thorough surface of extrasolar planets around solar neighbourhood, so that it
may give the host rate of planets with different types of stars, the distribution of masses, semimajor axes, eccentricities and possibly inclinations of these planets; the relationship between
planet-host stars with their abundance of metallicities, etc.
15
2) A search of habitable planets in solar neighbourhood. Once they are found, try to characterize
their orbital and physical signature.
3) Followup observations of transit extrasolar planets discovered by Kepler and CoRoT.
References:
1.
http://extrasolar planet.eu
2.
http://www.tmt.org (and hereafter through the following 3 Sections)
4.1.2
Direct imaging of extrasolar planets
Key questions:
1. How can we receive the photons form a planet around a bright star?
2. Are planets in distance orbits or in brawn dwarf system common in planetary systems?
3. What can we infer from the planet image?
Introduction:
It is now possible, using adaptive optics, to directly image giant planets. The higher resolution
provided by TMT/PFI will extend the reach of these observations to the nearest star-forming
regions, making it possible to relate the properties of the planetary systems to the environment, and
to observe directly large planets forming within circumstellar disks. TMT will also be able to detect
planets that are close to their host star, probing for the first time scales that are comparable to the
size of the inner Solar System. Since planets in this region intercept and reflect more light from the
host star, it will be possible to image even cold Jovian planets directly by reflected starlight and
study the atmospheric composition of these planets. Via direct imaging, TMT will be able to
explore a regime not addressed by RV surveys.
Main science areas:
1. Imaging Young Giant Planets in Distance Orbits
Where Giant plant in far distance (>50AU) formed through gravitational instability or core
accretion model is still in controversial. For example, three planets of around 10 Jupiter-masses are
imaged in HR 8799 system in 2008 [1]. More samples of planets in distance orbits are very useful
to understand their formation scenario.
Using a combination of coronographic imaging and precision astrometry, TMT/IRIS can explore
the moderate-contrast (103 – 105) wide-separation (> 50 AU) regime that may be occupied by
young (< 1 Gyr), self-luminous Jovian planets. If such planets are detected, moderate-resolution
spectroscopic follow up will also be possible with IRIS.
2. Imaging evolved giant planets in mid-distance orbits
The detection of giant planets in closer orbits (~10AU) would be very helpful, compliment to
the radial velocity, which favours the planets in close-in orbits, and also to micro lensing technique,
in which the stars are mostly far away and the experiments are not repeatable. The increasing of
planets samples in mid-distance orbits will, of course, helpful to understand questions, such as
whether the Solar system is special among planetary systems.
To study more evolved planets closer to their parent stars, higher contrast imaging is necessary.
16
TMT/PFI is designed to enable 3 λ/D spatial resolution, high-contrast (108) imaging with low
spectral resolution (R ~ 70) followup at 1.65 μm. A moderate-resolution spectroscopic mode (R ~
700) will also be available. This will allow systematic surveys in the mass range 0.5 < MJ < 12 over
0.5 – 50 AU out to 100 pc or more.
3. Imaging giant planets around brown dwarfs
Planet around brown dwarf stars are relative easier to be imaged, since the brightness contrast
between the star and the planets are much easier. The first planet being imaged is mass with four
times of Jupiter orbiting a brown dwarf 2M1207 [2]. Planet Formation in brown dwarf system is
not known much by us. To have more planet samples in brown dwarf stars are helpful to the study
of planet formation in these systems.
Possible TMT programs:
1. A systematic image of planets in distance orbits so that a statistics may be available. By fitting
their internal structures, we may distinct whether they are formed through gravitational
instability or core-accretion scenario.
2. Search for more giant planets in moderate distance to the host star. Try to find planetary systems
similar to the Solar system.
3. Detect more planets around brown dwarf stars. Develop the theory of planet formation around
brown dwarf stars.
References:
1.
2.
Marois, C. et al. 2008, Science 322:1348
Chauvin G. et al. 2004, Astron. & Astrophys. 425 , L29
4.1.3
Planetary atmospheres from absorption line studies
Key questions:
1. What is the difference between the atmospheres of extrasolar planets and Earth?
2. Is there any bio-maker in other rocket planets?
Introduction:
The light received from distant planetary systems is a combination of light from the planets and that
from the host star. At optical wavelengths, the star is typically about a billion times brighter than the
planets, so the light from the planets cannot be distinguished. However, at mid-infrared
wavelengths, the brightness contrast between a planet and its host star is much smaller, making it
possible to distinguish spectral features in the radiation emitted by the planet, superimposed up on
the spectrum of the star. These features have a small wavelength shift due to the motion of the
planet around the star, which makes it possible to separate them from features produced by the star
itself by a process of spectral deconvolution.
For planets that pass in front of their host star, as seen from the Earth, another technique is
possible. During the transit, a small portion of the light emitted by the star passes through the
atmosphere of the planet. As a result, absorption features due to molecules in the planetary
atmosphere are superimposed on the spectrum of the star. These features are extremely weak since
only a very small portion of the light is affected by the atmosphere. However, they can be detected
with a thirty-meter telescope and high-resolution spectrometer. Simulations indicate that it should
be possible to detect oxygen in the atmosphere of an Earth-like planet orbiting in the habitable zone
of an M star, in about three hours with TMT/HROS. Detection of oxygen would be highly
significant since it is indicative of photosynthesis, and thus the presence of life.
17
Main science areas:
1. Water, Carbon Monoxide and Methane observation
Between 1 and 2.5 μm, strong water, carbon monoxide and methane features exist and should be
observable by TMT/NIRES. Since these features contain 10s to 100s of individual lines, the
effective SNR increases by an order of magnitude relative to the measurement of a single line like
Na D in the optical. Fast rotating, late M dwarfs are particularly interesting targets in this regard
since the amplitude of the Rositter-Mclaughin effect is directly proportional to the stellar rotational
velocity.
In the mid-IR (5 – 20 μm), such observations become easier for Jovian planets because the
contrast between the planetary and stellar spectral features is higher (103 – 104 ; see, e.g., [1,2]) and
the molecular features found in the planetary atmospheres are more distinctive. Richardson et al.
(2007) have published the first such mid-IR measurement based on Spitzer observations [3].
2. Search for Oxygen
The most exciting but perhaps more speculative high-resolution spectroscopic project is the search
for oxygen (a key bio-marker) in the atmospheres of rocky planets transiting M dwarf stars.
Simulations by Webb & Wormleaton [4] suggest such measurements are possible if R ~ 40, 000
spectra with SNR ~30,000 can be achieved for V = 13 M dwarf stars – well within the grasp of
TMT/HROS. More than 2500 candidate stars are visible from any given location on Earth. The
obvious approach is to narrow down this list using an RV survey and then focus on the most
suitable candidates. Of course, before investing this effort, the simulations of Webb and
Wormleaton must be confirmed and extended.
Possible TMT programs:
1. Detection of water, carbon monoxide and methane features in selected Jovian planets.
2. Search for Oxygen and other bio-maker in the atmosphere of rocky planets during the transiting
of M dwarf stars.
References:
1.
2.
3.
4.
Sudarsky, D., Burrows, A. & Hubeny, I. 2003, ApJ, 588, 1121
Burrows, A., Surdarsky, D. & Hubeny, I. 2004, ApJ, 609, 407
Richardson, L.J. et al. 2007, Nature, 445, 892
Webb, J.K. & Wormleaton, I. 2001, PASA, 18, 252
4.1.4
Protoplanetary disks
Key questions:
1. Why do some planetary systems contain hot Jovian planets close to their parent star while at
least one (the Solar System) has rocky planets in the same region?
2. Are planets formed mostly by gravitational instability or by core-accretion scenario?
3. How and when protoplanet and planet orbits are circularized?
4. Are there any pre-biotic molecules in disks?
Introduction:
Protoplanetary disks are flattened rotating disks of gas and dust surrounding newly formed stars.
The inner (r < 10 AU) regions of proto-planetary disks are particularly interesting since these are
18
the regions where most planets may form. Such inner disks are typically too distant to be spatially
resolved by TMT. However, the Keplerian rotation of disks can be used to separate disk regions in
velocity (and hence radius) and derive the radial variation of line intensity by fitting resolved line
profiles. Making such separations in velocity demands high-resolution spectroscopy. Given the
expected temperatures of these disks, atomic and molecular lines of interest will lie between 1 and
25 μm.
Of particular interest to astrobiology are organic molecules, the building blocks for pre-biotic
molecules such as amino acids, nucleobases, and sugar-related compounds, which have transitions
in this wavelength regime. High-resolution spectroscopy also provides data on two key parameters:
gas dissipation time scale (and how it relates to dust dissipation time scale measured by, e.g. Spitzer)
and gas viscosity. Knowledge of both parameters is important to understanding how and when
protoplanet and planet orbits are circularized – a key issue in planet formation theory. Yet, both
parameters are currently poorly constrained by observation.
Main science areas:
1. Probing gas dissipation timescales
The timescale for dissipation of gas in circumstellar disks governs the viability of plausible
giant planet formation mechanisms and consequently, the range of plausible giant and terrestrial
planet architectures. Moreover, the persistence of gas in the terrestrial planet region of the disk also
affects the masses, eccentricities and consequently the habitability of terrestrial planets. The
observation of disk dissipation timescale is useful to tell the two basic regimes of planet formation:
gravitational instabilities vs. core accretion mechanisms. If the timescale for gas survival is short (t
<< 10 Mir) at distances beyond several AU in most systems, gas giant formation via accretion onto
a rocky core is unlikely. Moreover, if the residual gas in the terrestrial zone is << 10-3 that of a
minimum mass solar nebula, it becomes difficult to understand how the Earth and its sister planets
in the inner solar system ended up in low-eccentricity orbits. TMT/MIRES will have the flux
sensitivity and spatial resolution needed to constrain this timescale across a wide variety of
environments using both molecular (e.g. H2) and atomic tracers.
2. Probing protoplanetary disk gaps
Forming extrasolar giant planets should produce tidal ‘gaps’ in the accretion disk. Optically thin
emitting gas in these gaps can be used to diagnose the presence of forming protoplanets and
quantify their orbital distances and masses. Making such measurements can provide insight into the
formation mechanism for giant planets, and via comparison with the architectures of mature
planetary systems, their dynamical evolution. Determining the Keplerian velocity of emitting gas
within the gap provides a measure of the planet semimajor axis, while their mass can in principle be
assessed from the gap width as inferred from the shape of the emission line arising from the gas
diagnostic. The targets are young stellar objects still surrounded by circumstellar accretion disks –
diagnosed via their infrared spectral energy distributions (large excess emission from dust
embedded within the disk) and from optical photometry (UV excess) and spectroscopy (line profiles
indicative of accretion along magnetospheric columns). A variety of gas diagnostics sensitive to
emission arising at different temperatures (300 K at 1 AU; 150 K at 5 AU) will be used; they
include CO fundamental (4.6μm), H2 (12μm; 17μm).
1. Pre-biotic molecules in disks
A large number of extra-terrestrial organic and pre-biotic molecules are known to exist both in the
Solar system and the interstellar medium, and a better understanding of the inventory and formation
of these molecules in star-forming molecular clouds and circumstellar disks is a key goal of
astrobiology. The extent to which interstellar organic material is destroyed or modified as it is
19
accreted into and processed within protoplanetary disks is an open question. While the similar
compositions of cometary volatiles and interstellar molecules strongly suggest a direct connection,
numerous differences indicate processing within the presolar nebula. In addition, complex organic
compounds could be synthesized in regions of protoplanetary disks by the similar processes that
operate in the interstellar medium.
Mid-infrared spectroscopy with MIRES will be an essential tool for investigating the inventory
and content of organic molecules in protoplanetary disks. Most simple and complex hydrocarbon
compounds have strong mid-infrared transitions and a majority of these are accessible to groundbased observations. High spectral resolution is essential, particularly in searching for more rare
molecular species. A given spectral region may be crowded with molecular lines, and lines of more
complex molecules will be weak due to lower abundances and the typically larger number of
transitions. Thus, the very high signal-to-noise observations obtainable only with TMT are required.
Broad wavelength coverage, providing access to large numbers of transitions, will also increase the
detectability of rare molecules.
Possible TMT programs:
1. Probing gas dissipation timescales by systematically observe the presence of disk at variation of
young stellar objects, and estimate;
2. Probing inner disk holes of disk, connecting the observation with RV detection of extrasolar
planets.
3. Detection of pre-bio molecules in disks. Compare them with the organic molecules in our solar
system.
China’s strengths and weakness in extrasolar planet research:
Extrasolar planet research is one of the most exciting topics in astrophysical research, with major
breakthroughs in the last decade. World-leading theoretical studies of planetary systems, their
formation and dynamics are being actively pursued at KIAA (PKU) and NJU. Observationally, this
area is still under-developed in China involving only two groups nationally. A group at NAOC is
surveying young stars with the 2.16 m telescope at the Xinlong station, under a pan-Asia joint
project with Japan and Korea. So far one extrasolar planet has been found by the NAOC group 3.
Yunnan Astronomical Observaory (YNAO) will also begin RV detection of planets with the 2.4m
telescope in Lijiang through an international collaboration between University of Florida, and
University of Science and Technology of China (USTC), and Nanjing University (NJU). These
observational searches can be significantly enhanced by joint activities with institutes heavily
involved in TMT (such as UC Santa Cruz, Berkeley and Caltech), through concrete research
projects and joint student training.
3
Internationally 417 extrasolar planet systems have been discovered as of Jan. 5, 2010.
20
4.2 Fundamental cosmology
Hu Zhan (NAOC), Pengjie Zhang (SHAO), Xinmin Zhang (IHEP), Xuelei Chen (NAOC)
Key questions:
1. What is dark matter?
2. What is the nature of dark energy?
3. Do the fundamental physical constants vary as a function of cosmic time?
Cosmology is one of the central drivers of astronomical research throughout the human history.
Advances in instrumentation have led to revolutions in our fundamental understanding of the
universe. Likewise, TMT will be well poised to shed light on the most profound questions in
cosmology of the new century, such as the nature of dark matter and dark energy.
4.2.1
Nature of dark matter
Even though dark matter was first proposed a long time ago (Zwicky 1933) to explain the missing
mass in galaxies and the Coma cluster, it has thus far evaded detection except indirectly through its
gravitational effects on astronomical bodies. Meanwhile, there have been tremendous advances in
particle theories of dark matter as well as laboratory experiments for detection and production of
dark matter particles. Properties of dark matter particles, such as its mass and interaction crosssection, can have a progressively larger effect on dark matter distribution on smaller and smaller
scales, which will be easily accessible by TMT.
The leading candidates of the dark matter particle studied widely in the literature are the weakly
interacting massive particles (WIMPs, e.g., Jungman, Kamionkowski, & Griest 1996), for example
the lightest neutralino in the supersymmetric standard model with R parity. During the evolution of
the Universe, the WIMPs can be generated thermally like the cosmic background of the photons or
non-thermally (Acharya et al. 2009).
Thermally generated relic WIMPs behave like the cold dark matter (CDM). They froze out of
equilibrium with other species when WIMP annihilation became inefficient (e.g., when the universe
cooled to a temperature of the order of the WIMP mass). Non-thermally produced relic WIMPs
could be realized in various ways, for instance by decay of cosmic string loops, or decay of longlived particles, or a low re-heating temperature in inflationary cosmology. Compared to the thermal
WIMPs, the non-thermal WIMPs allow a
larger annihilation rate into leptons and could
make dark matter warm due to the large boost
of velocity when generated. Another example
for warm dark matter is a light particle with its
mass around 1 keV in thermal distribution.
Figure 2: Power spectra of thermal WIMPs (dashed
line), non-thermal WIMPs (solid line), and warm
thermal WIMPs with a mass of 1 keV (dotted line).
From Lin et al. (2001).
21
Figure 2 shows the suppression of the smallscale matter power spectrum in non-thermal
and warm thermal WIMP models relative to
the standard CDM model. The amount of
suppression is of a few orders of magnitude at
scales of a few hundred kpc. Precise
measurements of the matter power spectrum
at these scales could potentially distinguish
different WIMP models, though the scales
involved are too small to
models, though
the scales involved are too small to be studied with galaxy two-point statistics. TMT will meet the
challenge through alternative routes described below. The results would be highly complementary
to any experiments for direct and indirect detections as well productions of dark matter particles.
Dark matter halo profile
It is well known from N-body simulations that CDM halos follow a universal NFW (Navarro,
Frenk, & White 1997) profile, which has a cuspy center. Depending on its scattering and selfannihilation cross-section, dark matter particles could form different amount of substructures and,
possibly, a central core. Hence, measurements of dark matter density profiles can be used to place
constraints on dark matter interaction cross-sections. Dark matter substructures are difficult to
detect to high completeness for an individual halo, so one may need a large sample for statistical
detection. To measure the central density profile, it is better to use galaxy clusters, dwarf galaxies,
and low surface brightness galaxies, whose center is less dominated by baryonic components,
though one could still model and fit different components of normal galaxies with sufficiently high
signal-to-noise data.
TMT can measure halo density profiles with lensing and kinematics. For galaxy clusters, strong
lensing on arcmin scale is the best way to probe the inner profile. For instance, the central image of
a multiply lensed galaxy can place a strong constraint the central density profile (see Tyson,
Kochanski, & Dell’Antonio 1998). Stellar kinematics in dwarf spheroidal galaxies offers another
unique probe of dark matter halos. It is found with a sample of 18 Milky Way satellites that these
galaxies have a common mass of 107 solar masses within 300 parsecs, which rules out thermal
warm dark matter candidates lighter than about 1 keV (Strigari et al. 2008). The TMT IRIS IFU
instrument will be well suited for such studies; it would extend measurements of dwarf spheroidals’
stellar velocity dispersion beyond the Milky Way and could place tighter constraints on dark matter
properties with a much larger sample.
Small-scale power spectrum from the Ly forest
As seen in Figure 2, one needs precise measurements of the matter power spectrum at scales of a
few hundred kpc to distinguish different dark matter candidates. So far, such small-scale
measurements can only be achieved with the Ly Forest, which is a series of Ly absorption lines
in QSO spectra with neutral-hydrogen column densities of 1012—1017 cm−2 (Rauch 1998). These
lines are resulted from absorptions by the diffusely distributed and photoionized intergalactic
medium, which traces the underlying mass field at low overdensities (e,g., Cen et al. 1994; Zhang,
Anninos & Norman 1995; Bi & Davidsen 1997). Therefore, the Ly forest can be used to measure
the matter power spectrum on small scales and constrain cosmology (e.g., Croft et al. 1998;
McDonald & Miralda-Escudé 1999).
For each line of sight to a QSO, the Ly forest can sample the density field nearly continuously in
one dimension. The TMT HROS instrument could sample enough QSO spectra in a large volume,
so that one could obtain a more complete picture of the universe and tighter constraints on
cosmological parameter using the Ly forest. It is noted that BOSS (SDSS III) and BigBOSS plan
to measure baryon acoustic oscillations with dense Ly forest sampling at spectral resolution of
2000. However, many observational and astrophysical effects on the Ly forest, e.g., continuum
fitting (Hui et al. 2001), the UV ionization background (Meiksin & White 2004), metal-line
contaminations, etc., would require much higher resolution spectra to be disentangled from
underlying matter fluctuations.
4.2.2
Nature of dark energy
The accelerated cosmic expansion (e.g., Riess et al. 1998; Perlmutter et al. 1999) has led to yet
another puzzle – dark energy. Like dark matter, dark energy is thought to be connected to
fundamental physics, maybe at even more profound level. However, unlike dark matter, whose
22
gravitational effect is detected in objects as small as galaxies, dark energy might not have
appreciable effects on scales much smaller than the Hubble radius. One must bear in mind that dark
energy is only one class of explanation for the cosmic acceleration. Another widely discussed
possibility is that laws of gravity might depart from four-dimensional general relativity (GR) on
very small or very large scales. TMT, with its powerful imaging and spectroscopic capabilities, can
realize a number of tests for gravity and dark energy models.
Very high redshift type Ia supernovae for dark energy and curvature
Dark energy constraints from type Ia supernovae (SNe Ia) depend the prior on the mean curvature
of the universe (Linder 2005; Knox, Song, & Zhan 2006), because the sensitivity of the luminosity
distance to curvature is somewhat degenerate with the sensitivity to the dark energy equation of
state (EOS) parameter wa where the EOS is parametrized as w = w0 + wa (1 - a) (see Figure 3) With
only low redshift SNe Ia, it is hard to break the degeneracy. However, as shown in Figure 3, high
redshift luminosity distances are mostly sensitive to curvature and, hence, can constrain curvature
very effectively, which in turn helps determine the
EOS parameters. The mean curvature itself is of
great theoretical interest as well. Even though
inflation flattens the universe, it generally predicts
a residual mean curvature of the order of 10-5.
There may be a 10% chance to have a mean
curvature of the order 0.001 (Freivogel et al. 2006),
which is within the reach of future surveys (Knox,
Song, & Zhan 2006).
Because of its huge light collecting area and IR
capability, TMT could easily obtain spectra of SNe
Ia up to z ~ 4. To discover these high redshift SNe
Ia, one would need a large field-of-view opticalto-NIR instrument to conduct a dedicated survey
Figure 3: Sensitivity of the luminosity of at least a few square degrees, which might not
distance to the mean curvature and dark be the best use of TMT. Fortunately, other surveys,
energy equation of state parameters w0 such as the Large Synoptic Survey Telescope
and wa. From Zhan (2006).
Deep Drilling Survey (ugriz ~ 28 magAB, y ~ 26.8
magAB) and the Chinese Kunlun Dark Universe
Survey Telescope (in NIR bands), could provide alerts of potential candidates for spectroscopic
follow up. Combining with low redshift SNe Ia from other surveys, TMT could investigate possible
evolution of SN Ia population, constrain the mean curvature to 10-3, and provide complementary
constraints on dark energy EOS parameters. Furthermore, even though dark energy is thought to be
subdominant at high redshift, there is no measurement to prove one way or another. SNe Ia from
TMT could in principal be used to study the behavior of dark energy at high redshift.
Testing GR at cluster scale and above
Testing GR in the cosmos is of crucial importance not only in understanding gravity, but also in
understanding dark matter and dark energy (Zhang et al. 2007). TMT is possible to perform several
tests of GR at cluster scale and above simultaneously, none of which has reached 10% accuracy.
With a dedicated spectroscopic survey of a few square degrees, overlapping with existing or then
concurrent lensing surveys such as CFHTLS, Pan-STARRS, and LSST, the discriminating power
between alternative gravity models will be significantly improved. The key requirement is to
measure spectroscopic redshifts of 105  6 galaxies to z~1-2 or higher and ~100 type Ia supernovae to
even higher redshift.
(1) Measuring the parameterized post-Newtonian (PPN) parameter gamma at cluster scale.
23
The post-Newtonian parameter gamma quantifies the difference between the cluster lensing mass
and dynamical mass, which vanishes in GR. In combination with lensing surveys such as CFHTLS,
Pan-STARRS, and LSST, TMT will be able to perform such test. With optical and near-IR
spectrometers, TMT can significantly improve the measurement accuracy of both masses and thus
improve the robustness of the PPN measurement.
To robustly measure the cluster dynamical mass, the rms error in member galaxy velocity
measurement should be controlled to be less than 100 km/s, corresponding to a ~1% statistical error
in cluster mass. A cluster at z  1 is of a few arc-minutes in diameter, with ~ 103 galaxy members.
WFOS, with 40 square arc-minute field of view and the ability to measure 1500 spectra
simultaneously, is suitable for this task. With redshift measurement of more than ~100 galaxy
members, the model uncertainties in cluster dynamical mass reconstruction will be significantly
reduced.
A big uncertainty in cluster lensing mass reconstruction through strong and weak lensing is the
redshift uncertainty of source galaxies, a large fraction of which lies at redshift beyond 1. With both
the optical and near-IR spectrometers, TMT will be able to measure the redshifts of galaxies along
the sightline of galaxy clusters and thus eliminate this uncertainty. Another uncertainty is errors in
the lensing kernel, which is sensitive to errors in the distance. In combination with the distance
measurement from SNe Ia in the same field, the lensing kernel can be calculated in a self-consistent
way.
With ~100 massive galaxy clusters in the field, TMT is likely able to push the gamma measurement
to the limit of systematical errors. Given the large number of member galaxy redshifts, it is possible
to diagnose and reduce some systematical errors, such as the galaxy velocity bias and the velocity
anisotropy.
(2) Measuring the structure growth and testing the GR consistency relation
With a million redshifts to z~1-2 in a few square degrees, the 3D galaxy power spectrum can be
measured with more than 1000 independent modes in the linear and mildly non-linear regime. This
allows robust measurement of redshift distortion and the reconstruction of large scale peculiar
velocity. The structure growth of the universe can be measured with ~10% accuracy. This
measurement alone is already able to test GR and discriminate it from several alternative gravity
models.
In combination with the measurement of the expansion rate from SNe Ia, one can perform a
direction test on the GR consistency relation between the expansion rate and the structure growth
rate. It reads
f 
d ln D
 0m.56 (a ) .
d ln a
The left hand side can be measured from the redshift distortion and the right hand side can be
measured from SNe Ia.
It is also possible to perform further tests of GR. For example, in combination with the weak
lensing measurement, we can test the Poisson equation at cosmological scales. This allows us to
~
measure the quantity EG  Geff / f and further probe the nature of gravity.
4.2.3
Constancy of fundamental constants
Constancy of fundamental physical constants, such as the fine structure constant and speed of light,
constitutes another class of interesting tests. Since the currently well established cosmological
frame work is based on the constancy of these constants, any definitive detection of departure from
constancy will lead to fundamental changes of our view about the universe. Note that running
24
coupling constants are no stranger to particle physics, but departure from constancy over time at the
same energy level would require an exceptional explanation.
The Ly forest mentioned above can be used for such investigations, though hydrogen Ly lines
become a contaminant (for a review, see Uzan 2003). For the fine structure constant , one ideally
needs a line transition that is not sensitive to for measuring redshift and another that is very
sensitive to for determining the change in . Current limit on the variation of below redshift 2.5
is  ~10-5, which requires spectral resolution R ~ 40000. With TMT HROS, one could extend
the measurements to much higher redshift.
Possible TMT programs:
1. A survey of stellar kinematics of several thousand dwarf spheriodals with TMT/IRIS to study
dark matter properties.
2. A legacy survey of TMT/WFOS of galaxies in a few squares of degrees to measure the growth
of structure and test General Relativity.
3. A survey of galaxy kinematics of several thousand clusters with TMT/WFOS to measure PPN
parameter gamma.
4. TMT/HROS of observations of selected quasars at very high resolutions to study the nature of
dark matter and the variation of fundamental constants.
China’s strengths and weakness in this area:
There are a healthy group of theorists (IHEP, NAOC, SHAO) working on the nature of dark matter,
dark energy, and modified gravity. However, observational expertise, particularly on the 8-10m
class telescopes is lacking, and further PhD students should be trained in these areas.
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25
4.3 The formation and growth of black holes
Qinjuan Yu (KIAA), Weimin Yuan (YNAO), Youjun Lu (NAOC), Zhiqiang Shen (SHAO), Jianmin Wang (IHEP), Tinggui Wang (USTC), Xuebing Wu (PKU), Feng Yuan (SHAO), Yefei Yuan
(USTC), Shuangnan Zhang (IHEP), and Hongyan Zhou (USTC)
Active galactic nuclei (AGNs) and their luminous manifestation, quasars, refer to the energetic
activity, such as enormous electromagnetic radiation and mass ejection in relativistic jets in the
nuclei of galaxies, which are believed to be powered by accretion onto massive black holes
(MBHs). The discovery of AGNs/quasars and the MBH explanation to their energetics motivated
the search for dormant MBHs at the centers of nearby galaxies as the remnants of nuclear activities.
In the past two decades, MBHs have been indeed found to be ubiquitous in the centers of nearby
elliptical galaxies and spiral bulges (including the center of our own Milky Way). The MBHs in
both active and normal quiescent galaxies are of great importance in our understanding of their
assembly history over cosmic time.
It has been shown that the masses of the MBHs in nearby normal galaxies are tightly correlated
with their host galaxy properties (e.g., effective stellar velocity dispersions, luminosities, stellar
masses of the spheroidal components), which implies that growth of MBHs is closely connected
with galaxy formation and evolution. The energy or momentum output from the nuclear activity
during the MBH growth is proposed to be responsible for switching off the star formation in the
host galaxy and shutting down further growth of the MBH, and may thus control the ultimate size
of the galaxy and the MBH. However, the detailed physical processes responsible for the observed
relationships are not clearly understood yet. Precise determination of these relationships over a
large range of masses and redshifts will be not only important for revealing the underlying physics
controlling these relationships but also a key step towards understanding the coevolution of MBHs
and galaxies.
AGNs/quasars can be observed at redshifts as high as up to 6-7, rendering themselves to be valuable
laboratories to probe MBHs to large cosmic distances, which would not be possible for most
inactive galaxies with the current and foreseeable instruments. As the main phase of MBH growth,
AGNs/quasars is probably the most important stage to link the MBH to the evolution of their host
galaxies. Without proper knowledge on this stage, the understanding of the formation and evolution
of galaxies would not be complete.
The Galactic center is a unique laboratory for studying general relativistic effects and stellar
dynamics around a MBH due to its proximity. TMT will potentially reveal much more details on
the motion of central stars and the near-infrared radiation from the innermost region of Sgr A*,
which will lead to accurate measurements of the mass of the central MBH and possibly its spin, and
improve our understanding of physical processes occurring in the vicinity of a MBH.
With its unprecedented diffraction-limited spatial resolution and light collecting power, as well as
the 3-D spectroscopic capability, TMT is expected to advance our knowledge on MBHs and related
research, specifically, the demography of MBHs in external galaxies in the local universe (S4.3.1),
the MBH at the center of our Galaxy (S4.3.2), MBHs at higher redshifts and their coevolution with
galaxies (S.4.3.3) and the physics of AGNs (S4.3.4).
4.3.1
Massive black holes at the center of external galaxies
Key questions:
1. Do intermediate mass black holes exist in the centers of dwarf galaxies and/or globular
clusters? If any, do they follow the same black hole mass versus velocity dispersion (MBH-σ)
26
relationship as massive black holes do?
2. Are massive black holes in brightest cluster galaxies (BCGs) substantially larger than the
expectation from the currently determined MBH-σ relationship?
3. Is the MBH-σ relationship universal for galaxies with different morphologies?
4. Does the MBH-σ relationship evolve with cosmic time?
Introduction:
Over the past two decades, detections of MBHs in nearby normal galactic centers have been
accumulated to a number of more than three dozens. The masses of these MBHs are tightly
correlated with the velocity dispersions of the spheroidal components of their host galaxies, i.e.,
following the MBH-σ relationship MBH=1.5×108Msun(σ/200km s-1)4.02 (e.g., Tremaine et al. 2002).
The discovery of this fundamental relationship has triggered many studies on evolution of MBHs
and galaxies and advanced our understanding of it in recent years.
However, there still exist disputes in the determination of the slope, the normalization, and the
intrinsic scatter of this relationship. It is also unclear whether this linear relation (in the logarithmic
space) maintains at either the high-mass or the low-mass end, whether this relationship is universal
in galaxies with different morphologies, and how this relationship evolves with cosmic time (Barth
et al. 2005; Lauer et al. 2007; Gebhardt et al. 2005; Treu et al. 2006). To solve these problems, a
much larger sample of BHs spanning over a larger range of masses and redshift and locating in
various types of galaxies are required, which cannot be achieved yet by HST and other current
ground-based telescopes due to the limitation in their resolution and sensitivity.
To confidently measure the mass of a MBH, it is necessary to probe the region within which the
MBH dominates the gravitational potential, and this region is usually defined by the radius of the
sphere of influence of the BH: reff=GMBH/σ2=13pc(MBH/108Msun)0.5. With the adaptive optics
system, the Infrared Imaging Spectrograph (IRIS) for TMT can achieve a spatial resolution close to
the diffraction limit, i.e., ~8mas(λ/μm), which surpasses the spatial resolution of the HST by almost
one order of magnitude. Hence TMT is capable of detecting a MBH with mass MBH at an angular
distance up to DA=335Mpc(μm/λ)(MBH/108Msun)0.5, which corresponds to a distance of 1Mpc or
3Mpc for MBH=103Msun or 104Msun and corresponds to a redshift of z=0.1 or 0.4 for MBH=108Msun or
109Msun.
Determination of the BH masses in quiescent galaxies relies on the detection of the positional
variation of the stellar kinematics/velocity dispersion within the sphere of influence of the BH. The
relatively large collecting area of TMT will enable detection of objects with faint stellar surface
brightness, e.g., the center of M87; and the IFU spectrographs on TMT can reveal the intrinsic
dynamical structures in galactic centers better than long slit spectroscopy used before, especially for
those with complex kinematics. For some radio galaxies with large central BHs (>3×109Msun)
similar to M87, the IRIS on TMT may be able to detect their central BH mass out to somewhat
higher redshifts through the Keplerian rotation of their nuclear gaseous disks inferred from the
positional variation of their emission lines (e.g., Macchetto et al. 1997).
Main science areas:
1. The existence of intermediate-mass black holes in dwarf galaxies and/or globular clusters
Intermediate-mass black holes (IMBHs) are the missing link between stellar-mass BHs and MBHs,
which are of great importance as possible seeds of MBHs. The probable hosts of these IMBHs are
globular clusters and/or dwarf galaxies. Some dynamical evidence was reported for the existence of
IMBHs in globular clusters, such as G1 and M15. However, the existence of IMBHs is still
controversial partly due to the insufficient spatial resolution and sensitivity of current telescopes.
27
With the great spatial resolving capability of TMT, IMBHs at the centers of globular clusters or
dwarf galaxies in the local group should be detectable if they exist.
2. The most massive black holes in brightest cluster galaxies
The most massive BHs are probably hosted in brightest cluster galaxies (BCGs), whose progenitors
correspond to those BHs powering the brightest QSOs in the distant universe. It appears that MBH
masses expected from the MBH-σ relationship conflict with those from the BH mass versus galaxy
luminosity relationship for BCGs. However, the reason to cause the conflict and whether this
conflict is real or not is still uncertain, as the most massive BHs elude from the dynamical detection
mainly due to its rarity within the small cosmic distance explored by HST and ground based
telescopes. TMT will enable dynamical measurements of 109Msun MBHs up to z=0.4 and 1010Msun
MBHs throughout the whole universe. Measuring the masses of these most massive BHs will
provide powerful constraints on the growth history of MBHs and greatly improve our understanding
of the demography of QSOs and MBHs.
3. Black hole demography with different galaxy morphologies
The masses of MBHs are correlated with the properties of the hot components of their host galaxies.
The hot components of galaxies with different morphologies may have different origins, in which
the MBH-σ relationship is not necessarily the same. Some observations have indicated that the
normalization of the MBH-σ relationship for pseudo-bulges may be substantially smaller than that for
early-type ellipticals. TMT will enable dynamical measurements of MBHs in different types of
galaxies, including ellipticals, spiral bulges and dwarf spheroids, and thus help to understand how
much difference exists in the MBH-σ relationship among different galaxy types and further provide
important clues to the driving force for this relationship.
4. Evolution of the relationship between MBH masses and their host galaxy properties
The MBHs detected through stellar dynamics in normal quiescent galaxies so far are within a
distance ~100Mpc, and hence the related BH demography is limited only to the nearby universe.
The spatial resolving power and sensitivity of TMT enable direct dynamical measurements of
MBHs with masses 108Msun (or 109Msun) up to redshift z=0.1 (or 0.4). An IRIS IFU spectroscopic
survey may provide a sample of MBHs in quiescent galaxies up to redshift z=0.4. and reveal the
evolution of the MBH-σ relationship for normal galaxies. Dynamical detections of the MBHs located
at redshift z=0.1-0.4 will also help to calibrate the MBH masses measured by the reverberationmapping technique (see S4.3.3) and further explore the evolution of the relationship at even higher
redshift.
Possible TMT programs:
1. IRIS IFU spectroscopic surveys of galactic centers to obtain the kinematics and velocity
dispersion distributions of their central stars (or nuclear gaseous disks for some radio galaxies).
A meaningful sample needs to be carefully selected to cover (i) both the low-mass and highmass ends of MBHs (or both the low-σ and high-σ ends of galaxies), (ii) different galaxy
morphologies (including dwarfs and globular clusters), and (iii) galaxies at high redshift.
Through the survey, it is expected to have a better determination in the slope, the normalization,
and the intrinsic scatter of the correlation between the black hole mass and galaxy properties,
and its evolution.
China’s strengths and weakness in this area:
28
There are already a large number of people working in the black hole area, at IHEP, NAOC, PKU,
SHAO, THU, USTC, and YNO. Some of the works have significant impacts in the international
community of the BH research.
However, so far a limited number of people in China have experiences working in the area of
detailed modelling of stellar dynamics and gas dynamics around a MBH. We need to train more
Ph.D. students to work on both dynamically modelling of stellar systems with central BHs and
using observational data to measure BH masses. Another potential area that we require substantially
more expertise is the modelling of co-evolution of galaxies and their central MBHs.
References:
1.
2.
3.
4.
5.
6.
7.
TMT Science Advisory Committee, Thirty Meter Telescope Detailed Science Case: 2007
Tremaine, S. et al. 2002, ApJ, 574, 740
Gebhardt, K., Rich, R. M., Ho, L. C. 2005, ApJ, 634, 1093
Barth, A. J., Greene, J. E., Ho, L. C. 2005, ApJ, 619, L151
Lauer, T. et al. 2007, ApJ, 662, 808
Treu, T., Koopmans, L. V. E., Bolton, A. S., Burles, S., Moustakas, L. A. 2006, ApJ, 650, 1219
Macchetto, F. et al. 1997, ApJ, 489, 579
4.3.2
Black hole at the Galactic center
Key questions:
1. What is the mass of the MBH at the Galactic center and what is the mass distribution around
the MBH?
2. Does the MBH at the GC spin and how fast?
Introduction:
The MBH located in the center of our Galaxy, Sgr A*, is unique due to its proximity. Probably it
offers us the best laboratory to study strong gravitational field and its related general relativistic
effects, stellar dynamics around a MBH, and some other physical processes in the vicinity of a BH,
such as accretion and ejection. Investigations of these processes in the Galactic center (GC) will
help us understand many similar phenomena in other environments, such as in AGNs, and further
provide insights on the formation and evolution of galaxies and their nuclei.
In the past decade, probably the most important progress in the GC is the determination of the orbits
of some individual S stars moving within 0.04pc from Sgr A*, which provides the strongest
evidence for the existence of a central MBH in the GC and the best mass measurement among the
MBHs detected in galactic nuclei. The study of the stellar kinematics in the GC can be further
improved by the IRIS and WIRC on TMT with the following distinguished characteristics: (i) the
limit of K band can be 22 mag, much fainter than that of Keck; (ii) the astrometric precision limit
can be as small as 50-100μas; and (iii) the accuracy of radial velocities can be within ~ 10kms-1.
Taking those advantages, TMT can detect a larger number of (fainter) stars surrounding the MBH
(e.g., ~100) with better astrometric accuracy. It is likely to discover stars on highly eccentric orbits
with pericenter distances closer to the MBH, approaching the region where general relativity takes
effect.
Main science areas:
1. Mass of the central MBH and the mass distribution around the MBH
The mass of a BH is one of its basic parameters. The current mass estimate of the BH in the GC is
(4.1±0.6)×106Msun (Ghez et al. 2008) or (4.3±0.4)×106Msun (Gillessen et al. 2009), achieved by
29
Keck and VLT (8-10m). By detecting the orbital motions of a larger sample of central stars with
better kinematic accuracy to be obtained by TMT, the accuracy of the MBH mass to be determined
is expected to be less than 0.1% at the 99.7% level. Accurate measurements of the BH mass,
together with the kinematics of the central stars, can provide important constraints on the mass
distribution around the MBH in our own Galaxy.
2. Spin of the central MBH and its general relativistic effects
The general relativistic effect and the spin of a BH may be manifested in motions of stars if they
can move sufficiently close to the BH, e.g., by measuring their orbital pericenter advance due to the
relativistic prograde precession or detecting the frame dragging effects due to the spin of the BH.
Currently the smallest pericenter distances of the detected central stars is ~70AU, and the shortest
orbital period is ~15.8yr (Ghez et al. 2005; Gillessen et al. 2009). TMT can detect fainter stars with
accurate orbital motion measurements and thus enables the possibility of discovering some moving
closer to the central MBH.
3. Near-infrared flares of Sgr A*
One of the most interesting findings recently in Sgr A* is its multi-waveband flares, ranging from
radio, sub-millimeter, infrared, to X-ray (e.g., Eckart et al. 2008; Yusef-Zadeh et al. 2009). Radio
observations indicate that the flare is very likely associated with ejection of blobs. The phenomenon
of multi-waveband flare and ejection is common in BH systems, including AGNs, BH X-ray
binaries, and Gamma-ray bursts. Recently it is proposed that they are due to a similar physical
mechanism as that for the solar flare and coronal mass ejection in the Sun (e.g., Yuan et al. 2009).
However, we still know little about the origins of the flare and ejection. Some key observations at
infrared will be extremely valuable in this context. Continuously varying NIR flares from the
central region have been detected by both VLT and Keck. However, it is still a debate on whether
there is a quasi-period oscillation in the detected flare on a timescale of ~20min (i.e., QPO
phenomena, Genzel et al. 2003; Do et al. 2009), roughly the period of the innermost stable circular
orbit (ISCO) around the BH. TMT can detect the time evolution of the IR flux, the polarization, and
the spectrum of the flares. This will help to solve some important issues (such as, whether the
claimed QPO phenomena exist or not), improve our understanding of the underlying physics of the
flares and ejection. If the flare is due to some hot spot orbiting the BH near the ISCO, it would be
also possible to extract the BH mass and spin by observing the centroid path of the hot spot with the
high astrometric precision of TMT.
Possible TMT programs:
1. Monitor the kinematics of the stars in the Galactic center. Monitor some innermost stars for
more than one orbital period (e.g., >10yr).
2. Monitor the innermost region of Sgr A* in the GC to possibly detect IR flares, and coordinate
with other monitoring at X-ray and radio/sub-millimeter wavelengths.
China’s strengths and weakness in this area:
Quite a few people in China have done theoretical work and radio observations of the GC at
NAOC, PKU, SHAO, USTC, making some important contributions in this area.
However, we lack people who can utilise state-of-the-art large facilities, such as Keck and VLT in
NIR bands to monitor the central region of the GC. We need to train more Ph.D. students to work
on these.
References:
30
1.
2.
3.
4.
5.
6.
7.
8.
9.
TMT Science Advisory Committee, Thirty Meter Telescope Detailed Science Case: 2007
Ghez, A. M. et al. 2008, ApJ, 689, 1044
Ghez, A. M. et al. 2005, ApJ, 620, 744
Eckart, A. et al. 2008, A&A, 492, 337
Yusef-Zadeh, F. et al. 2009, ApJ, 706, 348
Gillessen, S. et al. 2009, ApJ, 692, 1075
Yuan, F. et al. 2009, ApJ, 703, 1034
Genzel et al. 2003, Nature, 425, 934
Do et al. 2009, ApJ, 691, 1021
4.3.3
Black holes at higher redshift
Key questions:
1. Do AGNs follow the same black hole mass—bulge relation as normal galaxies? Did this
relation evolve with cosmic time?
2. What is the relation between AGN and star formation?
3. How are AGNs fuelled and triggered?
4. When did the first MBHs form?
5. What is the lower-end of the mass function of black holes at galactic centers?
6. Can black hole masses be directly measured for a larger sample of AGNs?
Introduction:
At higher redshifts direct dynamical studies of most MBHs at the centers of normal galaxies are
hampered by their projected sizes of the sphere of influence being too small to be spatially resolved.
Fortunately, MBHs reveal their presence in AGNs with rich observational signatures, enabling their
studies out to large cosmic distances. Quasars at redshifts as high as up to 6-7 have been discovered,
indicating that supermassive black holes (SMBHs) with masses up to 109 Msun had already formed
even at only a few percent of the age of the Universe. It has long been noted observationally that
AGN and starburst in host galaxies are most likely linked to each other. Recent advances in
observational studies of MBHs established a surprisingly tight relationship between black hole mass
and the bulge of the host galaxy, suggesting coevolution of the two. In terms of the growth of MBH
and the coevolution with galaxies, AGN is the most important phase when MBHs gain their masses
rapidly by accretion and interact with their host galaxies with various ways of feedbacks. However,
the detailed physical processes of how this happens are not clear.
The high sensitivity and spatial resolution of TMT are very useful to study the host galaxy
properties of low-redshift AGNs, such as morphology, dust properties, metallicity, and star
formation history. The intergalactic environment of AGN can also be explored in great detail at
both low and high redshifts. Quasar elemental abundances can be measured using intrinsic narrow
absorption lines, narrow emission lines, or broad emission lines. This will provide unique probes to
high-redshift star formation and galaxy evolution. Dual AGNs as a probe of the galaxy merging are
also expected to be directly resolved spatially with TMT. Some of the most important topics are
outlined below.
Main science areas:
1. Coevolution of AGN and galaxy
1) AGN host galaxies and the black hole--spheroid connection
The well-known black hole--bulge relation was established mainly with observations of local
normal galaxies (Magorrian et al. 1998; Tremaine et al. 2002). Whether or not this relation is
applicable to active galaxies and quasars needs obviously more accurate determinations of host
galaxy properties for a large sample of AGNs and quasars. The bulge properties of host galaxies of
low-redshift quasars were studied only in several previous studies in the optical and near-IR bands
31
with HST and largest ground-based telescopes, and the results are very inconclusive due to the large
uncertainties, however (Dunlop et al. 2003; Guyon et al. 2006). Future near-IR imaging and
imaging spectroscopy of TMT will definitely improve this kind of study. The AO-fed IFU in the
NIR will be able to observe the host galaxies by removing the bright AGN core or performing high
dynamic range observations for both the core and galaxy at the same time. The CO absorption
feature in the near-IR H band can directly probe the stellar velocity dispersion of host galaxies of
low-redshift quasars.
At high redshifts, the black hole--bulge relation study is much more difficult than in the local
universe. As the UV/optical emission lines move to the near-IR band, only spectroscopy in near-IR
can be used to measure both the broad and narrow emission lines. The kinematics of broad lines can
be used to estimate the black hole masses while that of narrow lines to probe the stellar velocity
dispersion. At present, such study can only be done for several high-redshift quasars with the largest
ground-based telescopes such as VLT and Keck (Willott et al. 2003; Barth et al. 2003). With TMT,
we will be able to investigate the M-bulge relation for AGN over a large cosmic epoch from local to
high-redshift universe.
2) AGN and star formation connection
The physical connection between the starburst and triggering SMBH has a long study history
(Kauffmann et al. 2003). The connection was greatly enhanced recently by the well-known
Magorrian relation and M-sigma relation which are suggestive of the co-evolution of MBHs and
galaxies. High spatial resolution observations of circumnuclear star forming regions were started
with ESO VLT to explore the relation between the stars forming and the fueling of AGN activities
(Davies et al. 2007). The role of the starburst in triggering AGN activities has been drawn much
attention, in which massive stars are driving the strong turbulence to transport the angular
momentum outward (Wada & Norman 2002; Chen et al. 2009). We need to measure the
supernovae-driven turbulence of interstellar medium and the inflow velocity in nearby galaxies. The
detail study may allow us to determine the time lag between starburst and triggering AGN. TMT is
expected to advance studies in this field by taking advantage of its AO-fed spectroscopic and IFU
systems in near-IR.
3) AGN fueling and triggering mechanism, AGN intergalactic environment
AGNs must be fueled by gaseous material supplied by galaxies, but it is currently not known how
the gas in the galaxy is transported inward into the central sub-pc scale in the nucleus. There is
tentative dynamical evidence of gas streaming inward along the spiral arms in several galaxies with
low-luminosity AGNs from AO-aided 10m-class telescope observations (e.g. Storchi-Bergmann et
al. 2007). With the high spatial resolution and large light collecting power, the IRIS IFU in the
near-infrared provides the best tool for further mapping the 3D velocity field of the gas within an
unprecedented small region from the nucleus, possible down to pc or tens of pc scales. These
observations may be sensitive enough to trace the inward gas flow, and possibly yield estimation of
the amount of gas transportation rate. Furthermore, 3D near-infrared spectroscopic observations
will provide useful data to constrain the stellar populations in the very center of the nuclei, as well
as their star formation rates, etc.
The trigger of AGN may also have something to do with the intergalactic environment, but no
consensus has been reached so far. The WFOS of TMT is an ideal instrument to carry out such
tests, given its wide field and a large number of spectra taken simultaneously, combined with the
large aperture of TMT.
2. Black holes at the highest redshifts
Detection of MBHs at the highest possible redshifts traces back the cosmic epoch when the first
generation of MBHs were formed, and is therefore important for understanding the formation of
MBHs. The best way to detect such ancestors of monster is to discover quasars at high redshifts.
32
These objects are also important for probing the re-ionization of the Universe. Currently there are a
handful of quasars discovered in the highest redshift range (z~6), found using the SDSS and 10mclass telescopes. The current selection form SDSS must have missed faint objects, AGNs with
substantial reddening and objects at even higher redshifts (completely blank in optical). The future
multi-wavelength surveys, such as VISTA, Pan-STARRS, LSST, SKA, eROSITA are expected to
provide candidates. TMT’s extreme light collecting power and infrared capability will be suitable
for identifying more quasars, faint or bright, at redshifts around 6 and beyond. Furthermore, highresolution spectroscopy of high-redshift quasar will shed light on the metallicity in these systems at
early epoch of the universe.
3. Small black holes in the active nuclei of dwarf galaxies
Intermediate mass black holes (IMBHs) less than a million solar masses are expected to be present
at the centers of dwarf galaxies. Due to their small sphere of influence only very nearby dwarf
galaxies can be searched for black holes by the stellar- dynamical and gas-dynamical method. The
problem is even more severe for IMBHs with small black hole masses. However, they can reveal
themselves more easily if they are in the AGN state, i.e. accreting gas fast enough to make the
nuclei bright and produce AGN signatures. AGNs with small BH masses in dwarf galaxies are
difficult to detect. Currently only about two hundred candidates (~105-6Msun; Greene & Ho 2007,
Dong et al. 2010) have been identified, mostly from the SDSS. Yet those with truly small black
hole masses (~105solar masses or below) are to be found. The difficulties lie in that the AGN
luminosity and signatures produced by a small black hole are too weak to stand out from the central
galaxy star light recorded within seeing-limited slit aperture (~1arcsec) for ground-based
spectroscopy. The diffraction-limited IR imaging and spectroscopic capability of TMT (IRIS and
IRMS) is able to isolate active nuclei by cutting out as much the host galaxy star light as possible,
and make AGN emission lines in near-infrared detectable by virtue of the large light collecting
power of TMT. If any broad emission lines can be detected, the mass of the black hole may be
estimated using the widely used scaling relation, or even measured using reverberation mapping as
the BLR time delay may be as short as hours. Thus TMT is expected to detect accretion-powered
IMBH with ~105 solar masses or below and offers some constraints on their population, which is
otherwise difficult to probe by stellar- and gas-dynamics.
The stellar dynamics of the bulges can also be obtained from the starlight spectrum. Moreover,
bulge morphology and luminosity can be obtained with high spatial resolution imaging. These data
will make it possible to investigate whether the BH mass—bulge relation extends to the
unprecedented small end of BH mass range.
4. Direct black hole mass measurement in AGN
The HST discoveries of spatially resolved, round planar disks of ionized gas with dust in the centers
of some nearby AGN (radio galaxies, for example, M87, NGC4261) and subsequent studies of their
gas dynamics have led to the finding of black holes and measurement of the masses in local AGNs
(Harms et al. 1994, Ferrarese et al. 1996). Currently the sample of AGNs with HST gas-dynamical
measurements is small, limited by the spatial resolution as well as the small light collecting power
of HST. This is because it is essential to well resolve the projected radius of influence, and the
surface brightness of such disks is low. Advances in this field are expected to be made with TMT.
The diffraction-limited IFU IRIS would be extremely useful to map the 3D velocity field
efficiently. Though the HST observations were performed in the optical, this method should work
in the near-IR, as demonstrated by ground-based observations (Marconi et al. 2001). If some AGNs
can be found in which both the gas disk dynamics and the broad line reverberation mapping
measurements can be applied, this would be very important as it provides direct calibration of the
AGN reverberation masses, instead of calibrated using the M-sigma relation. This would be
valuable for investigating the M-sigma relation in AGN, in turn.
33
Possible TMT programs:
1. NIR IFU and slit spectroscopic observations of samples of AGN and quasars at low and high
reshifts, respectively, over a large range. The AO-fed IFU will be able to observe the host
galaxies by removing the bright AGN core or performing high dynamic range observations for
both the core and galaxy at the same time. This will provide data for studies of AGN-host
galaxy co-evolution, as well as the fuelling of AGN.
2. Deep imaging of the fields of quasars and subsequent spectroscopic observations of field
galaxies for a sample of quasars with a relatively large redshift range. This will study the
dependence of AGN activity on intergalactic environment on large scales.
3. Identification of high-redshift quasar candidates found in future multi-wavelength surveys. This
will enlarge the sample size of quasars at redshifts ~6 and push to higher redshifts. Highresolution spectroscopic observations will give abundance information of AGNs at the highest
redshifts.
4. AO-fed NIR imaging and spectroscopy of the nuclei of dwarf galaxies will discover AGNs with
the smallest IMBHs at the centers of galaxies (see also Section Error! Reference source not
found.).
5. Diffraction-limited imaging of more nearby AGN (radio galaxies) to search for central gaseous
disks similar to that found in the nucleus of M87; follow-up IFU observations of the gaseous
disks will lead to measurement of black hole masses using gas dynamics.
References:
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
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12.
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Magorrian, J. et al. 1998, AJ, 115, 2285
Tremaine, S. et al. 2002, ApJ, 574, 740
Dunlop, S., et al. 2003, MNRAS, 340, 1095
Guyon, O., et al., 2006, ApJS, 166, 89G
Willott, C., et al. 2003, ApJ, 587L, 15
Barth, A., et al., 2003 ApJ, 594L, 95
Kauffmann, G., et al., 2003 MNRAS, 341, 54
Davies, R. et al. 2007, ApJ, 671, 1388
Wada, T. & Norman, W. 2002, ApJ, 566L, 21
Chen, Y.-M. et al. 2009, ApJ, 695, L130
Storchi-Bergmann et al., 2007 ApJ, 670, 959
Greene, J. and Ho., L.C., 2007 ApJ, 670, 92
Dong, X., et al., 2010, in prep.
Harms, R. J. et al. 1994, ApJ, 435, L35
Ferrarese , L., et al., 1996 ApJ...470..444
Marconi et al. A., 2001, 2001 ApJ, 549, 915
4.3.4
Physics of active galactic nuclei
Key questions:
1. What is the structure and dynamics of broad line region and how it forms?
2. What is the structure and dynamics of narrow line region?
3. Does dusty torus exist in AGN? What is its structure and dynamics?
Introduction:
Since the discovery of the first quasar in the 1960’s, AGNs and quasars have been one of the most
dynamic areas of astrophysical research. Tremendous progresses have been made over the past
several decades in understanding the physics of AGN. The so-called standard unification paradigm
has been established to explain the diverse phenomena observed across the whole electromagnetic
wavelength. A gaseous accretion disk around a MBH is believed to be the ultimate energy engine to
generate the required energy budget to operate AGN. This engine is supposed to be surrounded by a
geometrically thick torus of obscuring gas and dust, obscuring some of the light in its direction. In
34
between the accretion disk and the dusty torus lie ionized gas clouds in fast orbital motion and
emitting strongly broadened UV/optical/NIR emission lines, the so-called broad line region (BLR);
while further outside the torus exist gas clouds emitting the observed narrow emission lines, the
narrow line region (NLR). In about 10% AGN highly collimated jets traveling at relativistic speeds
are seen in the radio and X-ray bands. Although the big picture of AGN has emerged, there are still
many questions, even some fundamental ones, remain open. The solution to many of these
questions requires extremely high spatial resolution and large light collecting power, simply
because AGNs are very compact in sizes and at enormous cosmological distances. Doubtlessly, the
advent of TMT will substantially advance our understanding of these enigma objects in the
universe.
Main science areas:
1. Understanding the structure and dynamics of AGN broad line region
The geometry of broad line regions and the nature of the emitting clouds remain intensive debates
in AGN studies (Krolik 1997; Laor 2006). A related, equally important issue is measurement of the
black hole mass for AGN with the reverberation mapping technique for the broad line regions
(BLRs), which relies heavily on the idealized assumptions that gases in BLRs are virialized and of a
simple geometry. Such assumptions certainly break down for many AGNs, and consequently the
state of art of reverberation mapping can only offer a mass accuracy within a factor of a few, with
perhaps some unknown systematic errors. Further significant improvements depend upon a
complete understanding of the kinematics of the BLRs in AGNs, including the geometries, density
and velocity distributions of all broad line emitting gases in the BLRs. Spectroscopic observations
with high resolution and high signal to noise ratio from visible to near-infrared are therefore
required to probe the kinematics and nature of line emitting clouds in the BLR. The physical
relation between the inflows and clouds remains open. In particular, the bloated stars as potential
emitting clouds will be explored by the TMT instruments.
2. Understanding the structure and dynamics of AGN narrow line region
The narrow line region (NLR) is much extended, up to hundreds of pc, and can be spatially resolved
for some nearby AGNs with the current instruments. Its dynamics is dominated by the gravitational
potential of the central galactic bulge. Observations show that its ionization state is stratified and its
dynamics is likely complicated by interaction with radio jets and outflows. The high spatial and
spectral resolution of IRIS is an ideal instrument for mapping the velocity field of the NLR, which
will update our understanding of the structure and dynamics of the NLR, especially the inner NLR.
Interestingly, in one of the nearby AGN NGC 4151, there is tentative evidence that the inner NLR
may have a component of planar motion dominated by the central black hole potential (Winge et al.
1999), suggesting a possibility of measuring BH using the gas dynamics of the inner NLR. This can
be explored by using TMT in greater confidence.
3. Understanding the structure and dynamics of AGN dusty torus
The BH accretion disk and BLR is supposed to be surrounded by a geometrically thick torus of
obscuring gas with dust. The dusty torus is important in AGN physics not only it is an important
component of the AGN unification model, but also it likely serves as the final stock of the fueling
material that feeds the accretion disk around the MBH. The dusty tori are typically on a scale of a
few parsecs (e.g. Soifer et al. 2003; Horst et al. 2009), and were only marginally resolved in the
thermal infrared (longward of 3 micron) with the state-of-art interferometric instruments such as
VLTI (Tristram et al. 2009). The near IR emission is expected to partially come from the inner part
of the torus, which is even smaller in size. With possible extension of its AO system to the midinfrared range, TMT (with WIRC and MIRES) may be capable of starting to (marginally) resolve
35
the dusty tori, by direct imaging and spectroscopy, though the goals are rather challenging the limits
of TMT.
Possible TMT programs:
1. TMT high resolution spectroscopic survey of AGNs with different luminosities, accretion rates,
host galaxies and perhaps at different redshifts. Such observations will answer key questions
like: (1) Is the BLR physically made of more than one component? (2) Are gases in the BLR
uniformly distributed or clustered? (3) What are the relative importance between virialized,
Keplerian-rotating, out-flowing and inflowing gases? (4) How is BLR effected by the host
galaxy, dust torus and accretion state? (5) How to calculate the black hole mass with
reverberation mapping in an accurate and robust way, by considering the details of the
kinematics of BLRs in different types of AGNs?
2. AO-fed IRIS IFU observations of the NLR of a sample of nearby AGNs in the near-IR to map
the 3D velocity field of the NLR.
3. Diffraction limited near- (K-band) and mid-infrared (3-18 microns) imaging of a sample of local
AGNs to attempt to directly resolve the dusty torus. In case of well-resolved tori, their gasdynamics can be studied using spectroscopy.
China’s strengths and weakness in this area:
There are already several groups working in these areas, theoretically or observationally using
mainly the SDSS data, archival data and a limited amount of observations using world-class
ground- and space-based telescopes. Most of the related observational programs have been carried
out using publicly available survey data (e.g. SDSS) and archival data. Few projects are based on 810m telescopes using state-of-art instruments. In particular there is a lack of expertise in making use
of AO-aided observations, near-infrared observations, as well as modeling of galactic dynamical
data.
References:
1.
2.
3.
4.
5.
6.
7.
Magorrian, J. et al. 1998, AJ, 115, 2285
Krolik, J.H., 1997 ASPC, 113, 459
Laor, A., et al., 2006ApJ, 636, 83
Winge, C., et al., 1999, ApJ, 519,134
Soifer, B. T., et al. 2003, AJ, 126, 143
Horst, H., 2009, A&A, 495, 137
Tristram K.R.W., et al., 2009, A&A, 502, 67
4.3.5
Stellar mass and intermediate mass black holes
Hua Feng (Tsinghua), Shuang-Nan Zhang (IHEP), Jifeng Liu (CfA)
Key questions:
1.
2.
3.
What is the mass distribution of stellar mass black holes?
Do intermediate mass black holes exist?
What is the nature of emission from accreting black holes in quiescence?
Introduction:
Stellar mass black holes are relics of massive stars created in the core collapse at the end of the
stellar evolution. Thus, their mass is determined by the mass and chemical abundance of their
progenitor stars, and will not exceed about 20 solar masses from the core collapse of a single star
with normal metallicity in the current universe. Currently a stellar mass black hole can only be seen
in a close binary system by accreting matter from its companion star. Matter falling onto a black
36
hole will form an accretion disk on which the gravitational energy is converted to the kinetic energy
and released into radiation, making accreting black holes one of the most efficient power plants in
the universe and thus allowing us to detect them in X-rays. Therefore, stellar mass black holes are
important labs for the study of stellar evolution and the physics of accretion; they are potentially
also the best labs for studying space time in strongest gravitational field, which cannot be obtained
on the earth or even in the solar system. Moreover, they are related to many other high energy
phenomena in astrophysics like gamma-ray bursts, hypernova explosion, relativistic jets, and
cosmic rays. Although they are brighter in X-rays, optical and infrared observations of these objects
have a remarkable priority that cannot be replaced by observations at other wavelengths. So far, the
most reliable means of weighing a stellar mass black hole is via optical measurement of the velocity
curve of its companion star. Also, the outer part of the accretion disk emits in the optical and
infrared band.
Main science areas:
1. Black hole binaries in the Milky Way and around it
In the Milky Way, current telescopes do not allow us to identify black holes in a binary system with
a faint companion, a short orbital period, or from dense interstellar environment with high
extinction. For example, surveys with INTEGRAL and SWIFT of the Galactic center have
discovered a large number of hard X-ray and soft gamma-ray sources [1]. The nature of many of
these sources is still unknown. Some of them are identified to be associated with a faint optical
object due to a large distance and high extinction. TMT will be able to see more optical
counterparts of these sources and measure radial velocity in some of them. This will help us
eventually discriminate their nature as being black holes or neutron stars. When the orbital period of
a black hole binary is short, large collecting area is needed to measure the line shifting from the
companion star at time scales much shorter than the orbital period. TMT will be able to discover
black holes in these short period systems, or the so-called compact or ultra-compact systems.
Identification of black holes in these systems will improve our view of the formation and evolution
of black hole binaries.
The Small Magellanic Cloud (SMC) and Large Magellanic Cloud (LMC) at the outskirts of the
Milky Way also harbor many stellar mass X-ray binaries. Although at larger distances than Galactic
sources, they do not suffer the significant interstellar dust extinction in the Galactic plane, which
makes optical identifications and further velocity curve measurements very difficult for Galactic
black holes. With TMT, optical identifications and precise mass measurements of many stellar mass
black hole in SMC and LMC will become accessible, and thus potentially providing a rich sample
of stellar mass black holes.
Therefore, with TMT the population of black holes with a dynamical mass determination will
explode, which will enable us to establish a statistically meaningful mass distribution of stellar mass
black holes, far beyond the currently only hands full of stellar masses black holes with reasonably
measured masses. Specifically we expect to increase the number of black hole mass measurements
by a factor of 5-10 in the Milky Way, and obtain at least about 10-20 black home mass
measurements in SMC and LMC. We expect to answer questions like: What is the smallest black
hole that could be formed in the core collapse? How will the metallicity of a massive star affect the
mass of the black hole it forms? What is the relation between stellar mass black holes and
intermediate mass black holes if the latter exist?
2. Ultra-luminous X-ray sources and black hole binaries in external galaxies
Accreting compact objects cannot exhibit a luminosity over the Eddington limit if the accretion
flow is spherically symmetric; otherwise the radiation pressure starts to balance the gravitational
37
force and no further accretion can continue. Although the accretion flow around black holes is
usually not spherically symmetrical, super-Eddington radiation is rarely observed from stellar mass
black hole binaries. Ultra-luminous X-ray sources (ULXs) are non-nuclear objects with luminosities
routinely exceeding the Eddington limit of a 20 solar mass black hole. They are therefore
speculated to harbor intermediate mass black holes of 102-103 solar masses [2]. Of course some
ULXs may contain stellar mass black holes accreting at super-Eddingtion rate, though rarely seen in
the Milky Way. If intermediate mass black holes do exist, they will play an important role in
connecting stellar mass black holes and supermassive back holes, and shed light onto the formation
of the latter, because supermassive black holes must be grown from seed black holes with masses
much larger than stellar mass black holes [3]. Also, it is an interesting topic in theoretical
astrophysics about how intermediate mass black holes are formed [4].
To securely determine the black hole masses is the “holy grail” of the ULX fields. Attempts have
been made to estimate the black hole masses in X-rays on the basis of their spectral and timing
behavior with comparisons to that from stellar mass black holes. However, these methods
themselves have not been perfectly established or tested, and cannot lead to an unambiguous
answer. Ultimately, the mass determination has to rely on the dynamical means, which has been
applied to successfully determine the masses for 22 stellar mass black holes in the Local Group [57]. This technique requires to measure the light curve and the radial velocity curve for the
companion star, and can be applied to companion stars brighter than B=20-21 mag with current 810m telescopes. For ULXs in distant galaxies, their optical counterparts are usually fainter than
22mag, and are very often contaminated by optical light from the X-ray irradiated accretion disk. It
is thus difficult to obtain the dynamical masses for ULXs with the current 8-10m telescopes, except
for the rare case when the companion star is a Wolf-Rayet star with strong emission lines [8].
TMT, with the 10 times larger collecting area and the diffraction-limited spatial resolution, will
push the limiting magnitude for the dynamical mass measurements down by up to 4 mag to 25 mag.
This enables us to measure the dynamical masses for a large sample of ULXs. Indeed, about half of
the known ULX counterparts are brighter than 25mag. Thus, with TMT, a survey of ULX optical
counterparts in nearby galaxies will result in accurate measurements of black hole masses in about
20-50 ULXs, better understanding of accretion physics at very high accretion rate, and may lead to
unambiguous discovery of intermediate mass black holes if they really exist.
3. The accretion disk at the low and quiescent states
The physics of accretion is much better understood when the disk flux is moderately high than in a
low or quiescent state. During the outburst of low mass X-ray binaries, the X-ray spectrum can be
well modeled as thermal emission from an optically thick accretion disk with temperatures
increasing toward the inner edge. At low and quiescent states, the X-ray radiation is non-thermal
and the accretion disk is suspected to be inefficient to convert gravitational energies into radiation.
Also, it is undetermined whether the emission at the quiescent state is dominated by the disk or by
the jets [9]. Therefore, it is important to obtain high quality disk spectrum at the low and quiescent
states to test different models and the structure of the disk. When the accretion rate is low, the disk
emission is weak and will release most of its energy in optical, where may be dominated by the
emission from the companion star. In infrared, the outer disk is still shining while the star light
declines. Therefore, large infrared telescopes are required to do the job. Statistically, most low mass
black hole binaries stay at the low and quiescent states. TMT will allow us to measure the
continuum spectrum of the disk from many of these sources, which will be useful to test models and
disentangle the nature of accretion in quiescence.
38
Possible TMT programs:
1.
Spectroscopic survey of ULXs to search for emission/absorption lines that could be possibly
used to measure the radial velocity curve of the companion star.
2.
A follow up survey of radial velocity curves of selected ULXs, based on the above
spectroscopic survey, aiming at determining accurately their black hole masses.
3.
A survey of radial velocity curves of selected black hole binaries in the Milky Way and in
nearby galaxies. The sample can be selected based on how interesting they appear in X-rays,
the brightness and type of their companions if already known, whether or not the orbital period
is measured in X-rays, what emission state they are in, etc.
4.
A spectroscopic survey of black hole binaries at the low and quiescent states.
These programs will provide a complete census of stellar mass and intermissive black holes in the
Milky Way and nearby galaxies, including the rich set of accretion physics from very low to very
high accretion rates, allowing much better understanding of stellar evolution, black hole formation
and physics at extreme conditions.
China’s strengths and weakness in this area:
A lot of scientists in China are interested in and have made significant contributions to observations
and theories about black hole binaries and the accretion physics. The weakness is that Chinese
people have not done a lot of optical observations of these objects which are usually faint in optical.
References:
1.
2.
3.
4.
5.
6.
7.
8.
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Bird, A.J., et al. 2009, ApJS to appear (arXiv:0910.1704)
Colbert, E. J. M., & Mushotzky, R. F. 1999, ApJ, 519, 89
Ebisuzaki, T., et al. 2001, ApJ, 562, L19
Portegies Zwart, S.F., et al. 2004, Nature, 428, 724
McClintock, J.E. & Remillard, R.A., 2006, Compact Stellar X-ray Sources, 157-213
Orosz, J.A., et al. 2007, Nature, 449, 872
Silverman, J.M. & Filippenko, A.V. 2008, ApJ Letters, 678, L17
Liu, J., 2009, ApJ, 704, 1628
Gallo, E., et al. 2007, ApJ, 670, 660
39
4.4 Star formation in the local universe and at high-redshift
How stars form in the Universe is a key unsolved question in astrophysics. We witness star
formation both in the local universe and at high-redshift, in quiescent disk galaxies such as our own
Milky Way and in violent interacting and merging galaxies. In the local universe, TMT, with its
unique capabilities, will be able to study the initial mass function down to much lower masses, and
probe the dynamics of star formation with its sensitive infrared instruments. At moderate redshift,
TMT can be used to trace the star formation rate with a new indicator, Paschen  line emission (see
section 4.4.2) in ultra-luminous infrared galaxies (ULIRGs). Extensive observations of star
formation by TMT at the key epoch z ~ 2 will directly probe how the stellar assembly occurs in
galaxies and how they are connected with active galactic nuclei activities. These observations will
also address how several fundamental scaling relations in disk and elliptical galaxies (the TullyFisher relation and the fundemental plane) evolve as a function of time.
4.4.1
Star formation in the local universe
Di Li (JPL), Qizhou Zhang (CfA)
Key questions:
1. What are the initial mass functions (IMFs) of local star formation regions? Are IMFs
universal?
2. What are the key dynamical processes in star formation regions?
1. Initial mass function: connectioning dark universe to visible matter
The formation of stars is the key process to construct most of the structures, including galaxies,
stellar clusters, and planets, in the universe. Heavy elements and dust are believed to have
originated in stellar nucleosythesis. The enrichment of matter further accelerates star formation.
Thus, understanding the cycle of star formation, especially, formation of massive stars given their
relatively short lifetime, is essential for understanding why we are living in the environment as
known to human today.
One of the holy-grail in studying star formation is understanding the initial mass function of stars
(IMF). Coupled with star formation efficiency, IMF is a crucial descriptive law for star formation,
which specifies what will be made out of how much matter. To connect cold dark matter
simulations to actual observable structures and to
explain galaxy evolution, the state of art treatment
of star formation in these simulations is still largely
a parameterized extrapolation based on galactic
IMF. Our understanding of galactic IMF, however,
is far from certain neither in terms of observational
facts nor of underlying physics. There are abundant
evidences for a relatively well accepted uniformity
of IMF (Parvel 2002). There are also evidences for
a flattening of IMF in special environment, such as
the high density regions near galactic center (e.g.
Figer et al. 1999). Since the majority of star
formation in early universe is likely to occur in
environments with enhanced density (e.g. mergers),
it is essential to correctly measure and understand
IMF under different conditions. Therefore half a
Figure 4.1 Three-color (J, H, Ks) composite
image of the Arches cluster with a field of view of
28” (Espinoza et al. 2009). The data were taken
with the NACO AO system on the VLT UT4. At
Ks band, the FWHM of PSF is at 0.09”
40
century after its discovery by Salpeter, the IMF is still at the forefront of star formation research.
TMT will have a direct and maybe game changing impact in IMF characterization.
The prime example of a top heavy IMF is the Arches cluster. Source confusion and large extinction
variation are main problems for converting JHK photometry data to the correct stellar population.
The image in Figure 4.1 represents the current state of art AO system on a 10 meter class system.
With TMT, the expected PSF core of about ~ 0.02” will be a factor of 4 improvement in resolution.
The sensitivity will expected to increase by 2 orders of magnitude. TMT will provide a dramatic
more comprehensive and more accurate count of stars in different star forming conditions (density,
metalicity, etc.) in the local universe.
Table 4.1 Limiting K-magnitudes and corresponding lower mass limit in Arches like clusters. This
table is based on radio profiles of the Arches cluster and adopted from TMT Detailed Science Case
(2007).
The resolving power and the sensitivity of TMT will allow for an unprecedented calibration of IMF
under different conditions. In the galaxy, the stellar population census will be complete down to
brown dwarf or even Jupiter mass objects. The characterization of IMF will be a major step in
understanding star formation, and in turn, provides essential parameters for simulations of largescale structures formation and galaxy evolution.
2. Dynamics and impact of star formation
Star formation is a complex process involving gravity, magnetic field, turbulence, etc. To move
beyond descriptive knowledge of laws of star formation (e.g. IMF), a direct probe of the dynamics
of the star forming cores is highly desirable. At R up to 120,000, TMT/MIRES achieves a 3 km/sec
spectral resolution in the mid-infrared bands. Such a spectral resolution along with the high spatial
resolution will be suitable to study the dynamical process of collapsing cores and protostars.
Compared with existing mid-infrared spectrometer, in particular, the Spitzer IRS instrument in the
same bands (see Figure 4.2), MIRES will increase sensitivity by about two orders of magnitude
while increasing the spectral resolution by 3 orders of magnitude. JWST will be slightly more
sensitive than MIRES, albeit at ~100 times worse spectral resolution. The capability of TMT will
be unique for the foreseeable future.
A primary example of spectral tracers are rotational transitions of H2, the most abundant molecule
in the universe. In mid infrared bands, warm molecular hydrogen produces S(1) transition at 17.04
m, s(2) at 12.28 m, and s(3) at 9.67 m. These lines traces gas with upper level energy of
hundreds to a few thousands K and both ortho and para population. The population ratio between
41
rotational levels and the ortho/para ratio are powerful diagnostic tools of the physical condition and
the evolutionary history of molecular gas.
The resolution element of TMT will be about 100 Au at a distance of 1 kpc. A comprehensive
survey of protostellar envelopes in this local volume will produce a systematic view of the
dynamical processes in star formation including collapses accretion, and jet/outflow.
Figure
4.2
Averaged
spectra toward positions in
NGC 1333 obtained using
Spitzer IRS in SH mode
(Maret et al. 2009). Note
that H2 lines are not
resolved in these spectra.
References:
3.
4.
5.
4.
Figer, D.F., Kim, S.S., Morris, M., Serabyn, E., Rich, R.M., & McLean, I.S. 1999, ApJ, 525, 750
Kroupa, P. 2002, Science, 295, 82
Maret, S., et al. 2009, ApJ, 698, 1244
Espinoza, P., Selman, F.J., & Melnick, J. 2009, A&A, 501, 563
4.4.2
Star formation at high redshift
Jiasheng Huang (CfA), Lin Yan (Caltech)
Introduction
“ORIGIN” is a key program identified by NASA and the international astronomy community for
the future space exploration, which includes studies of origin of earth, planets, stars and galaxies in
our universe. Studies of galaxy formation and evolution are an important part of the program. The
critical epoch in our universe is at z=2 when both the total star formation and AGN activities are at
their highest level. At present, astronomers began to observe formation process of galaxies with 8m
class telescopes and Hubble Space Telescope (HST). HST detected fine structures with a scale
down to 0.8kpc in galaxies at z=2. The new Near-infrared IFU, SINFONI, on VLT measures
velocities of H emission lines to study the dynamics of galaxies at 1<z<3 with stellar mass larger
than 1010 solar masses at a resolution of 1 kpc with adaptive optics and 4-5kpc with natural seeing.
The average size of galaxies at z~2 is rather small, thus a larger telescope with a better resolution
42
and higher sensitivity is needed to study the whole galaxy population. There are a number of
questions that TMT can make fundamental contributions.
1. Disk settling and star formation at z ∼ 2:
Key questions:
1. Are disks present at z ∼ 2 but thus far largely unseen because rest-UV images are dominated by
dust and young stars? If disks are in place, what are their radii, thicknesses, star formation
rates, and mass distributions? If disks are not present, is there an alternative, simple
description for the distribution of stars in these galaxies?
2. Did disks at z~2 have the Tully-Fisher relation? If so, how does it differ from the local one?
3. How does the star formation rate change during the formation of disks?
Theoretical models suggest that gas falling onto galaxies should settle quickly into disks, with radii
scaling with the size of the universe. Current hydrodynamic models produce disks readily, but they
are predicted to be very gas-rich and turbulent due to strong “cold flows” that feed them (e.g., [1]).
This suggests that the disk morphologies are more irregular and clumpier than present-day disks,
which is corroborated by the UV morphologies of the few disks that have been seen before z ∼ 1.5
([2, 3]). More observation on NIR morphologies for galaxies at z~2 will be done with the HST
WFC3 imaging survey. Dynamic evidence for disk formation at z=2 are lacking. Kassin et al. [4]
measured the Tully-Fisher relation at z~1, confirming that disks were already formed. SONFONI
observations reveal that only <1/3 star forming galaxies at 1<z<3, aka SINS galaxies, show
rotation signatures, but most have irregular velocity distributions. TMT will permit to measure
velocity distributions for large galaxy samples at z=2, and provide many key questions for disk
formation outlined at the beginning.
2. The emergence of massive spheroids at z ∼ 2:
Key questions:
1. What causes star formation to shut down, and what is the spectroscopic evidence?
2. Were spheroids formed in a single event, or did they grow slowly over time?
3. How are the spheroidal bulges of disk galaxies and E/S0’s related? How did E/S0 galaxies
move onto or along the fundamental plane?
Our current picture is that spheroidal galaxies – today’s E’s and E/S0’s – are the end-state in the
evolution of all massive galaxies (e.g., [5, 6]). Extremely low star formation rates make them look
“red-and-dead”, while dynamically “hot” kinematics give them their spheroidal shapes and smooth
brightness profiles. The shut-down of star formation leads to the build-up of a distinctive “red
sequence” [7] as colors redden after quenching.
A number of candidate mechanisms have been proposed for turning off star formation in galaxies.
Some are dynamical, such as quenching by mergers, which scramble disks into spheroids
dynamically and quench star formation by triggering a starburst or AGN which then drives out gas
([8, 9]). Others predict quiescent quenching, e.g., low-luminosity black hole feedback that prevents
gas (i.e., star formation fuel) from cooling onto galaxies (see [10-12] for an alternative scenario). In
the merger model, galaxies that are evolving red-ward are recent merger remnants with disturbed or
asymmetric morphologies [13], while red-and-dead spheroids should be old remnants with smooth
and symmetric profiles. In more quiescent quenching models, quenched galaxies can evolve without
merging, need not look disturbed, and may retain disks they had before quenching.
A slow build-up of stellar spheroids is suggested by NICMOS images of red galaxies at z ∼ 2,
which reveal sizes up to 5 times smaller than similar-mass spheroids today ([14-16], but see also
43
[17]). A series of gas-poor mergers may cause a gradual growth in size [18, 19], implying
continuous assembly of massive, quiescent galaxies over time. WFC3-IR will soon provide
unparalleled data for testing these theories. The Dynamic study of red galaxies will be able to
determine how red galaxies evolve From z=2 to z=0. Spectroscopy of red galaxies at z=2 are
extremely hard even with 8m class telescopes. Kriek et al. [20] performed ultra-deep exposure
spectroscopy for red galaxies at z~2 with VLT, could only measure the 4000A jump for their
redshift determination. A 30m telescope such as TMT is clearly needed for this study to address the
key questions in spheroidal formation.
3. The role of AGN at the peak of the QSO era:
Key questions:
1. Are mergers the main trigger of BH growth at z ∼ 2?
2. Does BH growth precede or lag star formation or the build-up of a spheroid?
3. Does BH feedback cause quenching in spheroidal galaxies?
The close connection between massive black holes (BHs) and spheroids (e.g., [21]) is puzzling
because only a few percent of galaxies appear to be building BHs at any instant – how is this
connection established and maintained? HST observations do not support dominant merger-driven
BH growth at z ∼ 1 (e.g., [22,23]) even though most models of AGN/galaxy co-evolution have
mergers as an essential component. These mergers may be occurring at higher redshifts, but very
little is known about the structure of AGN hosts when luminous optical QSO activity was at its
peak. The deep multi-wavelength data available in our fields will enable new tests of co-evolution
scenarios when WFC3 imaging is added.
QSOs at their birth are usually in very dusty environment, such as ULIRGs and
Dust Obscured Galaxies [24-27], which make their optical counterparts too faint for 8m class
telescopes. TMT will be able to identify faint AGN signature spectroscopically in the dusty
environment. By combining TMT spectroscopy and ALMA imaging for high-z Dusty galaxies and
ULIRGs, we will be able to study the birth of QSOs.
On the other hand, the relation between black hole mass and AGN hosting galaxies at high redshifts
are unclear at their dawn, while the local relation is well established. TMT will be able to measure
the dynamic masses of hosting galaxies which are impossible at all for 8m class telescopes.
Possible TMT programs:
1. A kinematic survey of disk galaxies at z~2 selected from other imaging surveys to study the
structure, dynamics and evolution of the Tully-Fisher relation.
2. A survey of spheroidal galaxies at z~2 to study their formation, and the evolution of the
fundamental plane.
3. A systematic study of selected QSOs to understand the interplay of star formation and the
formation of black hole.
China’s strength and weakness in this area:
China has several groups of people working on star formation at high-redshift at PMO and TJNU,
using multi-wavelength data, particularly in the radio. Several oversea Chinese astronomers are
playing leading roles in studying star formation using infrared data, e.g. from large ground-based
telescopes and space probes from SPITZER. However, closer collaboration between astronomers
inside China and overseas may be a fruitful way to gain access to current state-of-the-art ground
and space telescopes and further improve international visibilities in this area.
44
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4.4.3
Near-Infrared Emission Line Studies of ULIRGs
Zhong Wang (CfA)
Key question:
How do we measure the star formation rate independently in interacting galaxies?
Introduction:
Ultra-luminous infrared galaxies (ULIRGs), with their bolometric luminosities on the order of 1012
Lsun and beyond, represent some of the most active star forming sites on galactic scale, and have
significant implications to galaxy population and evolution. In addition to rapidly forming stars, two
other fundamental properties are now well-established for these systems: 1) most, if not all, of them
are interacting or merging galaxy pairs undergoing a drastic transition phase; and 2) nearly all
contain a large amount of gas and dust, often (but not always) near its central region (Sanders and
Mirabel 1996; Genzel et. al 2001; Tacconi et. al 2008).
Because of the high extinction in ULIRGs, conventional star formation probes in the optical tend to
be ineffective; longer wavelength measurements such as mid- and far-IR data, on the other hand,
are complicated by the presence of AGNs and black holes in the nuclei. The near-infrared Paα
emission is potentially one of the most useful for this purpose, and it is known to be very bright.
Unfortunately it cannot be used in ground-based observations of many well-known local ULIRGs,
due to the severe absorption of a telluric water-vapor absorption feature.
45
However, the K-band sensitivity of a TMT-class telescope can easily extend the near-IR emission
line study to a large number of more distant galaxies (those beyond z=0.15), whose redshifts move
the Paα line into a range where the water-vapor absorption is far less contaminating. This
potentially opens up an excellent window of opportunity for us to learn a great deal more about
individual ULIRGs in terms of their star formation activities and internal kinematics.
Paschen alpha line: a unique probe of star formation in dusty galaxies:
The Paschen  line emission is of particular interest to us for a number of reasons. First, being a
near-infrared line it provides a powerful probe of deeply embedded star forming clusters that are the
hallmarks of ULIRGs. It not only promises to reveal more underlying physics than the optical/UV
lines, but can also yield kinematic information on sub-angular scale (the average seeing in the NIR
for the TMT is expected to be around 0.3''). This is especially useful for the moderately distant (z
>= 0.15) ULIRGs, of which the existing longer wavelength (e.g., mid-IR) probes only measure
global properties. Secondly, basic atomic physics tells us that Paschen α has a fixed intrinsic flux
ratio to H which (along with the Hα line) is the conventional yardstick to gauge normal star
forming galaxies (Calzetti et. al 2007; Kennicutt et. al 2009). Therefore, by measuring this line we
will be able to make direct comparisons between the star forming activities in moderate to high
redshift ULIRGs and those in better studied local galaxies, while taking a more accurate account of
the average reddening in their central regions (Genzel et. al 2008).
Among the potentially observable hydrogen transitions beyond Hα line, Paα is also by far the
brightest. It is at least a factor of 10 brighter than Br the next brightest near-IR line observable (at
2.166 micron) from the ground (and often used as a substitute). Because of the strong atmospheric
absorption at its rest wavelength, unfortunately, it has been impossible to observe local objects
using Paα line from the ground. Nevertheless, its brightness and usefulness for studying starforming galaxies have been demonstrated with space-borne observations, in particular, those of
NICMOS aboard the Hubble (e.g., Scoville et. al 2000).
So far most ground-based NIR spectroscopic studies of local (U)LIRGs have concentrated on the
stellar absorption features in the H band. For example, Dasyra et. al (2006a, b) have carried out a
Keck-VLT legacy program to measure the velocity dispersion and stellar rotations of more than 50
local (U)LIRGs, using the CO bandhead absorption features. While this is an important and
worthwhile effort, many questions remain as to what extent these absorption features are
representative of the active star forming component at the nuclei of typical ULIRGs. Since most of
the ULIRGs we are studying have redshifts beyond 0.15, which moves the Paα line out
of the water-vapor absorption feature and into K band where a larger, NIR-optimized ground-based
telescope such as the TMT would have sufficient sensitivity up to z=1-2, we suggest that in these
cases, it would actually be far more efficient to measure the emission lines instead. [In fact, in one
of the earlier works of the Dasyra group, Paα line at z=0.164 was clearly detected in at least one of
the ULIRG's spectra with the Keck (Genzel et. al 2001). But they did not pursue this line of inquiry,
choosing to focus instead on the H band and targets which have lower redshifts (and thus brighter)].
ULIRG samples for observation with a large ground-based telescope
In collaboration with the GOALS (Great Observatory All-sky LIRG Survey) project (Armus et. al
2009), we have been studying a large number of LIRGs in the infrared and mm/submm
wavelengths. The GOALS sample starts with the Revised Bright Galaxy Sample (RBGS, Sanders
et. al 2003), selecting infrared bright ones based on a criteria of IRAS 60 micron flux of greater
than 5~Jy. A much more extensive collection, which includes largely ULIRGs (LFIR > 1012 Lsun as
opposed to LIRGs (LFIR > 1011 Lsun, is the 1 Jy sample of Kim and Sanders (1998). Because the NIR
emission line study focuses on the ULIRGs, we can draw sample galaxies primarily from the latter.
46
Limiting the sample to those ULIRGs with redshift beyond 0.15 would ensure detection and a
reliable calibration of the Paα line.
Besides the redshift range, we can rank the candidate galaxies based on total infrared luminosity
(LFIR), which for this sample ranges from approximately 1011 to 1013 Lsun. With the top-ranked
objects, we would further give higher priorities to those ULIRGs already studied in the other
wavelengths. This is done such that the emission-line study may be used to systematically compare
with the results of complementary studies and draw useful conclusions with regard to the
underlying physics from different probes.
There is a general consensus that reaching into a large population of ULIRGs at beyond z=0.15 is of
critical importance to the study of galaxy interactions and their impact on cosmic evolution. With
the powerful NIR capabilities the 30-m class TMT will allow precisely this type of quantitative
measurements. The rationale in ranking the sample ULIRGs primarily on total infrared luminosity
is that other measurements (e.g., visual magnitudes measured at shorter wavelengths) have little
bearing on the actual level of star forming activities in these extraordinary systems, given the high
level of average extinction. This is also consistent with our study of GOALS galaxies with Spitzer
and the radio telescopes. Our recent observations with the JCMT and CSO telescopes, for example,
have shown that when ranked in FIR luminosities, nearly all of the top 100 galaxies in the GOALS
sample turns out to be molecular gas and dust-rich interacting pairs, mergers, or post-merger
remnants.
Overall scientific goals:
Emission lines from ionized regions are the tell-tale signs of star forming activities in galaxies, and,
provided that the extinction factor can be reasonably evaluated, the most consistent and reliable
measure of the amount of star formation taking place (e.g., Kennicutt et. al 2009). In the case of
ULIRGs, the NIR emission lines are especially meaningful because of the heavy extinction. So our
first goal is to obtain a reliable measure of star formation rate, and compare that with the far-IR
luminosity, as well as the molecular gas content. This will provide not only overall rates but also
the star formation efficiencies in a large population of true ULIRGs.
The second goal is to measure the structural parameters of the star forming component in these
galaxies. With the expected spectral resolving power and a ~0.3'' seeing, we should be able to
obtain the position-velocity diagram from the Paα line, and thus to evaluate the rotational velocities
near the center of many ULIRGs. This can then be directly compared with the rotation curves
derived for local LIRGs. These kinematic information are very important in modeling the formation
and evolution of ULIRGs, widely recognized as a transitional phase from merging disk galaxies to
large ellipticals with central AGNs and black holes (Hopkins et. al 2007, 2008).
In many cases, we also expect to be able to resolve the Paα emission well enough to separate the
nuclear and disk contributions, thus assess the distribution of the starburst activities in these
systems. Because of the presence of AGNs in many of these systems, there has always been a
debate concerning the contribution of the non-thermal nuclear activities to the far-IR luminosity.
Since many of the longer wavelength diagnostics are only measuring global properties, being able
to spatially resolve the different components for a representative population of ULIRGs would be
critical to address such controversies.
Finally, the NIR emission line study of these moderately distant ULIRGs helps to bridge a gap
between the relatively well-studied nearby LIRGs and the large population of ULIRGs at truly high
redshifts. While the more familiar examples such as Arp 220 and NGC 6240 are indeed spectacular
and offer a close-up view of the merger-in-progress, questions remain regarding to what extent
47
these resemble the prominent ULIRGs around z=1-2 (for example, those from the 24-micron
selected sample) discovered with Spitzer (Le Floch et. al 2005, 2009; Yan et. al 2007). After all, it
is the latter population, formed when the universe was far younger and much more efficient in
processing its baryonic masses through star formation, whose evolutionary path would determine
the overall appearance of the universe of galaxies as we see today.
Possible TMT program:
1. Select a sample of ULIRGs both locally and at intemediate redshift and obtain their spectra
using TMT/IRMS to detect the Paα emission and measure their star formation rates.
China’s strength and weakness in the area:
This is an emerging new method where China can take lead in collaboration with oversea Chinese
astronomers through joint PhD student programs, particularly the pre-doctoral program at CfA.
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48
4.5 Galaxy formation and Evolution
Houjun Mo (UMASS), Xiaohu Yang (SHAO), Yipeng Jing (SHAO)
Overview of galaxy formation in the CDM
There is now much evidence that we live in a flat universe dominated by Cold Dark Matter (CDM)
with a total energy density 0~1, a total matter density m,0~0.3, a baryon density B,0~0.025h-2, a
Hubble constant (in units of 100 km/s/Mpc) h~0.7, a power-law index of initial perturbation n~1,
and an amplitude of the perturbation power spectrum, as specified by the rms of the perturbation
field smoothed in spheres of a radius 8Mpc/h, 8~0.85. This `standard' CDM model has been very
successful in explaining a variety of observations, such as temperature fluctuations in the cosmic
microwave background, the clustering of galaxies on large scales, and the clustering of the Lyman forest at high redshift (e.g. Spergel et al. 2007 and references therein).
In this paradigm, galaxy formation is believed to be a two-stage process (e.g. White & Rees 1978).
First, small perturbations in the density field believed to originate from quantum fluctuations in the
inflaton, grow and collapse to give rise to a population of virialized dark matter halos. Second, the
baryonic mass associated with these halos accumulates at the halo centers via cooling and cold
flows, causing the baryonic densities to become sufficiently high that star formation transforms the
baryonic gas into dense aggregates of stars, namely galaxies. Because of the hierarchical nature of
structure formation in a CDM cosmogony, dark matter halos merge, giving rise to halos containing
multiple galaxies (e.g. clusters), and to galaxy-galaxy mergers that are believed to be the main
formation channel of elliptical galaxies.
The first stage of this process, the formation and virialization of dark matter halos, has been studied
in great detail using the (extended) Press-Schechter formalism (e.g., Press & Schechter 1974; Bond
et al. 1991; Lacey & Cole 1993), spherical and ellipsoidal collapse (e.g., Gunn & Gott 1972;
Fillmore & Goldreich 1984; Bertschinger 1985; Sheth, Mo & Tormen 2001; Lu et al. 2006) and,
most importantly, numerical simulations (e.g., Efstathiou et al. 1985; Navarro, Frenk & White 1997;
Bullock et al. 2001a,b; Springel et al. 2005; Maccio et al. 2007). These studies have provided us
with an accurate description of the properties of the CDM halo population, such as their mass
function, spatial distribution, formation histories, and internal structure. This can be considered the
backbone for the formation of galaxies.
The second stage of the galaxy formation process is far less established, mainly because the
baryonic processes involved (cooling, star-formation and feedback) are poorly understood.
Although great progress has been made in the past two decades, the theory of galaxy formation and
evolution still faces several outstanding problems (see Primack 2009 for an up-to-date review). For
example, it remains challenging to fit the faint-end slope of the galaxy luminosity function (e.g.
Benson et al 2003; Mo et al. 2005), and the models typically predict disk rotation velocities that are
too high, unless adiabatic contraction and/or disk self-gravity are ignored (e.g. Cole et al. 2000;
Dutton et al. 2007). In addition, the models have problems matching the evolution of the galaxy
mass function with redshift (e.g., De Lucia & Blaizot 2007; Somerville et al. 2008; Fontanot et al.
2009), and typically overpredict the red fraction of satellite galaxies (Baldry et al. 2006; Weinmann
et al. 2006; Kimm et al. 2009; Liu et al. 2009). There are three main reasons that underly these
problems. First and foremost, it is more than likely that current models miss some vital ingredients
and/or the `recipes' used do not properly capture the underlying physics. Secondly, it has recently
become clear that the data used to constrain the models leaves significant degeneracies in the model
parameters that have thus far not been sufficiently explored (Henriques et al. 2009; Liu et al. 2009;
Lu et al. 2009). Thirdly, some of the outstanding problems may actually reflect inconsistencies in
the data itself. For example, it has been pointed out that the observed evolution in the stellar mass
49
function is inconsistent with the observed cosmic star formation history (Fardal et al. 2007; Gilmore
et al. 2009).
As is the case in virtually every field in science, in order to make progress we need more and better
data. During the last decade, there has been a dramatic increase of data on the nearby galaxy
population, mainly with the completion of large spectroscopic galaxy redshift surveys, such as the
Sloan Digital Sky Survey (SDSS; York et al. 2000; Stoughton et al. 2002) and the 2-degree Field
Galaxy Survey (2dFGRS; Colless et al. 2001). Although some spectroscopic redshift surveys out to
z~1 have recently become available as well, such as DEEP2 (Davis et al. 2003) and zCOSMOS
(Lilly et al. 2007), these surveys are much smaller than their low-redshift counterparts. At redshifts
significantly higher than one, very limited spectroscopic data of the entire galaxy population is
currently available.
With TMT, deep spectroscopic data will be possible, allowing us to study the galaxy population, its
relation to the dark matter component, and the properties of the gas reservoir for galaxy formation
for the entire history of the universe. It will allow us to probe the evolution of galaxy mass function
up to redshift z~2, to model the mass distribution in nearby galaxies with unprecedented precision,
to probe the chemical evolution of galaxies to redshift beyond z~1, to probe the gas state and
distribution span a large redshift ranges using QSO absorption lines, to probe the strong
gravitational lensing systems to redshift z~1.5, search for the highest redshift galaxies, etc. Below
we will address in detail these science cases for TMT.
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37. York, D. G., et al. 2000, AJ, 120, 1579
4.5.1
Evolution of galaxy luminosity/mass function
Xiaohu Yang (SHAO), Houjun Mo (UMASS), Yipeng Jing (SHAO)
Key questions:
1. Can one accurately measure the luminosity (stellar mass) functions from low to high redshifts,
especially their faint end behaviours?
2. What is their evolutionary trend?
3. What if separated into different colours and morphologies?
4. Can one obtain the luminosity (stellar mass) functions of galaxies in groups and clusters down to
very faint (low mass) end, i.e., their environment dependence?
Introduction:
In the current paradigm of galaxy formation, galaxies are formed within cold dark matter halos, i.e.
small galaxies in small halos form first, while massive galaxies in massive halos form later.
However, observations show that massive galaxies already formed at redshift z~2, and most of the
very small galaxies are actually newly formed. This is the so called ‘anti-hierarchical’ problem. To
answer this question one may first have a robust measure of the luminosity (stellar mass) functions
of galaxies at different redshifts. The luminosity (stellar mass) function describes the average
number density of galaxies of given luminosities (stellar masses) in the universe. It tells us the total
stars that formed at different epoch of the universe. However, because of the survey magnitude limit,
low luminosity galaxies can only be observed at low redshift. To get a clear picture of galaxy
formation, any efforts in the accurate measurement of luminosity functions to the very faint end at
high redshift are always appreciated. The unprecedented observation ability of TMT surely will
promote the research capability in this subject.
Main science areas:
1. Low mass end behaviour of luminosity (stellar mass) functions: from low to high redshifts
In the local Universe, thanks to the large sky coverage and highly completed observations, e.g., by
SDSS, we are now able to measure the luminosity function of galaxies to the very faint end [1]. One
of the major problems facing models of galaxy formation, however, is that cold dark matter models
in general predict larger numbers of low-mass galaxies than locally observed [2]. There are a
number of mechanisms have been proposed to suppress the star formation in small halos, e.g., the
UV background radiation, the SN and other feedbacks, e.g., the preheating. These processes have
different impact on the star formation in small halos at different redshift. Thus a key step to answer
this question is to have robust observational measurements of the faint (small) galaxies at high
redshifts (e.g. z~3). However so far at higher redshifts, most of the luminosity functions are
obtained from photometric redshift surveys where systematic error can be large, or mainly for a
specific population of galaxies, e.g., Lyman break galaxies, star forming galaxies or luminous red
galaxies.
51
The TMT/WFOS provides highly multiplexed, moderate resolution, slit spectroscopy in the optical
window (0.31– 1m). It is optimal for obtaining identification-quality spectra of the faintest
galaxies, and will provide us the opportunity to measure the luminosity function to redshift z~2. At
higher redshift, TMT/IRMS may be used: since some of the oldest and most massive galaxies
present at redshifts z > 2 are either heavily obscured by dust, or have little or no current star
formation, so that they may only be observed in the near-IR (there is no rest-UV flux).
2. Low mass satellite galaxies in groups and clusters
Since low mass satellite galaxies in groups and clusters were central galaxies of small halos before
their accretion into the massive halos, thus their population contains both the information about the
star formation in small galaxies and halos at higher redshift and their later evolution processes in
the massive host halos. Combing the observations of the low mass field galaxies at high redshift,
one can probe the evolution of satellite galaxies [3].
Although there are already quite a number of observational work regarding low mass galaxies in
clusters, most of them are based on the photometric redshift data. Studies show that the low mass
end slope of the cluster galaxies is about -1.3 to -1.4, much steeper than the field galaxies, and
varies between individual clusters. It is worth noting that deriving luminosity functions from
photometry alone requires background subtraction, an uncertain procedure that may bias one’s
estimates toward steep slopes. Popesso et al. [4] have used statistical background subtraction on the
photometric catalog to measure the optical luminosity function around X-ray clusters without
recourse to determining redshifts and found very steep low mass end slopes -1.6 to -2.1. Using the
SDSS observation, Yang et al. have obtained a low mass end slope ~ -1.6 for the stellar mass
function of galaxies in clusters [5]. This indicate that at higher redshift, the over abundant problem
of small galaxies may be less severe. Again, because of the current shallow observation, these
constraints are only carried for the galaxies in the local Universe. It will be a very good project for
TMT/WFOS to push this measurement to high redshift z~2.
3. Central massive galaxies in groups and clusters
Group and cluster halos are formed by the merger of smaller halos. The mass of the central galaxies
formed within them thus should be an ever increasing function as the increase of the host halo mass.
However, observations show that, according to the high mass end luminosity function of galaxies,
the most massive cluster galaxies have not grown at all after redshift z~2. Although AGN feedbacks
in clusters may prevent the star formation in central massive galaxies, the merge of satellite galaxies
into central galaxies will still raise their masses. Before try to solve this problem one shall have a
better understanding of the structure, size and luminosity of most massive galaxies at different
redshifts. As pointed out in recent studies, there are a significant number of diffused stars around
the cluster BCGs, e.g., central galaxies we observed contain only 10% to 20% stars that are really
associated with the them [6]. In such a scenario, there is no ‘anti- hierarchical’ problem anymore.
The TMT/IRIS integral-field spectroscopy over contiguous regions can provide near-IR spectral
information on physical scales as fine as 50 – 70 pc at any redshift in the range z = 1 – 6. Together
with the high resolution deep image of most massive galaxies either observed by LSST, KDUST or
other resources, will provide us the opportunity to have a better decomposition of the total stars that
associated with them to high redshift z~6 depending on the quality of the image.
4. Luminosity functions for galaxies of different colour, morphology and spectral types
Galaxies of similar masses at the same redshift may still differ in their colours, spectra,
morphologies, etc. These quantities hold important information regarding the status of the galaxies,
e.g., their ages, gas reservoirs, star formation rates, evolution histories, etc. Luminosity functions
for galaxies of different colour, morphology and spectral types in the local Universe has been
carried out for almost all the large redshift surveys, e.g., in 2dF [7] and SDSS [8]. Obviously in the
TMT observation, such kind of measurement can be carried for galaxies at higher redshifts.
52
Combined with high resolution AO imaging and redshift measurement, one may also investigate the
evolution of Hubble consequence.
These measurements can be used in semi-analytical models to constrain the galaxy formation
processes and various feedbacks to higher redshifts, or adopted by the conditional luminosity
function models to constrain the colour dependent relation between galaxies and dark matter halos.
Possible TMT programs:
1. Together with the high resolution AO imaging provided by other resources like CFHT, Subaru,
LSST or KDUST, TMT can be used to trace the morphological studies of galaxies to high
redshift and to understand how the Hubble consequence arise.
2. A few deep pencil beam surveys of galaxies to high redshifts with highly completed
spectroscopic observations carried out using WFOS and IRMS are needed for the accurate
luminosity function measurements to the very faint end.
3. Spectroscopic measurements for those potential member galaxies at the locations of some
candidate groups and clusters which will be identified from LAMOST or BIGBOSS project.
China’s strengths and weakness in this area:
There are already quite a number people and groups working in this area, at SHAO, NAOC, USTC,
PMO, and TJNU. In theoretical modelling, image processing, spectra analysis of galaxies, we are
already internationally competitive. Our weakness is the lack of the first hand observational data
and quite weak in building/running the state-of-the-art facilities. In addition, we do not have
telescope which can provide us the high resolution wide field image, unless the KDUST is launched.
We still need many more students or stuff members that are willing to deal with those raw data,
doing image processing, spectroscopic analysis etc.
References:
1.
2.
3.
4.
5.
6.
7.
8.
Blanton M. R., Lupton R. H., Schlegel D. J., Strauss M. A., Brinkmann J., Fukugita M., Loveday J., 2005, ApJ,
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Yang X., Mo H.J., van den Bosch F.C., 2009, ApJ,693, 830
Popesso P., Böhringer H., Romaniello M., Voges W., 2005, A&A, 433, 415
Yang X., Mo H.J., van den Bosch F.C., 2009, ApJ,695, 900
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Nakamura O., Fukugita M., Yasuda N., Loveday J., Brinkmann J., Schneider D.P., Shimasaku K., SubbaRao M.,
2003. AJ, 125, 1682
4.5.2
Probing the evolution of galaxies with kinematic studies
Zuhui Fan (PKU)
Key questions:
1. What are the kinematic properties of galaxies of different types in clusters of galaxies?
2. What are the environmental effects on the transformation of galaxies between different
types, and possible kinematic signatures of different formation mechanism of red sequence
galaxies in clusters?
3. How do the Tully-Fisher relation for rotation dominated galaxies and fundamental plane for
velocity dispersion dominated galaxies evolve as a function of redshift?
53
Introduction
From the evidence of the existence of dark matter to the discovery of the Tully-Fisher relation for
spiral galaxies and the Fundamental Plane for elliptical galaxies, kinematic studies have played
important roles in our studies of the formation and evolution of large-scale structures in the
Universe. Concerning gravities, the kinematic properties of dynamical tracers of a galaxy depend on
its mass distribution, and thus are excellent probes in studying the dark matter distribution of its
host halo [1]. Furthermore, as galaxies evolve, the kinematics of the galaxies are expected to change
accordingly. Therefore in addition to photometric and spectroscopic information, kinematic
analyses can provide important clues in understanding the physical mechanism underlying the
galaxy evolution. The recent advances in 3D spectroscopic observations have shown the exciting
potentials in probing relatively high redshift galaxies kinematically [2,3]. The much larger light
collecting area of TMT comparing to the existing facilities offers an excellent opportunity for us to
study detailed kinematics of high redshift galaxies, which in turn will improve greatly our
knowledge about how galaxies are shaped during their evolutionary history.
Main science areas
1. Kinematic studies of lening galaxies in conjunction with strong lensing studies
Kinematic studies offer important complementary information to strong lensing studies. The joint
analyses of the two can provide much better constraints on the mass distribution of the lensing
galaxies, and further shed lights on probing the nature of dark matter particles. Meanwhile, the
dynamical evolution of the galaxies, such as the evolution of the Tully-Fisher relation and the
Fundamental Plane can be probed for lensing galaxies up to high redshift z~2 with much higher
signal-to-noise than unlensed galaxies. This will be an integrated part of the strong lensing studies
with TMT (see 4.5.5).
2. Kinematic studies of galaxy transformations in clusters of galaxies.
While majority of the knowledge is currently obtained from the photometric observations of light
distribution and color information, and the spectroscopic studies concerning physical processes
related to star formation, it has been realized that kinematic studies can provide additional and
important complementary information in understanding the physics behind the galaxy evolution.
One of the most notably example is the discovery of large disks around high redshift star forming
galaxies from 3-D spectroscopic analyses [2]. For nearby galaxies, the SAURON project has done
extensive integral field spectroscopic studies, aiming at understanding the formation and evolution
of elliptical and S0 galaxies from the kinematic point of view. It is shown that kinematically, these
red galaxies can be divided into two categories of fast rotators and slow rotators. Some elliptical
galaxies can be fast rotators. To some extent, these kinematic differences may reflect the
differences in their formation process more directly than that of the visual classifications purely
based on light distribution of images [4]. For high redshift galaxies, such detailed kinematic studies
regarding the galaxy transformation are less explored due to the limited observing power of the
current facilities. TMT will be superb in making up this shortage.
Particularly, TMT can target at clusters of galaxies. It is well known that clusters of galaxies nurture
the transformation of their residing galaxies, and thus are the best ones to be investigated to probe
the evolutionary path of galaxies [5]. Many important ‘downsizing’ related phenomena, such as the
morphology-density relation, the color-magnitude relation for red sequence galaxies, the deficiency
of faint red galaxies in high redshift clusters comparing to that in local ones, were revealed firstly
from the studies of clusters of galaxies of different masses and at different redshifts [6,7]. Extending
the observations to the surrounding area of a massive cluster can also allow us to probe the
54
environmental effects on galaxy evolution at different conditions, from field, to group and to high
density environments [8]. The main questions we particularly concern here include when and where
the star formation activity is efficiently suppressed and the galaxy starts its journey to reach the red
elliptical end; How the morphology of a galaxy changes during its evolution; the possible different
mechanisms involved in the formation of luminous red galaxies and the formation of the faint red
galaxies. Besides the ability that TMT can detect fainter galaxies, red or k+a, it can make detailed
kinematic maps for a subsample of galaxies of different types at different places around clusters. It
is expected that different processes involved in galaxy transformations, such as major mergers,
minor mergers, or interactions with intracluster medium, should result distinctly kinematic
signatures. Therefore such kinematic information from TMT will improve our understanding on the
galaxy transformation tremendously.
Possible TMT programs:
The Spitzer Adaptation of the Red-sequence Cluster Survey (SpARCS) has yielded hundreds of z
>1 cluster candidates [9]. Potential clusters for TMT observations can be chosen from this
catalogue. As examples, SpARCS J163435+402151 and SpARCS J163852+403843 are the two
spectroscopically confirmed clusters in northern hemisphere ([10], see Figure 4 and Figure 5). The
following two figures are taken from [10]). The redshifts of the two clusters are z=1.1798 and
z=1.1963, respectively. The corresponding masses are ~1014 Msun and 2x1014 Msun. At redshift z
~1.2, the angular extend of of a cluster including surrounding regions is about a few arcminutes,
and the light distribution of a typical galaxy is ~1.2 arcsecond (10kpc). They are ideally suited to
the observations of IRMS and IRIS (imaging and IFU) mounted on TMT. Much larger number of
faint galaxies are expected to be detected with TMT in comparison with current 10m-level
observations, which will help to understand the problem of the deficiency of red galaxies at faint
end. It will also be very important to detect faint k+a galaxies to learn the evolutionary path from a
blue galaxy to a red one. The current observations seem to see fewer k+a galaxies than expected,
indicating some channels other than through k+a phase for galaxy evolution might be needed [11].
TMT can go much fainter magnitudes, and will help pin down the problem in a much more solid
ground. Kinematic mappings are needed for a subsample of galaxies of different types (blue, k+a,
S0 and ellipticals) and at different densities within the observed field. It is expected that such
kinematic observations will provide us a comprehensive picture about the transformation of
galaxies in clusters of galaxies, so that to improve considerably our knowledge on the underlying
physics.
Figure 4: Left: Rz ~ 3.6 μm color composite of the cluster SpARCS J163435+402151 at z = 1.1798.
The R and z-images have been convolved to match the 3.6 μm PSF. The FOV of the image is ∼3.5
arcmin across. Right: same as left panel but with spectroscopically confirmed cluster members
marked as white squares and spectroscopically confirmed foreground/background galaxies marked as
green circles. Taken from [10].
55
Figure 5: Same as Figure 4, but for cluste SpARCS J163852+403843 at z=1.1963. The FOV of
the image is 4.5 arcmin across. Taken from [10].
China's strength and weakness
There are a handful of people working in this field, and having experiences in IFU analyses.
Numerical simulations to explore the kinematic signatures related to different physical processes
can be pursued in China or through international collaborations. Kinematic studies are starting, but
need to be strengthened considerably.
References:
[1] Binney, J., Tremaine, S. 2008, Galactic Dynamic, 2 nd edition, Princeton University Press
[2] Genzel, R. et al. 2006, Nature, 442, 786
[3] Wright, S. A. et al. 2009, ApJ, 699, 421
[4] Emsellem, E. Et al. 2007, MNRAS, 379, 401
[5] White, S.D.M. et al. 2005, A&A, 444, 36
[6] De Lucia, G. et al. 2007, MNRAS, 374, 809
[7] Tanaka, M. et al. 2008, A&A, 489, 571
[8] Patel, S. G. et al., 2009, ApJ, 694, 1349
[9] SaARCS: http://www.faculty.ucr.edu/~gillianw/SpARCS/
[10] Muzzin, A. et al. 2009, ApJ, 698, 1934
[11] De Lucia et al. 2009, arXiv: 0907.3922
4.5.3
Chemical evolution of galaxies
Chenggang Shu (SNU), Xu Kong (USTC)
Key questions:
1. How do star formation and chemical evolution of high-z bright galaxies proceed?
2. What are the early stages of galaxy formation and chemical evolution?
3. How do chemical enrichments take place in quiescent objects and what is the enrichment
history of the inter-galactic and intra-cluster media (IGM/ICM)?
Introduction:
Chemical abundances of galaxies provide us important clues to the evolution of galaxies. They are
crucial to understanding the current star formation activities, star formation history, gas processes
including inflows and outflows, and the environments of galaxies, which are of primary interests of
galaxy formation and evolution theories.
56
From observational point of view, there are two ways to obtain the chemical information of
galaxies. The first is by observing galaxies are intrinsically bright so that their spectrum can be
easily obtained. The second (indirect) method, is by studying absorption features of bright
background sources induced by intervening galaxies that are by themselves too faint to observe
directly.
Currently, there have been many observational results available, mainly for bright galaxies in the
local universe. But we still lack sufficient and accurate data for galaxies at redshift z>1 and faint
galaxies at low z (such as low surface brightness galaxies) to establish a global picture of the
chemical (hence the dynamical) evolution of galaxies in the hierarchical structure formation picture
(see the Introduction). TMT, with its large aperture and high-resolution, will play a key role in this
field in many different ways.
Main science areas:
1. The chemical evolution of high-redshift “bright” galaxies
“Color dropouts” has been a very successful method to find galaxies at high-z (Chapman et al 2005;
Shapley et al 2005; van Dokkum et al 2006; Bouwens et al 2007; Casey et al 2009) as demonstrated
by the pioneering work by Steidel at al (1995) for Lyman Break galaxies at z~3. Since these
galaxies are active, they provide important clues to the chemical and dynamical evolution of present
day massive galaxies. For example, several studies have been outflows of several hundreds km/s are
common with important implications for chemical enrichment of the intergalactic medium.
Unfortunately, few observations have been done and no systematic sample with chemical
information are available, except a few that have been gravitationally lensed (CB58, Pettini et al.
2001). Currently it is difficult to measure their chemical composition because they are too faint
(usually R~23.5) to obtain a sample of high resolution spectra even by Keck (Teplitz et al 2000;
Matsuoka et al 2009). Since thousands of these galaxies have already been identified in imaging
survey, a relatively complete sample with chemical information of these galaxies can be establish
by TMT down to magnitude R~25.5, which is very important to understanding their chemodynamic evolution.
2. Star formation and gas process of emission-line galaxies at z>5
As mentioned above, there are many emission-line galaxies found at very high redshifts (z>5), e.g.,
Lyman-alpha emitters (Willis & Courbin 2005; Kovac et al 2007; Stanway et al 2007; Fernandez &
Komatsu 2008; Ota et al 2008; Goto et al 2009; Overzier et al 2009; Taniguchi et al 2009).
Although it is still difficult to have an unbiased sample, they display very special physics properties
such as active star formation activities, diverse morphologies, high velocities in kinematics,
ionization stages, etc, which are important to understanding the early stage of galaxy formation and
how the universe becomes re-ionised. Currently both ground-based and space telescopes are not
easy to obtain high signal-to-noise ratio spectra of these objects in order to probe their chemical
compositions and kinematics, TMT will be a suitable telescope to do the required deep
observations.
3. Chemical evolution of faint galaxies
From the modern cosmological theory and from observations, there exist small galaxies whose
number is several orders of magnitudes larger than ordinary galaxies (such as the Milky Way).
These faint galaxies have low star formation and other physical activities so that the chemical
evolution should be slow. Low surface brightness galaxies (LSBs) (Roberts et al 2004; Kuzio et al
2006; Coccato et al 2008; Pizzella et al 2008; Adami et al 2009) and QSO absorption systems (such
57
as Ly-alpha forests and damped Ly-alpha systems) are typical (Rao & Turnshek 1998; Chen et al
2000; Pettini et al 2001; Chen & Lanzetta 2003; Pettini & Pagel 2004; Bowen et al 2005; Wild et al
2006; Curran et al 2007, 2009; Pettini et al 2008). Since they are the neutral gas reservoir in our
universe and relatively quiescent, they provide good laboratory for us to understanding the physical
prescriptions we adopt for bright galaxies. Nerveless, because of their relatively shallow
gravitational potential wells, they are the dominating sources that may provide most of the metals to
the IGM/ICM metal enrichments. As one of the key projects of HST, QSO absorption systems have
been observed with chemical compositions and kinematics data available. But we are still far from
fully understanding their evolution especially the chemical evolution. For LSB galaxies, there are
still very few observations available. TMT can provide us a relative complete sample of LSBs and
more accurate measurements of the QSO absorption systems (such as the internal kinematics,
chemical abundances and dust properties).
Possible TMT Program
1. A spectroscopic survey to establish a complete sample of chemical abundance of high-redshift
bright galaxies and a detailed study of star formation and chemical evolution for these galaxies.
2. A detailed spectroscopic study of the Lyman-alpha emitters found at z>5 to provide their
chemical compositions and kinematics. Combining observations in other wavelengths such as
radio, sub-mm, this sample can be used to investigate the early evolutionary stage of galaxies
and probe the re-ionization epoch.
3. An ambitious program to accurately measure the kinematics and metallicity of LSBs and QSO
absorption systems (especially damped Ly-alpha systems) using TMT and to investigate their
chemo-dynamic evolution (hence the enrichments of IGM/ICM).
Chinese strengths and weakness in this area:
There are several groups at NAOC, SHAO, PKU, USTC and SNU, working in this field. The
research activities are mainly theoretical (numerical simulations and semi-analytical studies), and
internationally competitive. Form the observational side, few people are familiar with the data
reduction. So, more expertise in this area is urgently required; joint student training with
internationally-leading centers will be particularly useful.
References:
1.
2.
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5.
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8.
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Adami, C., Pello, R., Ulmer, M. P., et al 2009, AA, 495, 407
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Bouwens R. J., Illingworth, G. D., Franx, M., Ford, H., 2007, ApJ 670, 928
Casey, C. M., Chapman, S. C., Beswick, R. J., et al, 2009, MNRAS 399, 121
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Chen, H-.W., Lanzetta, K. M., Fernandez-Soto, A., et al, 2000, ApJ 533, 120
Coccato, L., Swaters, R. A., Rubin, V. C., et al 2008, AA 490, 589
Curran, S. J., Tzanavaris, P., Darling, J. K., et al, 2009, MNRAS accepted, (astro-ph/0910.3742)
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Fernandez, E. R., Komatsu, E., 2008, MNRAS 384, 1363
Goto, T., Utsumi, Y., Furusawa, H., et al 2009, MNRAS accepted (astro-ph/0908.4079)
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Haehnelt, M. G., Steinmetz, M., 1998, MNRAS 298, L21
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Ota, K., Iye, M., Kashikawa, N., et al, 2008, ApJ 677, 120
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4.5.4
The intergalactic medium
Renyue Cen (Princeton)
Key questions:
1. Where are the missing baryons and how do galaxies and the IGM interact?
2. How do we use the Lyα forest to obtain cosmological information?
Introducton:
In the remarkably successful standard cosmological model (Krauss & Turner 1995; Bahcall et
al.1999; Spergel et al.2007) most of the mass-energy density in the universe is in dark energy and
dark matter. At present neither of these two unknowns with the astronomically inferred amounts
can be explained by fundamental theories. While the exact nature of dark matter and dark energy
remains enigmatic, in the simplest models baryons and dark matter are tightly coupled on large
scales under the common action of gravity, with the overall temporal evolution also depending on
the nature of dark energy. Since the evolution of the intergalactic medium (IGM) is directly
observable over a long timeline to high redshift, it potentially provides a powerful probe of dark
matter and dark energy. In the past one and a half decade our understanding of the evolution of the
IGM has been dramatically enhanced with steadily improving cosmological hydrodynamic
simulations. Briefly stated, the evolution of the IGM is consistent with the growth of structure seen
in the formation and distribution of galaxies. That is, gravitational instability plays the dominant
dynamic role on large scales (r > 1 Mpc), while other physical processes, including hydrodynamics,
microphysics and galaxy formation feedback, become increasingly important on smaller scales.
Among notable successes, hydrodynamic simulations helped establish the current working theory
for the Lyα forest (Cen et al.1994; Zhang et al.1995; Hernquist et al.1996; Miralda-Escude et
al.1996) and have successfully predicted the existence the Warm-Hot Intergalactic Medium (WHIM;
Cen & Ostriker 1999; Dave et al. 2001) to account for the missing baryons.
Main science areas:
1. Where are the missing baryons and how do galaxies and the IGM interact?
Most of the ordinary, baryonic matter, which altogether makes up about 10-20% of the total matter
in the universe (as observed, for example, in clusters of galaxies), seems to be unaccounted for in
the present-day universe. At the last scattering (z ∼ 1000) the latest cosmic microwave background
experiments (Komatsu et al. 2009) indicate
Ωb,CMB(z ∼ 1000) ≥ (0.02265 ± 0.00059)h−2 = 0.046 ± 0.001,
59
(1)
where Ωb,CMB(z ∼ 1000) is the baryonic density in units of the critical density extrapolated to z = 0
and a Hubble constant h ≡ H0/(100km/s/Mpc) = 0.70 is adopted throughout. At redshift z = 2−3, the
amount of gas contained in the Lyman  forest (Rauch et al.1997; Weinberg et al.1997) is
Ωb,Lyα(z = 2− 3) ≥ 0.017h−2 = 0.035,
(2)
in units of the critical density extrapolated to z = 0. Independently, the observed light-element ratios
(in particular, the deuterium to hydrogen ratio) in some carefully selected absorption line systems at
z = 2−3, interpreted within the context of the standard light element nucleosynthesis theory, yield
the total baryonic density (Burles & Tytler 1998; Kirkman et al. 2003)
Ωb,D/H(z = 2− 3) = (0.019 ± 0.001)h−2 = 0.039 ±0 .002,
(3)
again in units of the critical density evaluated at z = 0. The agreement between these three
completely independent measurements is remarkable. But, at redshift zero, after summing over all
well observed contributions, the baryonic density appears to be far (by a factor of three) below that
indicated by equations (1), (2) and (3) (e.g., Fukugita, Hogan, & Peebles 1997):
Ωb(z = 0)|seen = Ω∗ +ΩHI +ΩH2 +ΩXray,cl ≈ 0.0068 ≤ 0.011 (2σ limit), (4)
where Ω∗ , ΩHI, ΩH2 and ΩXray,cl are the baryonic densities contained in stars, neutral atomic
hydrogen, molecular hydrogen and hot X-ray emitting gas in rich cluster centers, respectively, in
units of the critical density. Thus, unless three independent errors have been made in the arguments
that led to equations (1), (2) and (3), there is a sharp decline of the amount of observed baryons
from high redshift to the present-day; i.e., most of the baryons in the present-day universe are yet to
be detected.
Cosmological hydrodynamic simulations suggest that a warm-hot intergalactic medium (“WHIM”)
in the temperature range 105−7 Kelvin may contain most of the “missing baryons” (Cen & Ostriker
1999; Dave et al. 2001). It is therefore of fundamental importance to probe this gas and make an
accurate and reliable interpretation in the current cosmological context. The reality of the WHIM, at
least the low temperature portion of it (T ≤ 106 K), has now been fairly convincingly confirmed by a
number of observations from HST and FUSE, through the O VI λλ1032, 1038 absorption line
doublet in the FUV portion of QSO spectra (e.g., Tripp & Savage, 2000; Danforth & Shull, 2008)
and Ne VIII (e.g., Savage et al., 2006).
The fundamental link between galaxy formation and the IGM lies in the WHIM, which provides a
conduit for exchange of matter and energy. This galaxy-IGM interaction is extremely complex and
difficult to pin down theoretically. Therefore, mapping out the physical state of the ISM/IGM from
central regions of galaxies to the WHIM regions of several hundreds of kpcs will be a key
observational challenge in the next decade. A coordinated campaign to probe the multi-phase
medium will be required that provides diagnostics for gas at different temperatures, from X-rays,
UV to optical. TMT will play three keys roles on this. First, TMT will be able to detect galaxies to
very low luminosities, which is important for associating interesting IGM features (such as metal
enrichment) with responsible galaxies. Second, TMT will be able to provide a much denser sample
of background quasars that will then provide an essentially 3-dimensional absorption grid of cold
material (such as Mg II line) around galaxies, which, when combined with other complementary
measures of warm gas by HST/COS and of hot gas by X-ray observations, will form a quantitative
and much more complete picture of the galaxy-IGM interaction regions in the present universe.
Third, TMT will be able to do map out the galaxy-IGM interaction essentially throughout the entire
moderate redshift universe, for example, with C IV, O VI and other absorption lines. In summary,
TMT will provide a panoramic view of the galaxy-IGM interaction history, which will help us
understand the evolution of the IGM and formation of galaxies, and find the missing baryons.
60
2. How do we use the Lyα forest to obtain cosmological information?
The standard theory is that the observed Lyα forest is a natural consequence of the gravitational
growth of small-scale density perturbations (Cen et al.1994; Zhang et al.1995; Hernquist et al.1996;
Miralda-Escude et al.1996). TMT observations will provide the next quantum leap in the study of
the Lyα forest, over what SDSS and Keck observations have provided us, in two ways. First, it will
provide a much larger quasar sample than Keck has. Second, it will provide a quasar sample that
will cover a larger redshift range and much higher spectral resolution than SDSS III will.
Essentially, we will have a 3-dimensional Lyα forest as well as the forests for various prominent
metal lines, such as C IV, N V and O VI lines, over the entire redshift range up to z ∼ 6. While
numerous cosmological and astrophysical applications will be performed, we highight two
important cosmological applications.
First, CMB experiments (e.g., WMAP and Planck) do not have the necessary leverage to precisely
constrain the slope of the power spectrum, because of the lack of accurate normalization points at
small scales. The Lyα forest flux distribution provides the only competitively accurate measurement
of the matter ower spectrum at small scales (Croft et al.1999). The statistical accuracies to
determine the amplitude, slope and curvature of the density power spectrum using Lyα forest from
large SDSS QSO samples have reached unprecedented 1-3% level (e.g., Mandelbaum et al.2003),
complementary to WMAP and in the future PLANCK. Thus, with Lyα forest observations by TMT
and CMB observations one may be able to jointly nail down the matter power spectrum to a subpercent level that may test inflationary theories (Seljak et al.2005).
Second, observations by Type Ia supernovae (e.g., Riess et al., 1998; Perlmutter et al., 1998; Astier
et al., 2006) and Cosmic Microwave Background (CMB; (e.g., Komatsu et al., 2009)) both suggest
an accelerating expansion of the universe, driven by the mysterious “dark energy”. Characterizing
the nature of Dark Energy (determining its w — ratio of pressure to energy density) is perhaps the
most exciting problem in cosmology today. Baryon oscillations are believed to be the method “least
affected by systematic uncertainties” (Dark Energy task Force report: (Albrecht et al., 2006)). The
baryonic acoustic oscillaton (BAO) in the early universe provides a unique and precise scale that
should remain little changed with time until the present and can be used as a standard ruler, tightly
constrained by the WMAP CMB observation. The overall expansion history of the universe
depends on w. If one can measure the sizes of the universe at a few different redshifts precisely, one
will be able to place important constraints on w. The extremely valuable improvement of the Lyα
forest provided by TMT over what SDSS III BOSS survey is a much higher spectral resolution, a
larger redshift coverage and a denser quasar sample. If expectation comes true, BOSS will be able
to yield absolute distance measurements with statistical precision of 2−3% at z ∼ 2.5. TMT is likely
to achieve significantly better constraints. Is w different from −1? TMT will help answer this
fundamental question.
Possible TMT programs:
1. TMT/WFOS observations of selected fields with known Lyman-break galaxies or quasars, to
study the metal enrichment of the IGM, and to map the small-scale power-spectrum.
2. TMT/HROS observations of selected quasars with very high resolutions.
China’s strengths and weakness in this area:
There are limited expertise (NAOC, SHAO, and USTC) in China in this important area for TMT.
We need to significantly strengthen both in terms of observational expertise on large telescopes, and
theoretical modelling. Joint PhD training with international centres of excellence in this area will be
highly desirable.
61
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& Ostriker, J.P. 1997, ApJ, 489, 7
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4.5.5
Strong gravitational lensing
Shude Mao (Manchester/NAOC)
Key questions:
1.
2.
3.
4.
How can we identify the highest redshift galaxies using strong lensing clusters?
Are the flux ratio anomalies in gravitational lenses due to substructures?
How do the internal structures of lensing galaxies evolve as a function of redshift out to z~1.5?
What are the chemical abundances of Galactic bulge dwarf stars?
Introduction:
Strong gravitational lensing refers to the fact a background source is multiply imaged, strongly
distorted or magnified by a foreground lens. Microlensing, multiply-imaged quasars/galaxies and
giant arcs are manifestations of strong lensing by stars, galaxies and clusters respectively. Strong
lensing has diverse applications [1], ranging from the discovery of extrasolar planets (with
microlensing), determining the cosmological parameters (such as the Hubble constant from time
delays), and mass distributions in the Milky Way, external galaxies and clusters. The determination
of mass profiles for moderate redshift (z~0.5-1.5) galaxies is likely the most important application
of strong lensing in the next decades. Notice that TMT, with its limited field of view, may not be
very competitive in the area of weak lensing.
Main science areas:
1. The discovery of highest redshift galaxies
62
An effective way to search for high-redshift galaxies is to identify faint objects close to the critical
curves where the galaxies are highly magnified. For example, systematic searches for
gravitationally lensed Lyman break “dropouts” can be conducted via very deep imaging through
foreground clusters. A recent survey undertaken with the Hubble and Spitzer space telescopes has
yielded 10 z-band and two J-band dropout candidates beyond redshift 7 [2]. This method can be
extended to TMT to search for the highest redshift galaxies, in particular IRIS and IRMS can be
used to provide spectroscopic confirmations of these galaxies.
Such a systematic search may be attractive for another reason: if we can confirm the membership of
the foreground cluster with LAMOST, then we can combine gravitational lensing and kinematics to
constrain the cluster mass distribution, in addition to identify the highest redshift galaxies.
2. Substructures in lensing galaxies
Substructures are a generic prediction of the cold dark matter (CDM) hierarchical structure
formation theory. In this theory, galaxies are surrounded by massive dark matter haloes. Big haloes
are formed by the merging of smaller ones. In this process, the cores of smaller haloes often survive
the tidal disruption and dynamical friction, which then manifest as subhaloes (substructures). The
number of subhaloes appears to exceed the number of observed satellite galaxies in a Milky-Way
type halo by a factor of ~10 (e.g. [3]). One solution is that many of the substructures may be dark
due to the suppression of star formation, and so may escape detection from any light-based method.
Gravitational lensing provides a promising way to detect these since lensing does not depend on
whether the lens is luminous or dark. In particular, substructures may affect flux ratios in lenses,
which has been called “anomalous flux ratio problem.” A detailed study of the highest resolution
simulation using AQUARIUS seems to indicate CDM models actually under-predict the
substructures required by the CLASS survey (see [3] and references therein), although this underprediction may be due to small number statistics (22 lenses in the survey).
Equally puzzling, many of the anomalous lenses seem to host faint luminous satellite galaxies. So
far, these satellite galaxies (I~24.5) are too faint for spectroscopic observations even by KECK. So
it is unclear whether they are associated with the main lens, or lie along the line of sight [4]. TMT
will be able to securely determine the redshift of these faint galaxies and thus their nature. Such
observations will not only solve the anomalous flux ratio problems, but also provide insights on the
formation of satellite galaxies.
Evolution of dynamical properties in lenses out to z ~ 1.5
The HST lens survey SLACS have found ~100 new gravitational lenses. This sample, combined
with kinematics from KECK, provides the strongest kinematical constraints at moderate redshift
(out to z~0.7). A recent study found that these lensing galaxies are well modelled with singular
isothermal spheres [5]. In the next decade, many large-area photometric surveys, using, e.g. PanSTARRS and LSST, will discover orders of magnitudes more gravitational lenses. These new
lenses can potentially reach redshift of 1.5 to 2.
An IRIS IFU survey of a representative sample of lenses out to z~1.5-2 will offer us a chance to
have detailed studies of kinematics of lenses (extending the current limit of z~0.7), and thus provide
key insights in how the mass profiles change as a function of redshift: does the isothermal sphere
model still hold for these higher redshift lenses? The epoch of z~1.5 may be particularly interesting
because of the significant star formation and AGN activities at this epoch (see Section 4.4.2) that
may provide vital clues for the establishment of dynamical entities.
63
3. Spectroscopy of highly magnified lenses in the Galactic bulge
In the next decade, upgraded survey networks will discover several thousand microlensing events
per year; most of these will be discovered in real-time. Some of these may reach a magnification as
high as of ~3000. Target-of-opportunity observations of such highly lensed stars will make the
TMT as a telescope with an effective aperture of ~1500m! Building on earlier attempts [6], KECK
and Magellan high-resolution spectroscopies of microlensed targets already revealed potential
surprises in the stellar evolution [7, 8], although the sample is still too small. TMT observations of
highly microlensed stars may provide extremely high S/N ratio spectra for a sample of bulge stars.
WFOS/IRMS can also obtain the spectra of many other stars within the field simultaneously, and
thus establish a significant sample of stars with chemical abundance of faint dwarf stars (rather than
mostly giants as in the current samples), potentially provide important clues about the different
formation histories of the Galactic bulge and disk.
Possible TMT programs:
1. A detailed study of a sample of cluster lenses at moderate redshift to search for lensed, very
high redshift background galaxies; detailed membership classification (z~0.2) may be obtained
with LAMOST.
2. A spectroscopic survey of known anomalous flux ratio lenses with IRMS IFUs to obtain the
kinematics of the central lens, and obtain redshifts of the faint luminous satellite galaxies. This
will provide a definitive solution to the anomalous flux ratio problem.
3. Target-of-opportunity observations of a sample of microlensed bulge stars in the Galactic centre
to obtain extremely high S/N ratio spectra with IRMS/WFOS, which can be used to obtain
detailed abundance patterns of faint stars in the Galactic centre.
China’s strengths and weakness in this area:
There are already several people working in this area at NAOC, PKU, SHAO and SNU. In
theoretical (statistical) studies of galaxy-scale and cluster-scale lenses, we are already
internationally competitive, although in observations, we still lag behind due to the lack of access to
state-of-the-art observing facilities.
We still need to train more PhD students, especially in the area of detailed modelling of complex
cluster lenses in order to identify highest redshift galaxies; some promising work in this area has
been performed at SNU. Another area that we require substantially more expertise is in dynamical
modelling that can combine integral field unit and gravitational lensing data (through either the
Schwarzschild method or the made-to-measure method, see also Section 4.5.2).
References:
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3.
4.
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Kochanek C. S., Schneider P., Wambsganss J. 2003, Gravitational lensing: strong, weak and micro. Saas-Fee
Advanced Course 33.
Richard J., Stark D. P., Ellis R. S., George M. R., Egami E., Kneib J.-P., Smith G. P., 2008, ApJ, 685, 705
Xu D. D., Mao S., Wang J., Springel V., Gao L., White S. D. M., Frenk C. S., Jenkins A., Li G. L., Navarro J. F.
2009, MNRAS, 398, 1235
Metcalf, B. 2005, ApJ, 629, 673
Koopmans L. V. E. et al. 2009, ApJ, 703, L51
Lennon D. J., Mao S., Fuhrmann K., Gehren T. 2006, ApJ, 471, L23
Cohen J. G., Huang W., Udalski A., Gould A., Johnson J. A., 2008, ApJ, 682, 1029
Cohen J. G., Thompson I. B., Sumi T., Bond I., Gould A., Johnson J. A., Huang W., Burley G., 2009, ApJ, 699, 66
64
4.6 Near Field Cosmology and Stellar Astrophysics
4.6.1
Resolved stellar populations and kinematics in nearby galaxies
Martin Smith (KIAA), Eric Peng (KIAA), Richard de Grijs (KIAA), Jinliang Hou (SHAO)
Key questions:
1. Can we understand the origins of the first galaxies and study dark matter distribution on the
smallest scales?
2. How does accretion affect the evolution of galactic discs and how important are these effects in
comparison to secular processes?
3. How accurately can CDM simulations match observations, especially when they are confronted
with detailed analyses of the kinematics and chemistry of stars in nearby galactic haloes?
Introduction:
Theories of galaxy evolution have converged on the hierarchical growth scenario, where structures
are built up from the aggregation of smaller building blocks. Whilst this general picture has been
reaching a consensus, the next crucial step it is to identify and then analyse the signatures which
remain today. This process of ‘digging for fossils’ is not a trivial one, especially in distant galaxies.
In recent years, the most promising avenue for finding such fossils has been in our very own
Galaxy; only for our own Galaxy is it possible to study large numbers of individual stars in detail.
Spectrographs on 10m-class telescopes have enabled such studies to tentatively explore M31, but
further progress is hampered by the limitations of the current-generation of telescopes. TMT will
revolutionise this field by allowing us to probe more distant galaxies and in much greater detail than
is currently possible. Such work will allow us to push theoretical understanding to its limits,
moving the field into an era where progress is driven by observational discoveries.
Main science areas:
1. Ultra-faint satellites of the Milky Way
One of the most exciting recent developments in this field has been the discovery of a new class of
satellite galaxies around the Milky Way, the so-called ultra-faint (UF) dwarf galaxies [e.g. 1]. These
galaxies have very low-luminosity and as a consequence are believed to be the most dark matter
dominated objects in the universe, with mass-to-light ratios claimed to be as high as 1,000 [2]. This
means that they are important laboratories for studying dark matter on the smallest scales. They
play a crucial role in our understanding of the processes which control galaxy evolution, in
particular the influence of feedback and reionization on the gas content (and hence star formation
histories) of such low-mass galaxies.
However, spectroscopic analyses using the current generation of telescopes are limited to only a
handful of stars in these objects. With TMT it will be possible to obtain velocities for much larger
samples, allowing robust mass-to-light ratios to be determined from the analysis of velocity
dispersions. For example, TMT will allow us to extend at least two magnitudes below the mainsequence for UFs within 100 kpc. Their small sizes (half-light radii of 1 to 20 arcmin) are ideal for
the WFOS field-of-view. Furthermore, in the coming years photometric surveys such as Pan-Starrs
will identify these UFs out to much larger distances; only telescopes such as TMT will be able to
carry out spectroscopic analyses of these objects. In addition to the kinematic analyses, these
spectra will be provide metallicity information, which will be vital if we are to understand the
formation histories of these UFs. For example, recent studies are finding that these galaxies host
some of the most metal-poor stars in the universe [3]. Large samples of medium-resolution spectra
from WFOS will allow us to identify many candidate metal-poor stars which can be followed up
with, for example, HROS (see Section 4.6.2).
65
2. Dissecting the Andromeda galaxy
Although the Milky Way is an very good target to carry out detailed studies of the stellar
populations, there is one major drawback - because we are located within the galaxy is it difficult to
build up a full panoramic understanding of its structure. However, this is not a problem for more
distant galaxies, such as Andromeda (M31). This is an ideal target for TMT because M31 is the
Milky Way’s closest neighbour and, as TMT will be the only 30m-class telescope located in the
Northern Hemisphere, it will be the only one which will be able to study this important galaxy.
The current generation of large telescopes are only beginning to unveil the full complexity of M31
[4]. Some of the most important spectroscopic work in this field has been carried out using the
DEIMOS instrument on Keck [e.g. 5, 6], although these studies are only able to probe the tip of the
giant branch (which, at a distance of 785 kpc, lies at the limit of what is capable with a 10m
telescope). Given its comparable field of view with the DEIMOS instrument, a survey with WFOS
will be ideal to illuminate the processes which are governing the evolution of M31. Using TMT, a
study similar to the previous DEIMOS analyses will be able to measure the velocities and [Fe/H]
for much fainter depths, possibly extending to the red clump region. For the brightest stars WFOS
will be able to estimate the alpha-element abundance, which can be used to further illuminate the
chemical history of this galaxy (see [7] and also Section 4.6.2). Therefore TMT will be able to
amass a vast and rich set of data, considerably greater than what is possible with the current
generation of telescopes. With this we can make detailed studies of the structures in the halo of
M31, for example deconstructing the numerous streams and investigating how the recent encounter
between M33 and M31 has shaped the evolution of these two galaxies. For the first time we will be
able to understand the complexities of the M31 disc, which is known to extend out to ~70 kpc [8]. It
is unclear how the violent merger history of M31 has influenced the disc evolution, in particular
how the disc reacts to (or is formed by) these numerous accretion events. Only through detailed
kinematic and chemical studies of large samples of stars will we finally be able to decipher the
disc’s history, assessing the importance of accretion versus secular processes. By constructing
metallicity distribution functions for the disc at different locations we can constrain the star
formation and accretion history across the disc, simultaneously investigating the significance of
stellar radial migration which is currently an area of much debate.
Another important area of study for M31 will be its satellite population. Owing to the large distance
to M31, it is very difficult to determine reliable mass-to-light ratios for these objects using velocity
dispersions. One of the most comprehensive recent studies [9] looked at one cluster with DEIMOS
on Keck, but they were only able to place a very tentative upper limit on the mass-to-light ratio due
to their restricted sample size of only six stars. The satellite population of M31 is especially
important because of the differences to that of the Milky Way; one notable discrepancy is the
presence of recently-identified extended clusters around M31 [10]. The nature of these objects
(which intriguingly do not appear to be present around the Milky Way) is unclear, in particular the
question of whether they posses dark matter, i.e. whether they can be classified as dwarf galaxies or
star clusters. TMT will be able to obtain samples of the order of 100 stars, from which dispersion
profiles and mass models can be constructed.
Lastly, TMT will provide a major advance in the study of the M31 globular cluster
system. Globular clusters (GCs) are the oldest and most prominent stellar remains of some of the
earliest star formation in galaxies. These compact, massive star clusters are generally simple stellar
populations, older than 10 Gyr, and have played a crucial role in a wide range of astrophysics,
including the distance scale, the age of the Universe, stellar evolution, dynamics, and as tracers of
stellar halo formation. With a few exceptions, however, the ability to resolve GCs into stars and
study their detailed colour-magnitude diagrams has been limited to the Milky Way and its nearby
satellites. Even with HST, colour-magnitude diagrams of GCs at the distance of M31 are plagued
66
by crowding. With adaptive optics, the spatial resolution and the field of view of TMT/IRIS is
well-matched to GCs in M31 (comparable to ~1 arcsec seeing and a ~10 arcmin field of view at 12
kpc). TMT/IRIS will enable a deep, detailed study of halo GC population in M31, roughly
comparable to pre-HST studies of Galactic GCs, and complementing studies of the M31 field stars,
providing an independent measure of the star formation, merging, and chemical enrichment history
of M31.
3. Our more distant neighbours
For near-field cosmology, one of the most valuable aspects of a telescope such as TMT will be its
ability to study the kinematics and chemistry for resolved stellar populations in a number of
galaxies beyond the Milky Way. With velocity information for the streams in these systems it will
be possible to undertake modelling of a diverse array of streams in many different environments
[11]. For the first time we will be able to address the statistical significance of our findings,
comparing them to theoretical predictions from CDM simulations [e.g. 12, 13] across a wide variety
of galaxies. It will be possible to carry out spectroscopic surveys for many galaxies within the Local
Volume, for example building on the current work of the ACS Nearby Galaxy Survey Treasury [14]
which has constructed uniform photometric catalogues of resolved stellar populations in galaxies
out to 4 Mpc.
One such target is the M81 group which lies at 3.5 Mpc. This group is interesting because the
central members (M81, M82, and NGC3077) are known to be interacting [15], and so TMT will
allow us to see the effects of accretion using the kinematics of individual stars. Furthermore, at
these distances the WFOS field-of-view allows us to efficiently map large areas of the halo so that
we can use bright tracers to carry out detailed analyses of the mass distribution in these galaxies,
applying techniques developed for the Milky Way [e.g. 16] to more distant galaxies.
Possible TMT programs:
1. Velocity dispersion profile analysis of MW ultra-faint dwarf population, accurately determining
their mass-to-light ratios and investigating the chemical properties of these extreme galaxies.
2. Unravelling the accretion history of M31 by combining [Fe/H] and [alpha/Fe] with kinematics
for photometrically selected halo substructures.
3. Detailed dissection of M31 disc, using a number of well-chosen pointings along the major axis
to probe the kinematics and chemical history of the disc.
4. A spectroscopic survey of a number of more distant galaxies, probing the accretion history
across a diverse range of galaxies and placing strong constraints on CDM simulations.
China’s strengths and weakness in this area:
The science case that is presented here is a natural extension to work that is currently being
undertaken in the field of Galactic archaeology and near-field cosmology. Importantly, this is a
rapidly growing field in China. The current growth is being driven by the LAMOST telescope,
which will carry out a large spectroscopic survey of stars in the Milky Way and hence make a
significant contribution to the field. As a consequence, over the coming years the local knowledge
base will be built up so that by the time TMT reaches first-light this field will be flourishing in
China. The seeds of this growth are currently being planted through the recruitment drives such as
those funded by CAS, which aim to bring world-leading young scientists to China, although there is
still a deficiency in the number of experts that are crucial to train the current-generation of students;
it is these current students who will be the ones to lead the exploitation of TMT data and so, if they
are going to be able to make an impact on the international stage, it is vital for these to receive the
best possible training.
In the near-future LAMOST will carry out a survey of the M31 region. This will be particularly
67
important as it will allow Chinese students to actively carry-out science using a world-class dataset,
which is clearly the best way for them to learn how to excel in this field. Furthermore, this work
will build on the collaborations which already exist between the Chinese community and
international scientists who are at the forefront of M31 research, for example groups in the
Herzberg Institute of Astrophysics (Victoria, Canada) and the Institute of Astronomy (Cambridge,
UK).
At the present time the most active institutes are KIAA, NAOC and SHAO, all of which contain
numerous people working on multi-object spectroscopy. This list is rapidly growing and by the
middle of the next decade China has the potential to be a major power in the area of near-field
cosmology.
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Meeting of the Spanish Astronomical Society (SEA), Springer (arXiv:0812.3219)
Bullock & Johnston, 2005, ApJ, 635, 931
Cooper et al., 2009, MNRAS, submitted (arXiv:0910.3211)
Dalcanton et al., 2009, ApJS, 183, 67
Mouhcine & Ibata, 2009, MNRAS, 399, 737
Xue et al., 2008, ApJ, 684, 1143
4.6.2
Searching for first stars and cosmic stellar relics in the Galaxy
Haining Li (NAOC), Gang Zhao (NAOC)
Key questions:
1. How many metal-poor stars with [Fe/H] < -5.0 are there in the Galaxy?
2. Are there really two components of the Galactic halo?
3. Is there any first generation of stars or its relics in the Galaxy?
Introduction:
Metal-poor stars with atmospheric abundances of the heavy elements (such as iron) that are
substantially lower than the solar content provide fundamental constraints on numerous issues of
contemporary interests, from the nature of the Big Bang, to the astrophysical sites of neutroncapture element production. Specially, extremely metal-poor stars provide important clues to the
chemical history of our Galaxy, the role and type of early supernovae, the mode of star formation in
the proto-Milky Way, and the formation of the Galactic halo. Automatic spectral classification and
processing pipelines have made large scale stellar spectroscopic survey possible, and have already
obtained many metal-poor star candidates [1-3]. However, it is only possible, with high-resolution
spectroscopic observation by telescopes like TMT, that we can determine accurate elemental
abundances and chemical patterns to understand their nature, especially for stars with metallicity
[Fe/H] < -3.0.
68
Main science areas:
Searching for metal-poor stars
Stars with metallicities below 1/1000 of that of the Sun, i.e., [Fe/H] < -3.0, are referred to
extremely metal-poor stars (EMP stars), and are regarded the local relics of epochs otherwise
only observable at high redshifts [4]. These stars preserve, to a large extent, the chemical and
kinematic signatures of the gas clouds from which they formed, and hence they are important for
studying the formation and chemical evolution of the Galaxy, the properties (e.g., mass, rotation)
of the first generation of massive stars which exploded as type II supernovae (SN II),
nucleosynthesis and star formation processes in the early Universe. Age determinations of metalpoor, old stars also provide a lower limit for the age of the Universe [5].
Objective-prism or moderate-resolution (R ≤ 2000) spectroscopic and photometric survey, such as HK,
HES, SDSS/SEGUE, etc., have been quite successful in searching for metal-poor stars and systematic
researches including the tomography of the Milky Way and the metallicity distribution function of the
Galactic halo, as well as in collecting possible candidates for EMP stars. On the one hand, due to their
limited accuracy in metallicity measurements and elemental analysis, moderate-resolution
spectroscopic observations are unreliable in identifying or analyzing stars with [Fe/H] < -3.0, for which
follow-up high-resolution observations become rather crucial for constructing the metal-deficient “tail”
of the metallicity distribution function of Galactic halo [6]. On the other hand, there are still rather
limited high-resolution spectra available for metal-poor star candidates; only two stars with [Fe/H] < 5.0 have been detected so far, although several 8m telescopes are currently involved in follow-up
observations for metal-poor star candidates.
Therefore extremely large telescopes such as TMT are an unprecedented tool in identification and
analysis of metal-poor star candidates with [Fe/H] < -3.0, and in the future discovery of hyper metalpoor stars with [Fe/H] < -5.0 or even -6.0, which would undoubtedly help us explore the first generation
of stars and the very beginning of Galactic chemical evolution.
Possible TMT programs:
1. TMT WFOS/HROS follow-up observations of metal-poor star candidates with [Fe/H] < -3.0
identified by the LAMOST metal-poor star survey. Such a more complete metal-poor star
sample, combined with reliable kinematical measurements, will enable a systematic analysis on
the components of Galactic halo.
2. A special project with WFO/HROS aimed at finding metal-poor stars with peculiar chemical
patterns related to Population III (e.g., pre-enrichment by pair-instability SNe, etc.).
China’s strengths and weakness in this area:
We propose to carry out a new survey for metal-poor stars with LAMOST covering ~5500 deg2 of
the northern high-galactic latitude (|b| > 45°) sky down to B = 18.5 mag. Using the low-resolution
(R = 2000) LAMOST spectra in combination with SDSS photometry, we will determine the stellar
parameters Teff, log g and [Fe/H] of about 2,500,000 stars to identify the most metal-poor
candidates among them. Together with follow-up high-resolution observations, we expect to greatly
increase the total number of known metal-poor stars, including the discovery of ~1200-1300 new
extremely metal-poor stars, and ~12-13 hyper metal-poor stars.
Such a large scale survey needs to compete against a number of mature surveys including SEGUE,
it is thus important to have an optimal strategy for the LAMOST metal-poor star survey and followup observations that require coordination with these existing projects.
69
References:
1. Beers, T. C., Rossi, S., O’Donoghue, D., et al. 1999, in Astronomical Society of the Pacific Conference Series,
Vol. 165, The Third Stromlo Symposium: The Galactic Halo, ed. B. K. Gibson, R. S. Axelrod, & M. E.
Putman, 254
2. Christlieb, N. 2003, in Reviews in Modern Astronomy, Vol. 16, Reviews in Modern Astronomy, ed. R. E.
Schielicke, 191
3. Carollo, D., Beers, T. C., Lee, Y. S., et al. 2007, Nature, 450, 1020
4. Beers, T. C., & Christlieb, N. 2005, ARA&A, 43, 531
5. Frebel, A., Aoki, W., Christlieb, N., et al. 2005, Nature, 434, 871
6. Shoerck, T., Christlieb, N., Cohen, J. G., et al. 2008, ArXiv e-prints
4.6.3
The merging history of the Galaxy and stellar abundances of Galactic
globular clusters
Yuqin Chen, Gang Zhao, Jingkun Zhao (NAOC), Huawei Zhang (PKU)
Key questions:
4. What is the merging history of the Galaxy?
5. What are the abundance patterns of stars in Galactic globular clusters?
Introduction:
Mergers are expected to be a common phenomenon during the formation of galaxies. Mergers are
expected to have left behind a large amount of substructure in the phase-space distribution and
kinematics of stars, especially in the stellar halo, and probably also in the thick disc and bulge of
our Galaxy. As the Galaxy ages, the grouping phenomena in density or kinematics space overlap
more and more, and thus it is difficult to detect them in old streams and moving groups now. But
their member stars will be identifiable by their chemical imprints.
Some globular clusters may also have been driven by merger events, which may be extremely
common at the early time. The origin of the globular clusters is still rather unclear and but likely
involves connections with all the three other components of the Galaxy. The identification of their
origin from the abundance patterns of their stars in globular clusters would help to understand their
link with the thick disc, and their detailed elemental abundances will provide further information on
the diffusion theory for stellar astrophysics.
It is clear that nearby dwarf galaxies contribute to the formation of the Galactic halo, but halo field
stars have different compositions to the surviving satellites. Observations and modeling of such
differences may provide insights into the nature of satellites that may previously have been stripped,
such as their stellar and dark-matter masses, densities, spatial distributions and angular momentum,
compositions and kinematics.
Main science areas:
The evidence of merging history of the Galaxy
There are many stellar streams, such as Sagitarrius tidal trail, the Monoceros ring, etc, detected
outside the solar circle. The stars in streams are believed to have formed outside the Galaxy, via the
merging with nearby galaxies. The detailed kinematics and abundances of stars down to main
sequences of the tidal tails are not well studied with 8-10m telescope. It is interesting to obtain the
70
abundance patterns of dwarf stars in these stellar streams with TMT in order to check if they have
the similar origins in chemical space. In the solar neighbourhood, the Hercules stream associated
with the bar and its member stars have similar abundance and age. But several other streams found
in the solar neighbourhood have been shown to also contain stars of different ages and metallicities,
so they cannot be dissolving star clusters. They must have a dynamical origin probably associated
with spiral structure. TMT/HROS observations for selected stars down to main sequences in these
streams or moving groups are of high interest.
Meanwhile, the most outstanding result from chemical tagging of stars in the solar neighbourhood
is that some stars with low-alpha ratios detected by Nissen & Schuster (1997, 2009) and they are
thought to constitute the accretion component of the Galaxy from nearby satellite galaxies. In view
of this, it is interesting to search for low-alpha stars in the region far outside the solar
neighbourhood, especially in the outer halo, to investigate the merging history of our Galaxy. It is
probable that the accreted subpopulations exist throughout the halo, considering the number of
streams expected in ΛCDM is in the hundreds. These ‘missing satellites’ may have lose their space
and kinematical record of the parent cloud, but may eventually be detectable through the chemical
tagging. In addition, the outer halo formed from merging is considered to be younger than the
dissipative component. Therefore, the abundance analysis and age dating of stars in the outer halo
would provide crucial information on the merging history of the Galaxy.The best way to carry out
this study is that TMT/WFOS will be used to obtain the spectra of a significant number of stars in
outer halo, from which we can select interesting targets for HROS follow-up observations.
Finally, it would be interesting to compare the merging history of the Galaxy with those of nearby
galaxies (see Sect. 4.6.1) in order to understand the theory of galaxy formation.
The abundances of stars in globular clusters and surrounding fields
Globular clusters (hereafter GCs) play an important role in modern astrophysics because they are
the oldest objects in our Galaxy and some of them (e.g. young GCs in the outer halo) may originate
from merging satellites.
Of our great interests, some of the key issues involving GCs are listed as follows. Firstly, we want
to know to what extent there are multiple populations in GCs and to what extent are they chemically
homogenous. The chemo-dynamical information of stars in the lower main sequences of GCs with
TMT/WFOS will provide useful information on these issues. Secondly, it is commonly believed
that the globular clusters seen in the Galaxy today are only a subset of those existed; some may
have been shredded. It will be of interests to compare their chemical characteristics of GCs with the
field population in order to probe whether the Milky Way halo could have been made at least in part
by dissolving GCs and whether the GCs and the halo have the same metallicity distribution function,
and whether they have the same chemical inventory. Thirdly, tidal tails of GCs are extremely
interesting objects because they betray the existence of objects that were destroyed long ago and
thus reveal the Galaxy’s accretion history. Presently, at least 30 Galactic GCs are likely surrounded
by stellar tails as inferred from color-magnitude selected star counts (Grillmair et al. 1995; Leon et
al. 2000). A spectroscopy survey of stars in the tidal radius and the detected tidal tails of GCs with
WFOS/TMT is desirable. Finally, convincing measurements of [Fe/H] and key abundance ratios of
dwarfs in GCs, which will require high S/N, high-resolution spectroscopy obtained with
TMT/HROS, are important for tracing Galactic history (via the knowledge of early SN, star
formation, and formation of the Galactic halo) and can be used as a way of understanding the nature
and evolution of the diffusion history of elements during stellar evolution.
Possible TMT programs:
1. It is interesting to trace the merging history of the Galaxy by deriving chemical compositions of
71
stars in stellar streams, moving groups on the one hand and by searching for low-alpha stars (or
other stars with anomalous abundances) from the solar neighborhood to the very outer halo of
the Galaxy on the other hand. Some targets are selected from LAMOST low and middle
resolution spectra. The combination of WFOS and HROS will be efficient for tracing the
merging history of the Galaxy.
2. Chemical abundances of stars in different stages in a few globular clusters in order to probe the
diffusion history of elements and to investigate the characteristics of globular clusters
themselves. The abundance analysis of stars in the tidal tails of GCs with WFOS for thousands
of stars and HROS for limited sample of stars will be carried out.
China’s strengths and weakness in this area:
There are already several people working on abundance analysis of stars at NAOC and PKU. The
stellar spectroscopic survey with LAMOST will provide interesting targets for TMT/HROS
observations. Theoretically, we need to establish reliable chemical and dynamical evolution models
to reproduce the observational results.
References:
1.
2.
3.
4.
Grillmair C.J., Freeman K.C., Irwin M., Quinn P., 1995, AJ, 109, 2553
Leon S., Meylan G., Combes F. 2000, A&A, 359, 907
Nissen P.E. & Schuster W.J, 1997, A&A, 326, 751
Nissen P.E. & Schuster W.J, 2009, astro-ph/0807.3831
4.6.4
Isotope abundances
Jiangrong Shi, Gang Zhao (NAOC)
Key questions:
1. When did AGB stars begin to contribute to the Galactic chemical inventory?
2. Where are the n-capture elements produced?
3. Is there an 6Li plateau in metal-poor stars?
Introduction:
Measurement of isotopic ratios provides a completely new window into nucleosynthesis, galactic
chemical evolution, mixing within stars and stellar evolution. Such data, when available,
significantly improves our understanding of nuclear processes in various astrophysical sites. And
most isotope fractions, unlike elemental abundances, are very insensitive to the model atmosphere
parameters, and the isotopic fractions can be measured by detailed comparisons of an observed
absorption line profile to synthetic spectra of these line substructures. However, because the
isotopic shift is generally very small at optical wavelengths, very high spectral resolution (at least
90,000) is required to measure isotopic ratios in stellar spectra.
Main science areas:
1. The isotopic ratios of Mg in metal-poor stars with TMT/HROS
Magnesium (Mg) has three stable isotopes with atomic weights 24, 25 and 26, the lightest isotope
24
Mg is produced via burning of carbon in the interior of massive stars during their normal
evolution as a primary isotope, while the two heavier isotopes 25, 26Mg are secondary isotopes
produced primarily in intermediate mass AGB stars. Thus, the isotopic ratios 25, 26Mg/24Mg increase
with the onset of AGB stars. Therefore, Mg isotopic ratios in halo stars could be used to constrain
72
the rise of AGB stars in our Galaxy (Meléndzland & Cohen 2009).
It is important to know when AGB stars begin to enrich the halo in order to disentangle the
contribution of elements produced by intermediate-mass stars from the contribution of elements.
And the Mg isotopic abundances can be derived based on the analysis of MgH lines in cool metalpoor stars.
2. The isotopic ratios of n-capture elements with TMT/HROS
The elements heavier than the iron peak are mainly produced through neutron capture reactions in
two main processes, the s-process (slow) and r-process (rapid). Unlike other elemental abundances,
any n-capture element with multiple naturally occurring isotopes that are produced in different
amounts by the s- and r-processes can be used to assess the relative s- and r-process contributions to
the stellar composition. So, the isotopic abundances for these elements are more fundamental
indicators of n-capture nucleosynthesis, because they can be directly compared to r-process and sprocess predictions without the smearing effect of multiple isotopes (Mashonkina et al. 2003,
2006).
As supported by many observational and theoretical results, s-nuclei are mainly synthesized during
the thermally pulsing asymptotic giant branch phase of low-mass stars. The r-process is associated
with explosive conditions in SNe II, however, the precise astrophysical sites of the r-process have
not been identified. To reconstruct the evolutionary history of neutron-rich elements in the Galaxy it
is thus important to demonstrate that the r-process elemental abundance pattern extends to the
isotopic level, which would greatly strengthen the argument for a universal r-process mechanism
for the heavy n-capture elements.
The combination of Ba, Nd, Sm and Eu isotopic fractions can provide more complete knowledge of
the n-capture nucleosynthesis, constrain the conditions (e.g., temperature, neutron density, etc.) that
are required to produce the r-process elements, and determine the actual r-process path by
identifying the individual isotopes that participate in this process (Roederer et al. 2008).
3. The isotopic ratios of Li in metal-poor stars with TMT/HROS
Lithium has two stable isotopes, 6Li and 7Li. For the Big Bang nucleosynthesis theory, it is
important to determine the 6Li/7Li ratio in extremely metal poor dwarfs. Lithium is destroyed by
nuclear reactions at high temperatures. In the presence of mixing, material from the stellar
atmospheres can reach depths of high temperatures where lithium can burn or mix with material
that has burnt it. According to some stellar evolution models, little or no 7Li destruction is expected
for stars in the Spite plateau, so that these stars should retain their initial content. 6Li is, however,
more fragile than 7Li, so an observable content of the lighter isotope sets constraints on the degree
of depletion of the other one.
These are two challenging questions concerning the abundances of 6Li and 7Li in very metal-poor
stars: First, predictions of the nucleosynthesis by the big bang theory are tightly constrained because
the anisotropies of the cosmic microwave background determine the only free parameter in the
standard cosmological model. The predicted abundance of 7Li is about 0.5dex larger than the
measured abundance of lithium on the Spite plateau. So, the question is how does one bridge the 0.5
dex gap between observation and prediction? Second, the results from Asplund et al. (2006)
suggested that there may be a 6Li plateau parallel to the Spite plateau for 7Li, which implies a major
fraction of the 6Li may have been synthesized prior to the onset of star formation in our Galaxy. The
question is thus: how do we account for the high abundance of 6Li in some metal-poor stars?
Confusingly, the new results from Steffen et al. (2009) do not support the 6Li plateau. So it is
73
important to determine the 6Li/7Li ratios for more metal-poor stars with TMT/HROS to answer
these two questions.
While, the 12C/13C ratio is a sensitive indicator of the mixing processes experienced by carbonenhanced stars. As its ratio largely unaffected by uncertainties in the adopted stellar parameters, and
appears to be high in nearly primordial gas. Thus, any significant variation of 12C/13C should be due
to internal mixing processes in the stars. Thus, the measurements of this ratio are a particularly
powerful tool to constrain the models of internal mixing in giant stars, especially in extremely
metal-poor giants.
Possible TMT programs:
1) Determine isotopic ratios of Mg and Li in selected metal-poor stars.
2) Determine isotopic ratios of n-capture elements in a sample of halo and disk stars.
3) Determine iotopic ratios of C in metal-poor and Globular cluster giant stars.
China's strengths and weakness in this area:
In China, several teams, especially at NAOC and PKU, have experiences on analyzing the chemical
abundances of stars in our Galaxy, both for the disk and halo stars. Their works are well known
internationally in the field, and they have been involved in many international collaborations. They
have analyzed high-resolution echelle spectra of many stars observed with many telescopes 2m
(NAOC; Germany/Calar Alto), 6m (in Russia) and 8m (Sabura, VLT) optical telescopes. The
isotope abundance ratios for some important elements are determined taking into account non-local
thermodynamic equilibrium effects. Recently, these teams are working on LAMOST surveys,
which will provide huge database of spectra of Galactic stars. Together with follow-up highresolution observations, we expect to greatly increase the total number of (extremely) metal-poor
stars, thus making significant contributions to the field.
However, we have only limited opportunity to directly obtain high-resolution spectra with 8-10m
telescopes. TMT should remedy this situation. Combined with our theoretical expertise, we will be
able to make new, internationally competitive contributions in this area.
References:
1.
2.
3.
4.
Asplund, M., Lambert, D. L., Nissen, P. E., Primas, F., Smith, V. V., 2006, ApJ, 644, 229
Mashonkina, L., Gehren, T., Travaglio, C., Borkova, T., 2003, A&A, 397, 275
Mashonkina, L., Zhao, G., 2006, A&A, 456, 313
Meléndzland, J., & Cohen J. G., 2009,ApJ, 699, 2017
5. Roederer, I. U., Lawler, J. E., Sneden, C. et al. A., 2008, ApJ, 675, 723
6. Spite1, M., Cayrel1, R., Hill1, V., et al. 2006, A&A, 455, 291
7. Steffen, M., Cayrel R., Bonifacio, P., Ludwig, H.-G., Caffau, E., 2009, arXiv0910.5917
4.6.5
Chemical abundances of resolved giant stars and HII regions in Local Group
dwarf galaxies
Yanchun Liang (NAOC), Yuqin Chen (NAOC), Gang Zhao (NAOC), Jianrong Shi (NAOC)
Key questions:
1. How do we understand the evolutionary and chemical enrichment histories of local group
dwarf galaxies?
74
2.
3.
4.
What is the relationship between dwarf Irregulars (dIrr) and dwarf spheroidals/dwarf
ellipticals (dSphs/dEs)?
What are the [α/Fe] abundance ratios of stars in the dSphs?
How do the HII regions abundances correlate with the luminosities for dIrr galaxies?
Introduction:
The Local Group is a collection of dwarf galaxies dominated by two giant spirals, our Milky Way
(MW) and Andromeda (M31). The dwarfs of the Local Group provide a uniquely well-studied and
statistically useful sample of low-luminosity galaxies. This includes a number of different types:
early-type dwarf spheroidals/dwarf elliptical (dSphs/dE), late-type star-forming dwarf irregulars
(dIrrs), the recently discovered very-low surface brightness, ultrafaint dwarfs (uFds), and centrally
concentrated a actively star-forming blue compact dwarfs (BCDs). Tolstoy, Hill, Tosi (2009) [1]
and Mateo (1998) [2] have nicely reviewed the progress on dwarf galaxies in the Local Group (see
Appendix B). With ground-based 8-m telescopes, it is possible to obtain detailed chemical
abundances of the resolved red giant branch (RGB) stars in dSphs; the abundances of HII regions in
dIrrs can be obtained by even smaller telescopes since they are brighter. Great progresses have been
obtained for these subjects, but still lots of uncertainties exist. With the 30m TMT and efficient
instruments, such as HROS and WFOS, we will significantly improve our ability to study resolved
stellar populations in Local Group galaxies. Moreover, TMT has its unique opportunity to observe
the northern sky (from Hawaii) since GMT and E-ELT will be both located in Chile (in the
Southern Hemisphere).
Main science areas:
1. The chemical abundances of resolved RGBs in dSph galaxies using TMT/HROS
With the high resolution of TMT/HROS, it is possible to obtain the Echelle spectra of the resolved
RGB stars in the dSphs in Local Group in the northern sky. These can trace the stellar population
and star- formation history of the galaxies and study the statistic properties of this kind of galaxies.
More than one hundred RGB stars in the local dSphs galaxies mostly in southern sky have been
measured their chemical abundances, especially the [α/Fe], by using the 8-10m telescopes. The first
studies on detailed chemical abundances in dSphs galaxies are using Keck-HIRES (Shetrone et al.
2001[3], 17 stars in Draco, Ursa Min, Sextans) and VLT-UVES (Bonifacio et al. 2000 [4], 2 stars in
Sgr). These early works were shortly followed by similar studies slowly increasing in size. Most
recently, the DART survey determined kinematics and metallicities for large sample (>80)
individual stars in nearby dSphs by using the VLT/FLAMES multifiber facility with high spectral
resolution. There have been similar surveys by other teams on VLT and Magellan telescopes. All
these are for Sgr, Fnx, Carina, Sci etc. Aoki et al. (2009) [5] has used the Subaru/HDS to observe 6
metal-poor stars in Sextans. These stars typically have magnitudes in the range V=17-19, and the
exposure time is up to 14 hours even for 8-10m telescopes.
With the great 30-m TMT, and the high resolution spectrograph HROS, R~30000- 50000, we could
observe a number of resolved RGBs in dSphs in northern sky. There could be 28 such dSph within
distance of 2200 kpc [1, 2]. HORS will be able to obtain similar results for stars at V~20 in more
distant (~400-500 kpc) and hence more isolated systems, even can try for farther systems. The
optical measurements can also be supported by K-band measurement: TMT/NIRES can easily reach
below the tip of the red giant branch throughout the Local Group. These much extensive studies not
only allow us to obtain the abundance pattern in individual galaxy, but also are important for
statistical studies. The color-magnitude diagrams (CMDs) of nearby dwarf galaxies can be used to
find the targeted stars. Since 1950s, large numbers of detailed CMDs have been derived for star
75
clusters and nearby dwarf galaxies. To date, for many of the galaxies HST/WFPC2 data already
exist [1].
The α-elements abundances are good tracers for the chemical evolution histories of galaxies. They
can easily be measured in RGB spectra includes O, Mg, Si, Ca, Ti. Fig.11 of reference [1] shows
the Mg and Ca abundances of individual star in those dSphs with more than 15 measurements. It
shows that each of these dSphs starts at low metallicity, where its [α/Fe] ratios are similar to those
in the MW halo, and then evolve down to lower values than are seen in the MW at the same
metallicity. There is a "knee" in a plot of [Fe/H] versus [α/Fe]. The knee position indicates the
metal-enrichment achieved by a system at the time SNe Ia start to contribute to the chemical
evolution. The knee is not well defined in dSphs owing to a lack of data except in Scl dSph. With
HROS, we could study the [α/Fe] abundances of the resolved RGBs, and the "knee" in the
individual dSph in northern sky. We could further check whether the position of the knee correlates
with the total luminosity and/or the mean metallicity of the galaxy.
Moreover, [Na/Fe], [Ni/Fe], [s-process elements/Fe] abundances can also been studied to better
understand the chemical enrichment history of these dSphs. We could also compare the abundance
patterns between the dSphs around M31 and MW, and check if there exists any difference between
them, which could give hints for the environments and formation histories of M31 and MW.
Detailed chemical abundances of individual stars in M31, M33, M32 could also been studied and
compared with those in MW.
2. Chemical abundances of resolved stars in uFds with TMT/HROS
A number of Ultrafaint Dwarf galaxies (uFds) are being found by the SDSS around the MW[1].
Some of these new systems have had their stellar population analyzed using the synthetic CMD
method. The individual stars in the uFds have so far been little observed at high-resolution. This is
probably owing to the difficulty in confirming membership for the brighter stars in these systems.
Several groups (Koch et al. 2008[6], Frebel et al. 2009[7]) are currently following up confirmed
members (typically selected from low resolution Ca II triplet observations) to derive abundances.
With the TMT/HORS, we could observe the individual RGB stars in these uFds and to study their
chemical
abundances.
All
the
19
uFds
are
dSphs
(see
http://www.delphes.net/messier/more/local.html). The HROS observations will be good follow-up
for the middle resolution observation with WFOS (see Section 4.6.1).
3. Dwarf Irregular galaxies: abundances of HII regions with WFOS and abundances of resolved
supergiants with HROS
The dIrrs are all (except the SMC) located at rather large distances from the MW. So far, the only
probes that could be used to derive chemical abundances in these objects are HII regions and a few
supergiant stars. This limitation makes it difficult to gather relevant information to constrain the
chemical enrichment histories of these systems. However, abundances in HII regions and
supergiants are useful to understand how dIrrs fit in the general picture of dwarf galaxies, and how
they compare to larger late-type galaxies.
The well quoted work of Skillman et al. (1989) [8] provided the luminosity-metallicity relations for a
sample of local dIrr galaxies within ranges of 12+log(O/H) from 7.3 to 8.3 and MB from -10 to -19
mag. Candidate HII regions in the 7 nearby dwarf irregular sample galaxies were selected from an
Hα survey of dwarf irregular galaxies. Hodge et al. and Hunter et al. have made lots of efforts on
surveys on Hα in nearby irregular galaxies.
76
With TMT/WFOS (R<5000), it is possible to obtain the optical spectra of a number of HII regions
of dIrr galaxies in the Local Group in northern sky and study their HII region abundances. The
luminosity-metallicity relation could be checked further in the metal-poor and low-luminosity
ranges. This will extend the present SDSS luminosity-metallicity relation and stellar massmetallicity relations found for a large sample of metal-rich star-forming galaxies.
These local dIrrs will be good places to estimate the primordial He abundance. Their low
abundances of metals and helium, derived from HII region spectra in the dIrrs, allow the
determination of the primordial helium abundance with minimum extrapolation, and thus provide
insight into Big Bang nucleosynthesis.
A- to M-type supergiants are of further interests as they provide the present-day [α/Fe] ratios in
dIrrs. The first dIrr where abundances of stars were measured was the SMC. Similar studies in more
distant dIrrs needed efficient spectrographs on 8-10m telescopes at the expense of observing for
many hours. Venn et al. (2003)[9] study the A-type stars in NGC6822, and Tautvaisiene et al. (2007)
[10] study the M-type stars in IC 1613. This small sample shows that the dIrrs actually extends the
trends of dSphs. TMT/HROS will dramatically improve observations on individual stars in the dIrrs
and allow us to further compare the abundance patterns of dIrrs and dSphs.
Possible TMT programs:
1.
2.
3.
A survey to study chemical abundances of the resolved RGBs in dSphs.
A survey to study chemical abundances of the RGBs in uFds.
Surveys for chemical abundances of HII regions and supergiants in dwarf Irregular galaxies,
the primordial He abundance.
China's strengths and weakness in this area:
The science case mentioned in Sect. 4.6.1 about the resolved stellar populations and chemical
abundances in nearby dwarf galaxies are of fundamental importance in astronomy. Several teams,
especially at NAOC and PKU, have great experience on analyzing the chemical abundances of stars
in our MW, both in the disk and halo, and of the HII regions and Planetary Nebulae (PNe). Their
works are well known internationally, and they are involved in many high-profile international
collaborations. They have observed and analyzed the high-resolution Echelle spectra of many stars
and PNe using 2m (e.g., the 2.16m of NAOC), 6m (in Russia) and 8m (Subaru) optical telescopes.
For low-resolution spectra, a large sample of HII regions (and PNe) and star-forming galaxies,
especially from SDSS, have been analyzed to study the metallicities and stellar populations. All
these experiences will be useful for analyzing the observed spectra of the resolved stars and HII
regions in nearby dwarf galaxies. Also these teams are working on LAMOST surveys, which will
provide huge database of spectra of Galactic stars and extragalactic objects. These can also be
related to the chemical Evolution models of galaxies done at NAOC, SHAO, and SNU.
However, due to the lack of observational facilities, we have no opportunity to directly observe and
analyze the chemical abundances of resolved stars in Local Group galaxies with 8-10m telescopes.
TMT will allow the Chinese astronomers to compete globally in this area using first-hand data by
forming a coherent, stable research group including young astronomers and students.
References :
1.
2.
3.
4.
Tolstoy, E., Hill,V., Tosi, M. 2009, ARA&A, 47: 371-425
Mateo, M. 1998, ARA&A, 36: 435-506
Shetrone, M. D., Côté, P., Sargent, W. L. W. 2001, ApJ, 548 : 592-608
Bonifacio, P., Hill, V., Molaro, P. et al. 2000, A&A, 359: 663-668
77
5.
6.
7.
8.
9.
10.
Aoki, W., Arimoto, N., Sadakane, K. et al. 2009, A&A, 502: 569-578
Koch, A., McWilliam, A., Grebel, E.K., Zucker, D.B., Belokurov, V., 2008, ApJ, 688: L13-L16
Frebel, A., Simon, J.D., Geha, M., Willman, B. 2009, arXiv0902.2395
Skillman, E. D., Kennicutt, R. C., Hodge, P. W., 1989, ApJ, 347: 875-882
Venn, K. A., Tolstoy, E., Kaufer, A. et al. 2003, AJ, 126: 1326-1345
Tautvaišien, G., Geisler, D., Wallerstein, G. et al. 2007, AJ, 134 : 2318-2327
4.6.6
Mass distributions of the Milky Way and local group
XiangXiang Xue (NAOC),Gang Zhao (NAOC)
Key questions:
1.
2.
What is the total mass and dark matter distribution of our Milky Way? Are all the Milky Way's
satellites bound?
How does the mass distribute in the nearby galaxies?
Introduction:
The mass distributions of the Milky Way and the nearby galaxies are unclear as a result of limited
capability of current telescopes. Understanding the mass distributions of the Milky Way and nearby
galaxies is crucial for modeling the dynamics of the local group, connecting observations of
galaxies to large-scale cosmological dark matter simulations. Mapping the mass of the Milky Way
and the nearby galaxies demands good tracers with precise positions and velocities. Furthermore
these tracers should distribute in the galaxy as far from the center as possible. The standard candle,
such as blue horizontal branch stars (BHB, Mv=0 [1]), red clumps giant (RCG, 0.5<Mv<1.3 [2]),
super giants (SG, -9<Mv<-4 [3]), Cepheid variables (CV, -6<Mv<-2 [4]) and RR Lyrae variables
(Mv=0.61 [5]) are very luminous and their distances can be determined easily and accurately. TMT
will provide a chance to obtain such tracers in the distant Galactic halo and nearby galaxies.
Main Science areas:
1. Dark matter halo mass of the Milky Way
Our internal position permits the placement of unique constraints on the Galaxy’s stellar mass
content, its dark matter profile at large radii, and the three-dimensional shape of its dark matter
halo. Yet our location within the Galaxy also complicates some measurements, such as the extended
rotation curve of gas in its disk. As a result, the dark mass profile for the Milky Way beyond 20 kpc
and the total mass of the Milky Way have not been previously constrained to better than a factor of
2-3. The latest estimate of the Milky Way’s total mass was derived by the kinematics of 2400 BHB
stars drawn from SDSS DR6 [6], although these stars distributed only up to 60 kpc from the
Galactic center. SDSS DR8 provides about 4600 BHB stars up to 80 kpc from the Galactic center
[7]. The BHB sample has been enlarged two times, but many faint BHB stars are lost because of
bad quality spectra. TMT provide us a great opportunity to obtain high quality spectra of distant
halo stars. TMT observations of extreme distant halo BHB stars and RR Lyraes may provide unique
constraints on the Milky Way mass.
2. Mass distribution of nearby galaxies
In recent years many studies have been done to explore nearby galaxies, such as M31, M33, etc
[8,9,10,11,12], but it is hard to go further into M31 and other fainter nearby galaxies due to the
limitations of current telescopes. TMT enables spectroscopic observations of standard candle stars
(BHB, RCG, SG, RR Lyrae, CV) in the nearby galaxies even fainter ones (see also Sect. 4.6.1). The
78
distances and velocities of these stars can help us to explore the mass distributions of the galaxies,
and check the cosmological simulations of the galaxies.
Possible TMT programs:
1. A spectroscopic observation of faint BHB stars and RR Lyraes in the Milky Way with WFOS;
the faint BHB stars and RR Lyraes candidates may be obtained by SDSS/SEGUE and PanSTARRS.
2. The spectroscopic observations of standard candle stars of nearby galaxies with WFOS. The
radial velocities and precise 3D positions of these special stars enable mapping the mass
distributions in the galaxies.
China’s strengths and weakness in this area:
We have the experience on the research of mass distribution of the Milky Way. There are already
several people working on cosmological galaxy formation simulations. We are internationally
competitive for connecting observations to the theoretical simulations.
We still need to do more efforts on the simulations of nearby galaxies.
References:
[1] Yanny, B. et al. 2000, APJ, 540, 825
[2] Groenewegen , M. A. T. et al. 2008, A&A, 488, 935
[3] Slowik, D. J. et al. 1995, AJ, 109, 5
[4] Chiosi, C. et al. 1993, APJS, 86, 541
[5] Benedict, G. F. et al. 2002, AJ, 123, 473
[6] Xue, X.-X et al. 2008, APJ, 684, 1143
[7] Xue, X.-X et al. 2010, in prep.
[8] SarajediniAta, Yang, S. -C. 2009, arXiv0912.2863S
[9] Yang, S.-C.; Sarajedini, Ata, 2010, ApJ, 708, 293Y
[10] Fiorentino, G. et al. 2009, arXiv0911.0355F
[11] Forestell, A. D. et al. 2004, ASPC, 310, 99F
[12] Massey, P. et al. 2006, AJ, 131, 2478M
4.6.7
Spectroscopic study of low-mass stars, brown dwarfs and planets
L.F. Li, Z.W. Han, S. H. Gu, F.H. Zhang, X.F. Chen, R. S. Pokorny, J.H. He (NAOC/YNAO)
Key question:
1. How the stellar evolution theory is calibrated?
2. What is the physics that controls the production of magnetic fields of very low-mass fullyconvective stars and brown dwarfs?
3. What is the properties of brown dwarfs
4. How does a host star interact with its planet through the magnetic field?
Introduction:
This project would like to obtain the spectra of some low-mass, cool stars (including those with a
convective envelope and fully convective ones) or even brown dwarfs with high resolution and high
signal-to-noise (S/N) using TMT.
Main science areas:
79
1.The calibration of stellar evolution theory
Stellar evolution theory is sometimes considered to be a mature branch of astrophysics with
well-established results and only minor need for further research. This is a misconception. The
quality of a physical theory is always to be measured against experimental (observational) facts [1].
We do not understand the stars well enough. There are many physical processes which are either
not well understood, or not taken into account properly in the theoretical models of stars, such as
convection, rotation, diffusion, atmospheres, and mass loss, etc.
Convection is a very important means for the transport of both energy and matter. It is
intrinsically time-dependent, non-local, and 3-dimensional. It is a long-standing problem of stellar
evolution theory that it cannot be treated adequately in the calculations. The standard method still
assumes that the mixing due to convective flows is instantaneous and computes the resulting
temperature gradient in super-adiabatic layers according to a simple Mixing-length theory in which
only one length scale for transport of convection elements is assumed. Although the theory of
atmospheres is treated usually independently from that of the interiors of stars, the comparison of
the models with observations inevitably involves it. In particular the cool stars are difficult, because
of the formation of molecules and dust grains and the interaction with convection, pulsation, stellar
winds. Atmospheres should also provide mass loss rates, but so far only for hot luminous objects
the theory is far advanced [1]. Stars are known to rotate and some do so rapidly. The effects on the
stellar structure and evolution of are two-fold: the spherical symmetry is broken (the hydrostatic
equilibrium is modified due to the rotational forces), and large- and small-scale matter motions
might be induced, leading to rotationally induced mixing.
We would like to use TMT to observe the spectra with high quality for some low-mass single
stars with different spectral types (G and K) or detached binaries contained these cool stars. Then
we can derive the mass loss rate and improve atmosphere model for these cool stars through the
spectroscopic analysis. These results will be used to limit the treatment of convection and rotation
effects, since the atmosphere always interacts with the convective envelope. These results will in
turn be used in stellar evolution models to remove some uncertainties and improve the
determination of stellar age scales. These absolute and relative age-scales of the stars can offer an
opportunity to compare independently with cosmological models and parameters obtained from
other source (e.g. cosmic microwave background and SN Ia). Meanwhile, we analyse the spectra of
some detached binaries that contain cool components and derive their radial velocities and the
accurate physical parameters (masses, radii, luminosities and effective temperatures). By a
comparison between these observed results and the theoretical models we can calibrate stellar
evolution theory. Then, we use these results that have been calibrated to investigate the evolution of
the distant globular clusters and galaxies based on population synthesis.
2.The spectroscopic study of fully-convective stars and brown dwarfs
Very low-mass stars with late spectral types (M, L) are the majority constituent of the Galaxy
by number. Their lifetimes are much greater than the current age of the universe, and they therefore
serve as useful probe of Galactic star formation history in the local solar neighborhood [2]. They
also encompass many important regions of stellar parameter space, including the onset of
convection in the stellar interior, the onset of significant electron degeneracy in the core, and the
formation of dust and subsequent depletion of metals onto dust grains in the stellar atmosphere. Of
particular interest is the fact that many late type stars have strong surface magnetic fields [3] that
heat the outer atmosphere above the photosphere and lead to observable emissions from the
chromosphere (e.g. Ca II and H Balmer series), the transition region (e.g. resonance lines of
abundant ions such as CIV) and corona (e.g. thermal soft X-rays). The very low-mass fullyconvective stars (M3 to ~M6) and brown dwarfs (Later than ~M6) is very different from the solar80
like stars with a convective envelope. The solar dynamo theory is proposed on base of the structure
of the Sun which has a radiative core and a convective envelope. It suggests that the magnetic fields
of the solar-like stars usually origins on the base of convective zone, then it is enlarged by
differential rotation. Therefore, the physics of that controls the production of magnetic fields in very
low-mass fully-convective stars and brown dwarfs is not well understood [4]. Meanwhile, the
rotation evolution of very low-mass stars and brown dwarfs shows that the normalized magnetic
activity level does not weaken until spectral type mid-M (M 6-7), after that its diminishes rapidly
[5]. The physics that underlie the weakening of magnetic activity of stars with spectral types later
than M6-7 is not clear. Mohanty & Basri [6] suggested that the atmospheres of the stars with
spectral types later than M6-7 might become neutral, however it requires the supports of
observations.
We want to observe the spectra for some very low-mas stars and brown dwarfs with different
spectral types (from M to L) by using TMT, such as Gl 70 (M2.0, J=7.37mag), Gl 729 (M3.5,
J=6.22mag), Gl 876 (M4.0, J=5.93mag), Gl 299 (M4.5, J=8.42mag), Gl 905 (M5.0, J=6.88mag), GJ
1002 (M5.5, J=8.32mag), GJ 1111 (M6.0, J=8.24mag), LHS 3003 (M7.0, J=9.97mag), VB 10
(M8.0, J=9.91mag), LHS 2924 (M9.0, J=11.99mag), LHS 2065 (M9.0, J=11.21mag) and so on. By
analysis of these spectroscopic observations, we would investigate the atmospheres (including the
chemical elements and property of the atmospheres), the physics that controls the production of
magnetic fields and rotation in these objects. For example, the effective temperature of these very
cool dwarfs is very low and the atmosphere might be neutral. If so, no ionised components can be
found in the atmospheres of the stars with spectral type later than M6-7. The neutral atmosphere
would weaken the efficiency of magnetic braking, since the neutral matter is no longer lost at the
Alfven radius as the ionised gas of hot stars. The mass of dwarfs (~13-75MJup) locates a region
between the planets and low-mass stars, the systemic spectroscopic study in the brown dwarfs is
important not only for investigating the evolution of brown dwarfs, but also for investigating the
formation of planets (see below).
3. The magnetic interaction between the host star and its planet
For the hot Jupiters lied within 0.1AU to the host star, normally they are located within the
Alfven radius of the host stars. Such a condition permits the magnetic interaction between the host
star and its planet [7]. The study for such phenomenon can help us to understand the formation,
migration and evolution of hot Jupitors, and also provides a way to detect the magnetic field of the
planet, offering us a chance to explore the interior structure of extrasolar planet and understand the
hydrodynamic property of extrasolar planet's atmosphere.
Normally, such an interaction is weak, we can find the interaction signal from the variation of core
emission of CaIIHK lines, or using Doppler imaging to detect the magnetic variation in time series
of spectral profiles. Such observations will require large telescope like TMT/HROS.
Possible TMT programs:
1. Obtain high S/N spectra for low-mass stars with different spectral types, and then derive the
mass loss rate and improve atmosphere model for the cool stars through spectroscopic analyses.
2. Systematic spectroscopic study of a sample of brown dwarfs.
3. Select a group of stars with close-in planets, monitor the CaIIHK lines and detect the variation
of their core emission, and perform Doppler imaging of extrasolar planet systems to find the
change of magnetic field of the host stars due to the interaction.
Strengths and weakness of Chinese astronomy in this topic:
81
There are two groups working in this field at YNAO. One group mainly investigate the structure
and evolution of the single and binary stars by constructing their evolutionary models theoretically,
then, investigate the evolution of the globular clusters and galaxies by using population synthesis
based on stellar evolution theory. Therefore, the uncertainties in stellar evolution theory would have
a significant influence on the investigation of globular clusters and galaxies. However, we cannot
remove the uncertainties in the stellar evolution theory because of the lack of the support of
observational results with a high quality for cool stars and brown dwarfs. Another group working in
stellar magnetic fields, and they have already established a platform for such a research topic. They
need to further train PhD students, particularly on the magnetic interaction between host star and
planet so that we can perform research in this area more efficiently.
Reference:
1.
2.
3.
4.
5.
6.
Weiss A., 2002, ESASP, 485, 57
Gizis J.E., Reid I.N., Hawley S.L., 2002, AJ, 123, 3356
Johns-Krull C.M., Valenti J.A., 1996, ApJ, 459, L95.
West A.A., Hawley S.L., Walkowicz L. M., Covey K.R., et al., 2004, AJ, 128, 426
Schmidt S.J., Cruz K.L., Bongiorno B.J., Liebert J., Reid I.N., 2007, AJ, 133, 2258
Mohanty S., Basri G., 2003, ApJ, 583, 451
7. Shkolnik, E. Bohlender D.A., Walker G. A. H. et al. ApJ, 2008, 676, 628
4.6.8
Observational Studies of Neutron Star Systems
Zhongxiang Wang (SHAO)
Key questions:
1. Do fallback disks generally exist?
2. What is the optical/near-infrared radiation mechanism in magnetars?
Introduction:
While since their discovery in 1967, neutron stars (NSs) have been primarily targets at radio
frequencies, observational studies have been extended to optical, X-ray, Gamma-ray, and more
recently infrared (IR) frequencies because of various manifestations of them over the frequencies.
With TMT, much improvement on our understanding of certain aspects of them can be made.
Searching for fallback disks
Fallback occurs in core-collapse supernovae when the reverse shock, caused by the impact of the
shock wave with the outer stellar envelope, reaches the newly formed NS. Fallback could have
profound implications for the endgame of massive star evolution, particularly since it could lead a
newborn NS to collapse into a black hole. It is also a promising mechanism for producing the
debris disks necessary to form planets, which are known to exist around at least one pulsar. It was
once thought that young radio pulsars like those in the Crab and Vela supernova remnants were
prototypical of newborn NSs, but recently it has been realized that the young NS population is
unexpectedly diverse. For example, there are central compact objects (CCOs) that are mysterious
radio-quiet, non-plerionic X-ray point sources, and anomalous X-ray pulsars (AXPs) and
spectacular soft gamma-ray repeaters that are believed to have extremely strong (10^{14} G)
surface magnetic fields (“magnetars''). This diversity requires explanation, and the existence of
fallback disks could be part of the cause [see 1 and references therein].
Recently, Magellan near-IR and Spitzer mid-IR searches for fallback disks were carried out. The
preliminary targets were several AXPs and CCOs, and a few radio pulsars. One candidate fallback
disk was found around the AXP 4U 0142+61 [2]. The upper limits on other targets were
approximately 5 micro-Jy at K band, Spitzer 4.5 and 8.0 microns. Considering these results, the
82
searches are not conclusive because of the uncertainties on the source distance, interstellar
extinction, and (putative) disks' geometry and orientation.
With TMT, 100 times better sensitivity at near-IR wavelengths will be reached for point sources
such as neutron stars. By doing a similar search to that recently conducted and combining it with
JWST observations at mid-IR, the question whether or not fallback disks generally exist around
isolated NSs would be answered. The candidate fallback disk found around 4U 0142+61 needs
further confirmation. With TMT, IR spectroscopy searching for lines in its emission would provide
the answer.
In addition, as part of the targets, AXPs are found to in general have relatively bright optical and
near-IR emission (R>25, K>20), which is not seen in normal pulsars. Because of large distances
and high extinction to them, this part of emission is not well observed and not understood at all.
TMT will allow a detailed study of them, helping our understanding of the emission mechanism.
Pulsars and Pulsar Wind Nebulae
Radio pulsars are born spinning rapidly, with a large amount of rotational energy to expend. The
major part of the rotational energy loss of a pulsar will be carried out in a magnetized, high-velocity
stellar wind, which fuels an extended pulsar wind nebula (PWN) by shocking the surrounding slowmoving medium. Studies of PWNe are important for understanding astrophysical processes such as
magnetized relativistic flows, shock interactions, and high-energy radiation mechanisms.
Observations of them also offer ways of probing the interstellar medium [3]. Thanks to Chandra,
~40 PWNe have been detected and studied at X-ray energies. They appear as various types of
shapes, having fine structures that indicate distributions of the magnetic fields and high-energy
particles. Their spectra are power-law, arising from synchrotron radiation [4].
It has been considered that synchrotron spectra of PWNe rise from X-rays all the way to radio
frequencies, with a break somewhere between the radio and IR frequencies. Such a spectral break
provides information about the age and magnetic field strength of a PWN. However, recently it has
been realized through IR observations of PWNe that multiple spectral breaks may exist, indicating
the evolution of a PWN's magnetic field and the spectrum of particles ejected from the associated
pulsar [5]. In order to better study PWNe by obtaining their broadband spectra, a TMT survey to
cover optical and near-IR wavelengths would help. Previous targets that have been accessible to
study with current telescopes are only a handful of brightest pulsars.
With such a survey, it would be likely to detect pulsars as well. While it is generally accepted that
pulsars have a power-law optical/IR spectrum, arising from synchrotron radiation of particles in the
magnetosphere, only 9 of ~1000 pulsars have been detected and most of the detections had low
signal-to-noise ratios (S/N) [6]. It is important to firmly confirm the previous results and establish
this fact by detecting more pulsars. For a few middle-aged pulsars, a better determination of their
power-law emission will improve studies of thermal emission from the surface of the NSs, which
helps determine the thermal evolution of NSs and the equation of state of their super-dense
interiors. In addition, the spectral turn-over point due to synchrotron self-absorption has been
suggested to be within near-IR wavelengths [7]. Detections of the turnover points would probe the
magnetospheric structure of pulsars.
Observations of Low-Mass X-Ray Binaries
a. Masses of accreting neutron stars
One effort in observational studies of low-mass X-ray binaries (LMXBs) is to measure the masses
of NSs in them through time-resolved spectroscopy. NSs in LMXBs are accreting from their
companions, and thus may have mass values larger than the canonical 1.4 Msun value. The ultimate
goal is by collecting mass measurements of NSs, the long-sought equation of state of their
83
superdense interiors can be determined. For certain cases, it has shown that the current 6-10 meter
telescopes are still not sufficiently large, with low S/N detections of lines [e.g., 8]. A large
collecting area of TMT will greatly help in this area.
b. Properties of ultracompact LMXBs
Ultracompact LMXBs are those binaries with orbital periods lower than 80 min. Along with their
white dwarf analogues (the AM CVn binaries), they represent extreme and exotic endpoints in
binary and stellar evolution. For the persistent ultracompact systems, their optical emission is
dominated by that from their accretion disks. It is believed that the companions in ultracompact
binaries must be hydrogen-poor. In several sources, based on the orbital periods, the companions
are suggested to be C/O white dwarfs [9]. Spectroscopy of them with TMT will help find direct
evidence for the suggestion; they are too faint to the current large telescopes. In addition, there are
a few candidate ultracompact LMXBs, selected based on either their optical brightnesses or X-ray
spectral features. Time-resolved photometry with TMT will help determine whether they are
ultracompact and have orbital periods of ~10 min. The limiting orbital period that can be detected
by the current large telescopes is approximately >15 min for these sources (V=21).
The known transient systems, which outburst once in every few years, also contain a millisecond Xray pulsar (MXP). The discovery of the first MXP binary in 1998 first and finally confirmed the
long-sought connection between LMXBs and recycled millisecond radio pulsars [10]. Thus far, the
transient systems have only been observed in their outburst, with optical emission coming from an
X-ray heated accretion disk. What the companion stars are may be guessed from the obtained ultrashort orbital periods. Observations of the sources in their quiescence are required in order to learn
the detailed properties of them. Recent such effort with Very Large Telescopes failed [e.g. 11],
indicating the need of TMT.
Possible TMT programs:
1. Searching for fallback disks around a sample of isolated, young neutron stars.
2. Optical/near-IR observations of pulsars and their wind nebulae.
3. Spectroscopy of low-mass X-ray binary systems; Ultracompact X-ray binary identification.
China's strength and weakness in these sub-fields:
There are a few people working on observational and theoretical studies of the sub-fields. We are
internationally competitive, although much effort is needed to build up world-leading groups.
84
4.7 Early light houses and cosmic reionization
Haojing Yan (Ohio State), Liang Gao (NAOC), Xiaohui Fan (Arizona)
Key questions:
1.
2.
3.
4.
What are the origins of first stars, first galaxies, and first supermassive blackholes?
When and how did the first light sources cause the cosmic reionization, and what is the impact
of the reionization to the cosmic galaxy formation processes afterwards?
How were the early galaxies assembled?
What is the chemical enrichment history of the early Universe?
Introduction:
While we have not yet found the "first light sources", we are approaching this goal. Currently,
quasars and star-forming galaxies have been unambiguously identified out to z~6.4-7 ([1][2]), an
early epoch that is only ~ 700-800 million years after the Big Bang. Studies of galaxies at only
slightly later epochs (z~5.5-6.5) suggest that the Universe must have started actively forming
galaxies well before z~7 ([3][4]), and indeed we have found candidate galaxies out to z~10 ([5-13]).
Notably, there is also strong evidence that gamma-ray bursts (GRBs), which are believed to be
intimately connected to very massive stars, have been identified out to z~8.2 ([14,15]), an epoch
that extends well into the cosmic reionization. It is expected that more and better candidate objects
will be discovered to z~10 within the next decade; however, any detailed studies of such objects, or
even just the unambiguous confirmation of their high-redshift nature will have to wait for the next
generation of large facilities such as the TMT. The unprecedented light gathering power of TMT
will enable us to push to the very moment when the first stars in the Universe began to form, and to
study in detail the history of galaxy and supermassive black hole formation. With adaptive optics
(AO), TMT will have higher spatial resolution than ALMA and JWST, and will also be able to
carry out very high resolution spectroscopic studies which JWST will not be capable of. All this
will place TMT to a unique position in the synergy of future facilities in the high-redshift frontier.
Main science areas:
1. Direct detection of Population III stars
One of the most exciting subjects in the study of the early Universe is the detection of the very first
generation of luminous objects formed out of the primordial gas, or the metal-free ``Population III"
(Pop-III) stars. In the popular Cold Dark Matter (CDM) scenario, Pop-III stars could begin to form
at z>50 within dark matter halos that are as massive as 100,000 times of the Sun, and could have
formed in large number by z~10-15 (e.g., [16]) such that the characteristic He II 1640A line ([17])
could be strong enough for detection. IRMS, which is one of the first-light instruments at TMT, will
be sufficiently sensitive for such experiments.
Direct observations of Pop-III stars are not only of fundamental importance in understanding early
star/galaxy formation but could also be critical in constraining the nature of dark matter. For
instance, an attractive alternative to the CDM model is the so-called ``Warm Dark Matter" (WDM)
model in which dark matter has a larger intrinsic velocity than that in the CDM. In this model, the
very first stars would form in a huge burst along thin and straight filaments that could stretch over
some 24 million light years in length (about half the size of the Milky Way galaxy) ([18]). In
contrast, the CDM model predicts that first stars are distributed in quasi-spherical clumps of
individual dark matter halos. The high luminosity generated by such a huge star burst in the WDM
model would significantly boost the probability of the direct detection of the earliest firework of
85
star formation, and the distinct filamentary geometry, which will be discernable by the TMT with
AO, could be used to distinguish the two cosmological models.
2. Detailed spectroscopic studies of galaxies and quasars at z  6
By design, TMT will be uniquely suited for detailed spectroscopic studies of early galaxies and
quasars. The most obvious science to pursue at TMT is to do spectroscopic confirmation of welldefined galaxy candidate samples at z  6. Spectroscopic identification is the only unambiguous
way to confirm the high-z nature of such candidates and to obtain their precise redshifts. Such
samples at z ~ 6 are already in place, and similar samples at z ~ 7--10 are being gathered by the ongoing near-IR surveys from the ground (e.g., UKIDSS, VISTA) and from the space (HST/WFC3 IR
surveys). After JWST is launched, more and better samples will soon be available. The mere
identification of very high-z objects will already have great impact to a number of important
questions, e.g., whether low-luminosity star-forming galaxies could be the major sources of
reionization ([19]), when the precise epoch of reionization is, etc.
High-resolution spectroscopic observations of galaxies at very high-redshifts, at least for the
brightest ones, will be feasible at the TMT with a reasonable amount of time. It will be possible to
study their chemical abundances, and to put constraints to the IMF of early galaxies. Furthermore,
the IFU capability will enable us to study the kinematic of such galaxies (e.g., through [O II] 3727A
emission line), and hence provide invaluable constraints to galaxy formation models.
It is worth keeping in mind that TMT will eventually have PFI, a high-contrast AO imager and
spectrograph. While it is designed for extra-solar planet detection, it can also be used to detect
quasar host galaxies. The study of quasar hosts at very high-redshifts will reveal the growth of
supermassive black holes and its impact to the star-formation processes (a.k.a. "AGN feedback").
3. Spectroscopic studies of IGM through quasars and GRB afterglows
The studies of SDSS quasars at z  6 have revealed the presence of the Gunn-Peterson absorption
due to cosmic neutral hydrogen, and have shown that the reionization likely ended by z ~ 6
([20][1]). With TMT, similar studies can be expanded to many more sightlines and extended to
higher redshifts, using fainter and higher-redshift samples that will be available from the wide-field
near-IR surveys (e.g., UKIDSS, VISTA). Such studies will enable us to obtain the topology of the
IGM, and to probe the inhomogeneity of the reionization. Furthermore, high-dispersion
spectroscopy will provide us the chemical abundances of the IGM along the sightline, and hence
enable us to study the metal enrichment history of the Universe.
GRB afterglows provide a powerful alternative to quasars in the study of the IGM (e.g., [21]). As
long-duration GRBs are believed to be associated with the death of massive stars ([22]), they could
exist before the formation epoch of the first quasars and hence could probe earlier epochs than
quasars do. The design of TMT is capable of rapid response to Target-of-Opportunity requests, and
it is very likely that our understanding of the early IGM in the reionization epoch will come from
the observations of a large sample of GRB afterglows.
Possible TMT programs:
1.
2.
IRMS spectroscopic survey of well-defined samples of candidate galaxies and quasars at z~615, and establish their relation to the cosmic hydrogen reionization. It is possible to directly
detect Pop-III stars during such effort.
Kinematic studies of galaxies at z>5 using IRIS IFU.
86
3.
4.
5.
High-contrast AO observations of quasar host galaxies at z>7 using PFI to study the interplay
of early star-formation process and supermassive black hole growth.
High-resolution spectroscopic observations of quasars and GRB optical afterglows (through
target-of-opportunity requests) at z>6 to study the IGM topology and chemical abundance, and
the inhomogeneity in reionization.
Imaging and spectroscopy of the host galaxies of GRB at very high redshifts.
China’s strengths and weakness in this area:
Quite a number of groups in China (NAOC, KIAA/PKU, NJU, SHAO, PMO, USTC) already have
established record in the studies of the following subjects from theoretical aspect: Population-III
stars, reionization, early structure formation, AGN physics, and GRB mechanism. From the
observational side, there are a number of on-going or future projects that will complement the TMT
science at this forefront and we will seek synergy: the 21CMA experiment will provide valuable
experience in detecting the reionization signature, the search for extremely metal-poor stars at the
LAMOST will possibly reveal the relics of Pop-III stars, the time-domain study at the South Pole
Dome-A will likely result in a large number of GRB afterglows, and the millimeter/sub-millimeter
facilities at Dome-A will enable us to study the dust contents of very early galaxies.
Currently, China is still lacking the expertise of observational study of the high-redshift universe.
To take full advantage of the opportunity that joining the TMT will bring us in this frontier, we
should start fostering such expertise now.
References
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
Fan, X., et al. 2006, AJ, 132, 117
Iye, M., et al. 2006, Nature, 443, 186
Yan, H., et al. 2005, ApJ, 634, 109
Eyle, L., et al. 2005, MNRAS, 364, 443
Stark, D. P., et al. 2007, ApJ, 663, 10
Bradley, L. D., et al. 2008, ApJ, 678, 647
Bouwens, R. J., et al. 2008, ApJ, 686, 230
Zheng, W., et al. 2009, ApJ, 697, 1907
Oesch, P. A., et al. 2009, arXiv:0909.1806
Bouwens, R. J., et al. 2009, arXiv:0909.1803
McLure, R. J., et al. 2009, arXiv:0909.2437
Yan, H., et al. 2009, arXiv:0910.0077
Capak, P., et al. 2009, arXiv:0910.0444
Salvaterra, R., et al. 2009, arXiv:0906.1578
Tanvir, N. R., et al. 2009, arXiv:0906.1577
Gao, L., et al. 2009, arXiv:0909.1593
Schaerer, D. 2002, A&A, 382, 28
Gao, L & Theuns T., 2007, Sci, 317, 1527
Yan, H. & Windhorst, R. 2004, ApJ, 612, L93
Becker, R., et al. 2001, AJ, 122, 2850
Vreeswijk, P. M, et al. 2004, A&A, 419, 927
Woosley, S. E., & Bloom, J. S. 2006, ARA&A, 44, 507
87
Appendix A: Acronyms and abbreviations
ExAO
FOV
HROS
IRIS
IRMOS
IRMS
IFUs
JWST
LSST
MCAO
MIRAO
MOAO
NFIRAOS
NIRES
Pan-STARRS
PFI
PSF
WFOS
WIRC
Extreme Adaptive optics
Field Of View
High Resolution Optical Spectrometer
InfraRed Imaging Spectrograph
Infra-Red Multi-Object Spectrograph
Infrared Multislit Spectrometer
Integral Field Units
James Webb Space Telescope
Large Synoptic Survey Telescope
Multi-Conjugate Adaptive Optics
Mid-Infrared Adaptive Optics
Multi-Object Adaptive Optics
Narrow Field InfraRed Adaptive Optics System
Near Infrared Echelle Spectrograph
Panoramic Survey Telescope & Rapid Response System
Planet Formation Instrument
Point Spread Function
Wide Field Optical Spectrometer
Wide Field InfraRed Camera
88
Appendix B: Dwarf galaxies observable by TMT
Table 1. Local Group Member Galaxies observable by TMT
Name
RA
(2000)
00 01
58
00:20.4
00:26.1
00:33.2
DEC
(2000)
-15:27.8
Type
Subgroup
IrrIV-V
LGC
+59:18
-11:02
+48:31
M31
And II
NGC
185
NGC
205
And VIII
And I
And IX
00:35.4
00:39.0
+36:31
+84:20
00:41.3
+41:41
00:42.3
00:45.7
00:52.9
+40:37
+38:00
+43:12
dIrr
dSph/E4
dSph/dE
5
dSph
dSph/dE
3p
E5p/dSp
h-N
dSph pe
dSph
dIrr/dSp
h
LGS 3
IC 1613
01:03.8
01:05.1
+21:53
+02:08
IrrV
And X
And V
And II
UGCA
92
Leo A
Sex B
Leo I
Sex A
Sex dw
01:06.6
01:10.3
01:16.4
04:32.0
+44:48
+47:38
+33:27
+63:36
09:59.4
10:00.0
10:08.5
10:11.1
10:13.2
+30:45
+05:20
+12:18
-04:43
-01:37
dSph
dSph
dSph
Irr ? S0
?
dIrr
dIrr
dSph
dIrr
dSph
Leo II
11:13.5
+22:10
GR 8
UMi dw
12:58.7
15:08.8
Dra dw
WLM
IC10
Cet dw
NGC147
V
(mag)
10.9v
Dist
(kpc)
978
825
M31
10.3v
14.4
9.5v
M31
M31
13.5p
9.2v
760
616
NOT
M31
8.5v
824
NOT
M31
M31
9.1v
13.2
16.2
805
M31/LGC
15.4p
9.2v
620
721
M31
16.1
15.9
13.5
13.8
Inst for
CMD
WFPC2
Diameter
(arcmin)
12 x 14
7.3 x 6.4
5.0 x 4.3
15.0 x 9.4
725
4.5 x 3.0
14.5 x
12.5
19.5 x
12.5
45 x 10
2.5 x 2.5
5
WFPC2
WFPC2
2
20.0 x
18.5
7
2.0 x 1.5
3.6 x 2.5
2.0 x 1.0
5.1 x 3.1
5.1 x 3.5
9.8 x 7.4
5.9 x 4.9
30.0 x
12.0
12.0 x
11.0
1.2 x 1.1
41.0 x
26.0
51.0 x
31.0
2.5 x 2.0
5.0 x 2.7
525
MW/N3109
N3109
MW
N3109
MW
12.9
11.8
9.8
11.9
12.
800
1370
254
1320
86
ACS
ESO/2.2
WFPC2
WFPC2
dSph
MW
12.6
233
WFPC2
+14:13
+67:12
dSph
dSph
GR8
MW
14.5p
10.9v
2200
66
WFPC2
17:20.1
+57:55
dSph
MW
9.9v
76
INT/WFC
And VII
Peg dw
23:27.8
23:28.6
+50:35
+14:45
dSph
dIrr/dSp
h
LGC
12.9
13.2
919
WFPC2
And VI
23:51.7
+24:36
11.2
3.5 x 3.5
Table 2. Further possible members in the Local Group, mostly found by SDSS.
Name
UGCA 86
Willman 1
UMa I
UMa II
Vir Stream
CVn I
Boo dw
Leo T
Com dw
CVn II
RA
(2000)
03:59.9
10:49.4
10:34.9
08:51.5
12:24
13:28.1
14:00.0
09:34.9
12:27.0
12:57.2
DEC
(2000)
+67:08
+51:03
+51:55
+63:08
-01.1
+33:33
+14:30
+17:03
+23:54
+34:19
Type
Irr ? S0 ?
dSph? GC ?
dSph
dSph
dSph
dSph
dSph
dSph
dSph
dSph
V
(mag)
13.5
15.3
14.3
13.9
13.6
16.0
14.5
15.1
89
Dist
(kly)
6200
147
330
815
30
730
200
1370
143
490
Diameter
(arcmin)
0.8 x 0.7
1.75
30 x 10
8.4 x 5.0
13.0 x 8.7
1.4 x 1.4
5.0 x 2.5
3.0 x 2.1
Her dw
Leo IV
And XI
And XII
And XIII
And XIV
And XVI
And XV
And XVII
16:31.0
11:32.9
00:46.3
00:47.4
00:51.8
00:51.6
+12:48
-00:32
+33:48
+34:22
+33:00
+29:41
dSph
dSph
dSph
dSph
dSph
dSph
14.7
15.9
460
520
2900:
2900:
2900:
2900:
8.0 x 6.0
3.3 x 2.5
0.5
0.55
0.5
00:37.1
+44:19
dSph
16.0
2590
1.1 x 0.9
90
×

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