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2007 Spring Symposium Featured Talks

Stellar Relaxation Processes Near the Galactic Massive Black Hole

Dr. Tal Alexander (Weizmann Institute of Science)

The massive black hole and the stars around it are a unique laboratory for studying how relaxation processes lead to close interactions of stars and compact remnants with the central massive black hole. I will describe new results on the processes of loss-cone refilling by massive perturbers, resonant relaxation and mass segregation; describe observational evidence that these processes play a role in the Galactic Center; and discuss some of the cosmic implications.

Build-Up of Supermassive Black Holes

Dr. Kate Brand (Space Telescope Science Institute)

How did the mass of 10^9-10^10 solar mass super-massive black holes at the center of massive galaxies in the local Universe build up? Did the bulk of the growth happen in an optically luminous AGN phase? Or did a substantial fraction of SMBH growth occur in a dusty, obscured phase, visible as a luminous infrared galaxy? Has there been substantial SMBH growth in a radiatively inefficient regime after the more luminous AGN phase? These are particularly important questions given the tight relationship between the mass of galaxy bulges and their SMBHs, suggesting that the formation and evolution of galaxies and SMBHs are intimately linked. I will use the multi-wavelength data in the NDWFS Bootes field to address this issue. First, I will present an X-ray stacking analysis of ~20,000 red galaxies between z~0-1 to show that the nuclear accretion rates in these sources are either low or radiatively inefficient and are declining with time. I will then discuss the nature of an extreme, obscured population of AGN-dominated ULIRGs which are likely to be sources undergoing a period of rapid and substantial growth.

Gravitational Waves from Black Hole Mergers

Dr. Joan Centrella (NASA Goddard Space Flight Center)

The final merger of two black holes releases a tremendous amount of energy and is one of the brightest sources in the gravitational wave sky. Observing these sources with gravitational wave detectors requires that we know the radiation waveforms they emit. Since these mergers take place in regions of very strong gravitational fields, we need to solve Einstein's equations of general relativity on a computer in order to calculate these waveforms. For more than 30 years, scientists have tried to compute these waveforms using the methods of numerical relativity. The resulting computer codes have been plagued by instabilities, causing them to crash well before the black holes in the binary could complete even a single orbit. Recently this situation has changed dramatically, with a series of amazing breakthroughs. This talk will take you on this quest for the holy grail of numerical relativity, showing how a spacetime is constructed on a computer to build a simulation laboratory for binary black hole mergers. We will focus on the recent advances that are revealing these waveforms, and the dramatic new potential for discoveries that arises when these sources will be observed by LIGO and LISA.

Where to look for radiatively inefficient accretion flows in low luminosity AGN (and where not too look for them)

Dr. Marco Chiaberge (Space Telescope Science Institute)

We have studied the nuclear emission detected in HST data of carefully selected samples of low luminosity AGN(LLAGN)in the local universe. We find faint unresolved nuclei in a significant fraction of the objects. The nuclear emission is as low as 10-8 times the Eddington luminosity, indicating extremely low radiative efficiency for the accretion process and/or an extremely low accretion rate. When the Eddington ratio is plotted against the nuclear "radio-loudness" parameter, sources divide according to their physical properties. It is thus possible to disentangle between nuclear jets and accretion disks of different radiative efficiencies. This new diagnostic plane allows us to find objects that are the best candidates for hosting (and showing)radiative inefficient accretion and determine in which ones we cannot see it. I will show that the (extremely limited)information available in the HST archive to derive the nuclear SEDs strongly supports our results.

Birth of Supermassive Black Hole Binaries

Prof. Monica Colpi (Department of Physics, University of Milano Bicocca)

We present a study on the dynamics of massive black holes (BHs) in gas-rich galaxy mergers, obtained from a series of high-resolution N-Body/SPH simulations. We show that the presence of a gaseous component is essential for the rapid formation of an eccentric (Keplerian) BH binary. The binary resides at the center of a massive (10^9 M_sun) turbulent nuclear disc resulting from the earlier collision of the two gaseous discs present in the parent galaxies. Using physically and/or numerically motivated recipes, we follow the accretion history of the BHs during the merger. We find that (i) the mass of the BHs increases along the course of the interaction as central inflows of gas establish inside each galaxy at every close passage; (ii) the mass ratio q_BH varies with time indicating that the memory of its initial value may be lost. We then trace the BH binary orbit down to a scale of 0.1 pc modeling the nuclear disc, resulting from the galaxy collision, as an equilibrium Mestel disc composed either of gas, gas and stars, or just stars. Under the action of dynamical friction against the rotating gaseous and/or stellar background the orbit circularizes. When this occurs, each BH is endowed with it own small-size (<0.01 pc) accretion disc comprising a few percent of the BH mass. Double AGN activity is expected to occur on an estimated timescale of 10 Myrs, comparable to the inspiral time. The two nuclear point--like sources that may appear have typical separations of 10 pc, and are likely to be embedded in the still ongoing starburst.

Ultraluminous X-Ray Sources

Dr. Giuseppina Fabbiano (Smithsonian Astrophysical Observatory)

Ultra-luminous X-ray Sources continue to be a source of interest, as extraordinary X-ray emitters in galaxies, and potential candidates for Intermediate Mass Black Holes. While a definitive mass measurement for even one of these sources does not yet exist (and may well require HST observations), progress is continuously been made in understanding their emission characteristics. This talk will give an update on the spectral and X-ray variability properties of ULXs, that reinforce the connection between ULXs and BH X-ray binaries, in terms of the behavior of an accretion disk in very high accretion rate states.

Spins of Supermassive Black Holes

Prof. Andrew Fabian (University of Cambridge)

High quality X-ray spectra of many AGN and Galactic BH show broad iron lines. The red wing of the iron line is broadened by gravitational redshift, the extent of which is determined by the innermost circular stable orbit (ISCO) of the surrounding accretion disc. In several objects the broadening is extreme, indicating that the ISCO is at about two gravitational radii, meaning that the BH must be rapidly spinning. The details, prospects and limitations of such measurements of BH spin will be discussed.

The Black Hole at the Galactic Center

Prof. Reinhard Genzel (Max Planck Institute for Extraterrestrial Physics)

In the past decade high resolution measurements in the infrared employing adaptive optics imaging on 10m telescopes have allowed determining the three dimensional orbits stars within ten light hours of the compact radio source SgrA* at the Center of the Milky Way. These observations show that SgrA* is a three million solar mass black hole, beyond any reasonable doubt. The Galactic Center thus constitutes the best astrophysical evidence for the existence of black holes which have long been postulated, and is also an ideal 'lab' for studying the physics in the vicinity of such an object. Remarkably, young massive stars are present there and probably have formed in the innermost stellar cusp. Variable infrared and X-ray emission from SgrA* are a ew probe of the physics and space time just outside the event horizon.

Tidal Disruptions of Stars by Supermassive Black Holes

Dr. Suvi Gezari (California Institute of Technology)

A supermassive black hole lurking in the nucleus of a normal galaxy will be revealed when a star approaches close enough to be torn apart by tidal forces, and a flare of radiation is emitted as the stream of stellar debris plunges onto the black hole. The luminosity, temperature, and decay of a tidal disruption flare are dependent on the mass and spin of the central black hole, and can be used to directly probe dormant black holes in distant galaxies for which the sphere of influence of the black hole is unresolved, and a dynamical measurement of the black hole mass is not possible. I will present the discovery of tidal disruption flares in the Ultraviolet with the GALEX Deep Imaging Survey, and compare the properties of the flares to theoretical predictions.

Black Holes from Stellar Evolution

Prof. Alexander Heger (Los Alamos National Laboratory)

Massive stars are the progenitors of most, if not all, black holes in the universe. In this talk I will discuss what is the initial mass function for black holes and neutron stars as a function of the mass of the progenitor stars. This relation also changes as a function of other stellar properties like metallicity and rotation. One of the key uncertainties in determining the black hole initial mass function is our still poor knowledge of the supernova explosion mechanism, that is, even if we know the structure of the star at the time it undergoes core collapse, it is still hard to reliably and accurately predict the explosion energy of the supernova or whether the star will explode in the first place, and what remnant will result. To determine whether the remnant is a black hole or a neutron star further requires to know the neutron star equation of state to derive the maximum mass for neutron stars as bound for the least massive black holes. I will present results based on recent calculations involving more than hundred stellar models and more than thousand supernova models.

Black Holes, Entropy and Information

Prof. Gary Horowitz (University of California, Santa Barbara)

Although the basic classical properties of black holes have been understood since the 1970's, their quantum properties raise some of the deepest questions in theoretical physics. Some of these questions have recently been answered using string theory. I will review these fundamental questions, and the aspects of string theory needed to answer them. I will then explain the recent developments and new insights into black holes that they provide. Some puzzles remain, and I will discuss the prospects for further progress.

High Redshift Black Holes

Dr. Anton Koekemoer (Space Telescope Science Institute)

Searches for black holes at or beyond redshifts 6-7 have recently expanded significantly with the advent of deep, wide multiband surveys with Chandra, HST, Spitzer and large ground-based telescopes (such as GOODS, COSMOS and similar projects). I will describe the current status on searches for these objects, such as the optically undetected X-ray bright `EXO's and similar high redshift candidate black holes. Together with deep spectroscopy programs, the optical flux limits are combined with IR detections and X-ray fluxes to understand the spectral energy distributions of the sources and help constrain the properties of their central black holes. The results are used to examine the evolution of black holes up to high redshift, with corresponding implications for the co-evolution of galaxies and their central black holes.

Making Black Holes Visible: Accretion, Radiation, and Jets

Prof. Julian Krolik (Johns Hopkins University)

The fundamental problem in deriving energy from accretion onto black holes is the nature of angular momentum transport. Strong arguments link this process to MHD turbulence driven by the magneto-rotational instability. Using large-scale numerical simulations in full general relativity, it is now possible to trace MHD turbulence and the resulting accretion dynamics in considerable detail. Contrary to the central guess of the Novikov-Thorne model, magnetic stresses persist throughout the flow, and are particularly strong when the black hole rotates rapidly. Enhanced radiative efficiency and high-temperature emission at small radius are likely consequences. Large-scale magnetic field can be spontaneously generated in a cone around the rotation axis, creating a relativistic Poynting-dominated jet whose strength increases sharply with increasing black hole spin. The jet's kinetic luminosity can be comparable to the radiative luminosity of the accretion disk itself.

Black Holes at Future Colliders

Prof. Greg Landsberg (Brown University)

As was suggested a few years ago, production of mini-black holes at the LHC and other future accelerators may be the very first signature of low-scale quantum gravity. Large cross section, spectacular signatures, as well as small backgrounds would make black-hole samples an excellent laboratory to study Hawking radiation, general relativity, and quantum gravity. I will review various aspects of black hole phenomenology in models with large and warped extra dimensions and describe their detection and possible measurements.

Black Hole Masses from Gas Dynamics

Dr. Duccio Macchetto (Space Telescope Science Institute)

Since the advent of HST, the progress in studying and understanding black holes has been impressive. Early questions regarding the very existence of black holes have been replaced by questions regarding the role that they play in the formation and evolution of galaxies, particularly at early epochs in the universe. However, the apparently well established relationship between the mass of the black hole and the mass or luminosity of the galactic bulge rests on a relatively small number of direct observations, and while very few doubt that this relationship exists, it is essential to actually measure a the properties of a number of black hole over a range of masses and host galaxies. The direct methods adopted to measure MBH in the nearby universe use gas or stellar kinematics to gather information on the gravitational potential in the nuclear region of the galaxy. The stellar kinematical method has the advantage that stars are present in all galactic nuclei and their motion is always gravitational. The drawback is that it requires relatively long observation times in order to obtain high quality observations and that stellar dynamical models are very complex, potentially plagued by indeterminacy Conversely, the gas kinematical method is relatively simple, it requires relatively short observation times for the brightest emission line nuclei, even if not all galaxy nuclei present detectable emission lines. However an important drawback is that non-circular or non-gravitational motions can completely invalidate this method. Since the observed correlations are based on MBH masses obtained with different methods, it is important to check whether these methods provide consistent and robust results.Over the last several years we have carried out HST observations for a variety of ellipticals, Seyfert and spiral galaxies. In particular we have undertaken a major STIS survey of 54 late type spiral galaxies to study the scaling relations between black holes and their host spheroids at the low mass end. Our measurements of BH masses in late type spiral galaxies has shown that these measurements are very challenging and at the limit of the highest spatial resolution currently available. Nonetheless our estimates generally support the scaling relations between black holes and their host spheroids suggesting that (i) they are reliable and (ii) black holes in spiral galaxies follow the same scaling relations as those in more massive early-type galaxies. A crucial test for the gas kinematical method, the correct recovery of the known BH mass in NGC 4258, has been successful. I will review the key advantages and disadvantages of the gas kinematical method and will discuss the key results of these surveys.

Estimating the Spin of Stellar-Mass Black Holes

Dr. Jeffrey McClintock (Harvard-Smithsonian Center for Astrophysics)

Starting with Cygnus X-1, we now have mass estimates for a good sample of 21 stellar black holes. The next obvious step is to measure spin. Using the straightforward methodology of fitting the X-ray continuum spectrum of the accretion disk, our team has estimated the spins of three black holes. Most exciting among these is the microquasar GRS 1915+105, which has a spin that is between 98% and 100% of the theoretical maximum value. We plan to estimate the spins of a total of a dozen black holes during the next few years. We will describe work in progress, including new dynamical results for M33 X-7, the only known eclipsing black hole system. We will also discuss studies that we are undertaking in order to test our methodology and to assess sources of systematic error. Finally, we will comment on the importance of determining spin.

The Formation of Black Holes in Globular Clusters

Prof. Steve McMillan (Drexel University)

Dynamical evolution in star clusters naturally creates an environment in which interactions among massive stars, binaries, and compact remnants are common. Young clusters may temporarily contain a significant population of stellar black holes, and close encounters and physical collisions among stars in dense cluster cores may lead to the formation of very massive stars and high-mass black holes via runaway merging. Numerical simulations suggest runaway masses in the range commonly cited for intermediate-mass black holes. While our understanding of black hole formation and retention has improved greatly in recent years, substantial uncertainties remain in both the physics of the runaway merger process and the evolution of very massive stars. Direct and indirect observational evidence has been reported for massive black holes in globular clusters, although here too interpretations remain controversial. I will examine critically some details of the processes possibly leading to massive black holes in present-day globular clusters, and will discuss some observational constraints on the various theoretical scenarios.

Dynamics around Black Holes

David Merritt (Rochester Institute of Technology)

The structure of galactic nuclei reflects the presence of supermassive black holes in many ways. Single SMBHs act as sinks, destroying a mass in stars equal to their own mass in roughly one relaxation time and forcing nuclei to expand. Binary SMBHs displace roughly their combined mass in stars, creating low-density cores and hyper-velocity stars. Ejection of coalesced binary SMBHs via radiation recoil injects further energy into nuclei and creates the possibility of substantially displaced SMBHs. I will review these and related processes and discuss the implications for nuclear structure.

Microquasars

Dr. Felix Mirabel (European Southern Observatory)

I will review the the main conclusions from the VI international workshop on microquasars with particular emphasis on the accretion-ejection connection, and correlations among black holes of all mass scales.

Black Hole Event Horizon

Prof. Ramesh Narayan (Harvard University)

During the last decade, a number of groups have developed observational tests for the presence of event horizons in astrophysical black holes. Most of these studies have focused on stellar mass black hole candidates in X-ray binaries, though one test has been done also on the supermassive black hole candidate at the Galactic Center. The cumulative evidence from all the tests is now very strong: astrophysical black hole candidates do possess event horizons. The talk will summarize the current status of the field.

Black Hole Masses from Reverberation Mapping

Prof. Bradley Peterson (The Ohio State University)

Emission-line reverberation mapping explores the dynamics of gas at distances of order 1000 gravitational radii from the supermassive black holes in Type 1 (broad emission-line) active galactic nuclei. We will review the basics of the technique and demonstrate that reverberation data can be used to measure the mass of the central black hole to an accuracy of about a factor of three. We will discuss correlations between black hole masses and other observables. We will discuss the uncertainties and limitations of current results and how these can be mitigated.

Black Holes and Strong-Field General Relativity

Prof. Dimitrios Psaltis (University of Arizona)

Black-hole astrophysics has matured to the point that, in the very near future, various observatories will routinely detect strong-field gravitational effects. In this talk, I assess in a quantitave way the prospect of testing strong-field general relativity using observations of black holes. I show that the external spacetimes of black holes predicted by a wide variety of alternative gravity theories, including scalar-tensor, higher-order, and large-extra-dimension theories, are practically indistinguishable. This argues that it is virtually impossible to use observations of black holes to test alternative gravity theories, unless matter outside the black-hole horizon alters significantly the outcome of gravitational experiments. On the other hand, the robustness of the predicted external spacetimes allow us to test the validity of some of the most basic assumptions of gravitational physics.

Supermassive Black Hole Demographics

Dr. Douglas Richstone (University of Michigan)

We'll review the host-galaxy predictors of BH mass and estimates of the mass density and mass function of BHs at zero redshift. The high end of the mass function can be compared to the AGN population. The low end of the mass function is critical to predicting the LISA event rate. We believe that the bimodality in central structures in bulges and elliptical galaxies is real.

Black Hole Formation and Growth: Simulations in General Relativity

Prof. Stuart Shapiro (University of Illinois at Urbana-Champaign)

Black holes are found everywhere in our universe: in compact binary X-ray sources and GRBs, in quasars, AGNs and the cores of all bulge galaxies, in binary black holes and binary black hole-neutron stars, and maybe even in the LHC! Black holes are strong-field objects governed by Einstein's equations of general relativity. Accordingly, general relativistic, numerical simulations of gravitational collapse to black holes, binary black hole merger and recoil, black hole accretion, and other black hole phenomena may help reveal how, when and where black holes form, grow and can be detected. We summarize two such simulations: the collapse of hypermassive neutron stars to black holes following binary neutron star merger, and the formation and growth of supermassive black hole seeds in the early universe. A computer-generated movie highlighting some of the simulations will be shown.

Black Holes in Deep Surveys

Prof. C. Megan Urry (Yale University)

In recent years deep X-ray and infrared surveys have provided an efficient way to find accreting supermassive black holes in the young universe, otherwise known as active galactic nuclei (AGN). Such surveys can, unlike optical surveys, find AGN obscured by high column densities of gas and dust. In those cases, deep optical data show only the host galaxy, which can then be studied in greater detail than in unobscured AGN. Some years ago the hard spectrum of the X-ray "background" suggested that most AGN were obscured. Now GOODS, MUSYC, COSMOS and other surveys have confirmed this picture and given important quantitative constraints on AGN demographics. Specifically, we show that most AGN are obscured at all redshifts and the amount of obscuration depends on both luminosity and redshift, at least out to redshift z~2, an epoch of substantial black holes and galaxy growth. Deeper infrared surveys will be needed to reach higher redshifts and to probe fully the co-evolution of galaxies and black holes.

Black Hole Masses in Distant AGN

Dr. Marianne Vestergaard (University of Arizona)

Mass scaling relationships based on broad emission line widths and continuum luminosity measurements are commonly used to estimate the mass of the central black hole in distant active galaxies and quasars. I will outline and discuss this technique which is anchored in reverberation mapping data of nearby active galaxies. I will demonstrate the applicability of the mass scaling relationships to distant active galaxies and quasars and will discuss the accuracies and limitations as presently known and our efforts to improve these. I will present results of applying these relationships to large samples of distant active galaxies and quasars, including the Sloan Digital Sky Survey. One goal is to constrain how active black holes grow at different epochs by combining theoretical models with black hole mass functions.

Supermassive Black Hole Growth

Dr. Marta Volonteri (University of Michigan)

I'll discuss how massive black hole "seeds" may form in proto-galaxies, within a hierarchical cosmological framework. The growth from "seeds" to supermassive black holes, via accretion, mergers and dynamical interactions, as well as their implications, will be critically addressed.