Draft (Jan. 18, 2010) Executive Summary




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4.3The formation and growth of black holes


Qinjuan Yu (KIAA), Weimin Yuan (YNAO), Youjun Lu (NAOC), Zhiqiang Shen (SHAO), Jian-min 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).

4.3.1Massive 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) 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 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. 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 MBHrelationship 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 relationship is not necessarily the same. Some observations have indicated that the normalization of the MBHrelationship 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 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 MBHrelationship 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 reverberation-mapping 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 high-mass 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:

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. TMT Science Advisory Committee, Thirty Meter Telescope Detailed Science Case: 2007

  2. Tremaine, S. et al. 2002, ApJ, 574, 740

  3. Gebhardt, K., Rich, R. M., Ho, L. C. 2005, ApJ, 634, 1093

  4. Barth, A. J., Greene, J. E., Ho, L. C. 2005, ApJ, 619, L151

  5. Lauer, T. et al. 2007, ApJ, 662, 808

  6. Treu, T., Koopmans, L. V. E., Bolton, A. S., Burles, S., Moustakas, L. A. 2006, ApJ, 650, 1219

  7. Macchetto, F. et al. 1997, ApJ, 489, 579



4.3.2Black 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 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:


  1. TMT Science Advisory Committee, Thirty Meter Telescope Detailed Science Case: 2007

  2. Ghez, A. M. et al. 2008, ApJ, 689, 1044

  3. Ghez, A. M. et al. 2005, ApJ, 620, 744

  4. Eckart, A. et al. 2008, A&A, 492, 337

  5. Yusef-Zadeh, F. et al. 2009, ApJ, 706, 348

  6. Gillessen, S. et al. 2009, ApJ, 692, 1075

  7. Yuan, F. et al. 2009, ApJ, 703, 1034

  8. Genzel et al. 2003, Nature, 425, 934

  9. Do et al. 2009, ApJ, 691, 1021

4.3.3Black 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 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.

  1. 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. 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 10m-class 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, high-resolution spectroscopy of high-redshift quasar will shed light on the metallicity in these systems at early epoch of the universe.


  1. 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.



  1. 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.
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. High-resolution 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. Magorrian, J. et al. 1998, AJ, 115, 2285

  2. Tremaine, S. et al. 2002, ApJ, 574, 740

  3. Dunlop, S., et al. 2003, MNRAS, 340, 1095

  4. Guyon, O., et al., 2006, ApJS, 166, 89G

  5. Willott, C., et al. 2003, ApJ, 587L, 15

  6. Barth, A., et al., 2003 ApJ, 594L, 95

  7. Kauffmann, G., et al., 2003 MNRAS, 341, 54

  8. Davies, R. et al. 2007, ApJ, 671, 1388

  9. Wada, T. & Norman, W. 2002, ApJ, 566L, 21

  10. Chen, Y.-M. et al. 2009, ApJ, 695, L130

  11. Storchi-Bergmann et al., 2007 ApJ, 670, 959

  12. Greene, J. and Ho., L.C., 2007 ApJ, 670, 92

  13. Dong, X., et al., 2010, in prep.

  14. Harms, R. J. et al. 1994, ApJ, 435, L35

  15. Ferrarese , L., et al., 1996 ApJ...470..444

  16. Marconi et al. A., 2001, 2001 ApJ, 549, 915



4.3.4Physics 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 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.


  1. 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.

  1. 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 mid-infrared range, TMT (with WIRC and MIRES) may be capable of starting to (marginally) resolve 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 gas-dynamics 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 8-10m 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. Magorrian, J. et al. 1998, AJ, 115, 2285

  2. Krolik, J.H., 1997 ASPC, 113, 459

  3. Laor, A., et al., 2006ApJ, 636, 83

  4. Winge, C., et al., 1999, ApJ, 519,134

  5. Soifer, B. T., et al. 2003, AJ, 126, 143

  6. Horst, H., 2009, A&A, 495, 137

  7. Tristram K.R.W., et al., 2009, A&A, 502, 67



4.3.5Stellar mass and intermediate mass black holes


Hua Feng (Tsinghua), Shuang-Nan Zhang (IHEP), Jifeng Liu (CfA)
Key questions:


  1. What is the mass distribution of stellar mass black holes?

  2. Do intermediate mass black holes exist?

  3. 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 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 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 [5-7]. 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 8-10m 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.



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. Bird, A.J., et al. 2009, ApJS to appear (arXiv:0910.1704)

  2. Colbert, E. J. M., & Mushotzky, R. F. 1999, ApJ, 519, 89

  3. Ebisuzaki, T., et al. 2001, ApJ, 562, L19

  4. Portegies Zwart, S.F., et al. 2004, Nature, 428, 724

  5. McClintock, J.E. & Remillard, R.A., 2006, Compact Stellar X-ray Sources, 157-213

  6. Orosz, J.A., et al. 2007, Nature, 449, 872

  7. Silverman, J.M. & Filippenko, A.V. 2008, ApJ Letters, 678, L17

  8. Liu, J., 2009, ApJ, 704, 1628 

  9. Gallo, E., et al. 2007, ApJ, 670, 660



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