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Stars and Singularities
The physics of dense cusps
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| Speaker: Pau Amaro-Seoane (AEI) | Title: The distribution and capture of stellar-mass black holes around SgrA* and other galactic nuclei | Session: Thu 10/12/09 15:45-16:15 |
| Abstract: An analysis of the population of SBHs at the centre of a galaxy, in a near-Keplerian system (in which the gravitational potential is dominated by an SMBH) is crucial for understanding how SBHs distribute around SMBH and to predict the rate at which they are swallowed by the MBH through ``direct plunges'' (closer periapsis passages) or extreme-mass ratio inspirals (EMRIs). We present an approximate but self-consistent model for the population of stellar-mass black holes at the centre of a galaxy, in the vicinity of an SMBH. By combining this model with the LISA selection function, we can construct a function that determines the number of EMRI events we expect as a function of a small number of parameters characterising the black hole population. Also, rates for “direct plunges” are orders of magnitude larger than slow, adiabatic EMRIs. We show that a direct plunge will actually flash during many orbits before being swallowed by the central MBH. As a matter of fact, this flashing source can become a real, adiabatic EMRI in certain circumstances. Taking into account the event rates, addressing of this problem has significant bearing on LISA's applicability to this science. Accurate predictions of the event rate are important for preparing for LISA data analysis and design. | ||
| Speaker: Hendrik Bartko (MPE) | Title: Young stellar disks in the Galactic Center | Session: Wed 9/12/09 11:15-11:45 |
| Abstract: We present new observations of the nuclear star cluster in the central parsec of the Galaxy with the adaptive optics assisted, integral field spectrograph SINFONI on the ESO/VLT. Our work allows the spectroscopic detection of early and late type stars to mK≥16, more than 2 magnitudes deeper than our previous data sets. Our observations result in a total sample of 177 bona fide early-type stars. We find that most of these Wolf Rayet (WR), O- and B-stars reside in two strongly warped disks between 0.8″ and 12″ from SgrA*, as well as a central compact concentration (the S-star cluster) centered on SgrA*. The main clockwise system either is either a strongly warped single disk with a thickness of about 10°, or consists of a series of streamers with significant radial variation in their orbital planes. The mean eccentricity of the clockwise system is 0.36±0.06. The later type B stars (mK>15) in the radial interval between 0.8″ and 12″ seem to be in a more isotropic distribution outside the disks. The observed dearth of late type stars in the central few arcseconds is puzzling, even when allowing for stellar collisions. The stellar mass function of the disk stars is extremely top heavy with a best fit power law of dN/dm∼m-0.45±0.3. Since at least the WR/O-stars were formed in situ in a single star formation event ∼6 Myrs ago, this mass function probably reflects the initial mass function (IMF). The mass functions of the S-stars inside 0.8″ and of the early-type stars at distances beyond 12″ are compatible with a standard Salpeter/Kroupa IMF (best fit power law of dN/dm∼m-2.15±0.3). | ||
| Speaker: Holger Baumgardt (U. Bonn) | Title: Constraining the initial mass function of stars in the Galactic center | Session: Sun 13/12/09 9:00-9:30 |
| Abstract: We combine the results of stellar evolution models and recent observations on the total luminosity and mass of stars in the galactic centre to determine the most likely form of the IMF of stars in the Galactic Centre. We find that the observations of the Galactic Centre are well consistent with continuous star formation following a canonical IMF and that the observed luminosity in the central parsec is too high to be explained by a long-standing top-heavy IMF. We conclude that if the young stellar discs prove to follow a top-heavy IMF, the circumstances that led to their formation must be very rare, since these have not affected most of the central cluster. | ||
| Speaker: Laura Blecha (CfA / Harvard) | Title: Wandering black holes in merging galaxies | Session: Tue 8/12/09 16:00-16:30 |
| Abstract: It is well established that galaxies form hierarchically through mergers. In gas-rich mergers, large amounts of cold gas can be funneled into the central region of the merging galaxy, triggering a starburst. Strong evidence also exists that supermassive black holes (SMBHs) are ubiquitous in galaxies, and that SMBH feedback plays an important role in galaxy formation and evolution. However, gravitational-wave (GW) recoil events that result from SMBH mergers may alter this standard paradigm. GW recoil can set an SMBH in motion during a critical phase of the host galaxy merger, when the SMBH may be actively accreting and a dense stellar cusp may be rapidly forming in the galaxy center. We present the results of SPH/N-body simulations of galaxy mergers in which a GW recoil kick is given to the SMBH at the time of the black hole merger. Our simulations include a wide range of kick velocities, as well as several different galaxy models. We find that a single GW recoil event can have a substantial effect on the growth and feedback of the recoiling SMBH, albeit for a fairly narrow range of kick velocities. These events may also affect properties of the merged host galaxy, particularly in the central region. We discuss the prospects for observing signatures of GW recoil. | ||
| Speaker: Michal Bregman (Weizmann) | Title: Resonant torquing of accretion disks by stars | Session: Fri 11/12/09 8:45-9:15 |
| Abstract: Accretion disks around massive black holse (MBH) in active galaxies can exhibits warps, as observed in the maser disk of NGC4258. The physics driving the warp are still debated. We propose a new warping mechanism: resonant torquing of the disk by stars in the dense cusp around the MBH. We show that resonant torquing can induce such a warp and can affect the accretion rate of matter flowing into the MBH. The resonant torques, by acting on the disk, can also modify the MBH spin, and therefore its evolution. | ||
| Speaker: Rainer Buchholz (U. Köln) | Title: Structure of the late type component in the Galactic Center | Session: Sun 13/12/09 9:30-10:00 |
| Abstract: Investigating the structure and composition of the late type component of the nuclear stellar cluster of our galaxy can provide constraints for modelling several key processes in the vicinity of a supermassive black hole. These include cusp formation and long term stellar dynamics in general as well as star formation scenarios. To that end, we have developed a new method of separating early and late type stellar components of a dense stellar cluster based on narrow band filters and have applied it to the central parsec of the GC, using the results to conduct a population analysis of this area. For the first time, this new method allows for a very deep and efficient source classification, down to 15.5 mag in the K band. This represents a major improvement in the depth of the observations and reduces the needed observation time. In addition to exploring the late type component, we also find a large number of new early type candidates, with a total of 312 stars now classified as such out of a sample of 5914 sources. It appears that the late type stars indeed do not form a cusp, but in fact the late type density function shows an inversion in the inner 6″. We find that the distribution of the early type stars can be fitted with a steep single power law, alternatively with a broken power law, since we find a drop in the early type density at approx. 10″. We also detect early type candidates outside of 0.5 pc in significant numbers for the first time. The late type KLF closely resembles the KLF obtained for the bulge of the Milky Way, while the early type KLF has a much flatter power law slope. | ||
| Speaker: Monica Colpi (U. Milan) | Title: On the inspiral of binary black holes in gas-rich nuclear discs: spin evolution and gravitational recoil | Session: Thu 10/12/09 16:15-17:00 |
| Abstract: We describe the dynamics of dual black holes in gas-rich galaxy mergers, tracing their inspiral from the largest scale (∼100 kpc) down to the scale where they form a close binary (∼0.1 pc). We underscore the vital role played by gas dissipation in boosting and shaping their orbital decay. As soon as the black holes move inside the nuclear gaseous disc that forms in the aftermath of the merger, their initially eccentric orbit becomes circular. Surrounded by dense cocoons of low-speed gas that provide mass for the fueling, the black holes change their spin vectors due to their gravito-magnetic coupling with the small-scale accretion discs that grow oriented along the disc plane. Viscous torques then suffice to align the black hole spins with the angular momentum of their orbit well before they enter the phase of inspiral driven by gravitational wave emission. This has important astrophysical consequences, as the relic black hole, born in the fossil disc, receives a gravitational wave-induced kick with median much smaller than the median resulting from a random distribution of spins. Thus, coalescences in gas-rich fossil discs is a major pathway to black hole retention in galaxy mergers. | ||
| Speaker: Jorge Cuadra (MPA) | Title: Formation and Dynamics of Stellar Discs in the Galactic Centre | Session: Sun 13/12/09 10:30-11:00 |
| Abstract: Young massive stars orbit at sub-parsec distance from Sgr A*, the central black hole of our Galaxy. These stars are puzzling, since the huge tidal field of Sgr A* would prevent “normal” star formation. Instead, we argue that this is the first evidence for star formation inside a massive AGN disc, and that such a disc existed in the Galactic centre in the past. We show numerical simulations of the fragmentation of a massive gas disc into stars and the subsequent growth of the stars through accretion of the surrounding gas. We then evolve the dynamics of the stellar system and compare it to the observed stellar population in the Galactic centre. | ||
| Speaker: Katie Dodds-Eden (MPE) | Title: Flares from Sgr A* | Session: Wed 9/12/09 14:45-15:15 |
| Abstract: I will discuss the emission mechanism behind the NIR and X-ray flares observed from Sgr A*. A very important constraint comes from the multiwavelength observation of the simultaneous NIR and X-ray flare (the brightest yet observed) that was caught as part of a large multiwavelength effort on April 4, 2007. The spectral characteristics of this flare make it difficult to explain the X-ray emission through traditionally invoked inverse Compton mechanisms. I will present the case for X-ray synchrotron emission from electrons accelerated over a roughly half hour timescale in a magnetic field of about 30 Gauss. This model can quite easily explain the observed spectral characteristics as well as curious features of the simultaneous lightcurves under a very natural evolution of physical parameters in the flaring region. | ||
| Speaker: Steve Drasco (AEI) | Title: Parameter estimation errors for radiating binaries with non-trival mass ratios | Session: Mon 14/12/09 10:45-11:15 |
| Abstract: I describe estimates of the errors that should be expected when using gravitational-wave detectors to measure parameters that characterize binaries with non-trivial mass ratios. The estimates are based on Fisher-matrix calculations with waveform models expected to be more realistic than those used in current and previous mock data challenges. Understanding these errors will quantify limitations of the various approximations used for these binaries. This is important because full numerical relativity simulations will not be available for these systems, due to the large range in characteristic length and time scales, and because there is little understanding of which approximations should be used in the construction of their template waveforms. | ||
| Speaker: Frank Eisenhauer (MPE) | Title: Probing the Galactic center with GRAVITY | Session: Wed 9/12/09 10:45-11:15 |
| Abstract: GRAVITY is the second generation VLT Interferometer instrument for precision narrow-angle astrometry and interferometric phase referenced imaging of faint objects. With its targeted astrometric precision of 10 microarcsecond, GRAVITY will open a new domain for probing physical processes close to the supermassive black hole in the Galactic Center. In this talk we will give an overview of the tree main goals for GRAVITY: measuring the motion of gas at few event horizon radii from the Galactic Center black hole, imaging of stellar orbits within few light days, and measuring accelerations of stars in the stellar discs. These key measurements will allow us to address a number of question in fundamental physics including the validity of the no-hair theorem, to measure the fundamental properties of the black hole including its spin, to characterize the gaseous environment close to the last stable orbit, to constrain the stellar relaxation processes very close to the black hole, and to get a hand on the origin of the young stellar population in the central light month and in its surrounding stellar disc. | ||
| Speaker: Jonathan Gair (Cambridge) | Title: Science applications of gravitational wave observations of high mass ratio inspiral events | Session: Thu 10/12/09 9:45-10:30 |
| Abstract: LISA should detect between a few and a few thousand extreme-mass-ratio inspirals (EMRIs) of compact objects into black holes in the mass range from ∼ten thousand to ~ten million solar masses. Such inspirals are expected to be both eccentric and inclined to the equatorial plane of the central black hole, and the emitted gravitational waves have a correspondingly rich structure. This complexity encodes information that can be used for scientific inference. EMRI observations will have applications to astrophysics, through precise measurements of the system parameters including black hole masses and spins; to cosmology, through measurement of the Hubble constant; and to fundamental physics, through precise measurements of the spacetime structure in the vicinity of the central black hole. We will discuss how LISA EMRI observations can inform us about each of these areas. | ||
| Speaker:Reinhard Genzel (MPE) | Title: An overview of recent observational results | Session: Wed 9/10/09 9:00-9:45 |
| Abstract: ... | ||
| Speaker:Andrea Ghez (UCLA) | Title: Bringing our Galaxy's Supermassive Black Hole and its Environs into Focus with Laser Guide Star Adaptive Optics | Session: Thu 10/12/09 11:15-12:15 |
| Abstract: The proximity of our Galaxy's center presents a unique opportunity to study a galactic nucleus with orders of magnitude higher spatial resolution than can be brought to bear on any other galaxy. After more than a decade of astrometry from diffraction-limited speckle imaging on large ground- based telescopes, the case for a supermassive black hole at the Galactic center has gone from a possibility to a certainty, thanks to measurements of individual stellar orbits. The advent of adaptive optics technology has significantly expanded the scientific reach of our high- spatial-resolution infrared studies of the Galactic center. In this talk, I will present the results of several new adaptive optics studies on (1) our current understanding of the galaxy's central gravitational potential, (2) the puzzling problem of how young stars form in the immediate vicinity of the central black hole, (3) the surprising, apparent absence of the predicted central stellar cusp around the central supermassive black hole (an essential input into models for the growth of nuclear black holes), and (4) how future large ground-based telescope may allow these studies to test general relativity and cosmological models. | ||
| Speaker: Stefan Gillessen (MPE) | Title: The power of monitoring stellar orbits | Session: Wed 9/12/09 9:45-10:15 |
| Abstract: Adaptive Optics based imaging and spectroscopy in the near-infrared has shown that the Galactic Center hosts a dark compact object of about 4 million solar masses. The motions of individual stars as observed today are consistent with a simple Newtonian potential. Continuous monitoring of the stars should reveal corrections to that. Special relativistic effects and gravitational redshift modulate the observed radial velocities. The lowest order effect of the Schwarzschild nature of the gravitational potential is a prograde periastron shift, detectable astrometrically. Furthermore, a cluster of dark remnants in the vicinity of the massive black hole would yield retrograde periastron shifts. In this talk I will review the observational status, pointing out current limitations, and compare the accuracy achievable with current instruments to the expected sizes of the various effects. | ||
| Speaker: Alessia Gualandris (MPA) | Title: Young Stars in the Galactic Center | Session: Sun 13/12/09 15:30-16:00 |
| Abstract: Near-infrared imaging and spectroscopy of the Galactic center indicate the presence of over 100 young massive stars in the inner parsec of the Milky Way. These belong to two distinct populations: (1) Outside the central arcsecond, a group of roughly 40 stars, mostly O and Wolf-Rayet stars with an estimated age of 6Myr, move on approximately circular orbits in a thin and coherent stellar disk. (2) Inside the central arcsecond, a group of 20 B-type stars (the S-stars) move on eccentric and randomly oriented orbits around the supermassive black hole (SMBH). The existence of young massive stars so close to the SMBH is puzzling since tidal stresses from the SMBH are likely to suppress star formation in this region. If the S-stars formed at larger distances from SgrA* and were then transported inward, the migration would need to occur on a timescale of only a few Myr, given the apparent age of the stars. This constitutes the so-called paradox of youth. I will discuss different models for the origin of the young stars in the Galactic center and present recent results obtained by means of state-of-art numerical simulations. I will also discuss the short and long term effects of an intermediate-mass black hole on the orbits of the stars bound to the SMBH. | ||
| Speaker: Bence Kocsis (CfA / Harvard) | Title: The end state of resonant relaxation in the Galactic Center | Session: Sun 13/12/09 14:30-15:00 |
| Abstract: The distribution of massive young stars in the Galactic Center is anisotropic. The observations can be interpreted as showing one or two warped disks of radii around 0.1 - 0.5 pc. In this regime, resonant relaxation can reorient the orbits of stars on Myr timescales, shorter than the age of the stars. Using analytical models and numerical simulations, we examine the thermodynamic equilibrium distribution of stars interacting through resonant relaxation. We find that the equilibrium states can be warped disks with features that resemble the observed disks. | ||
| Speaker: Gábor Kupi (Weizmann) | Title: Numerical studies of resonant relaxation in galactic nuclei | Session: Sun 13/12/09 14:00-14:30 |
| Abstract: ... | ||
| Speaker: Jean-Pierre Lasota (IAP & Jagellonian U.) | Title: Seeing black-hole spin through radio-loudness | Session: Mon 14/12/09 10:45-11:15 |
| Abstract: I will discuss the implications of the radio-loudness dichotomy for black-hole spin evolution. | ||
| Speaker: Mario Livio (STScI) | Title: Jets from black hole systems | Session: Wed 9/12/09 15:45-16:30 |
| Abstract: I will review various mechanisms for the formation of astrophysical jets in general, and jets from accreting black holes in particular. | ||
| Speaker: Avi Loeb (CfA / Harvard) | Title: Sub-millimeter imaging of SgrA* and M87 | Session: Mon 14/12/09 9:00-9:45 |
| Abstract: The technology is currently available to obtain images of the black hole silhouette in SgrA* and M87 through sub-mm VLBI. I will review the latest theoretical and observational advances on this frontier. | ||
| Speaker: Avi Loeb (CfA / Harvard) | Title: Informal talk: An EM signature of galactic BH binaries that enter their GW induced inspiral | Session: Sun 13/12/09 Lunch 12:15-14:00 |
| Abstract: ergers of gas-rich galaxies lead to black hole binaries that coalesce as a result of dynamical friction on the ambient gas. Once the binary tightens to <103 Schwarzschild radii, its merger is driven by the emission of gravitational waves (GWs). I will show that this transition occurs generically at orbital periods of ∼1-10 years and an orbital velocity V of a few thousand km/s, with a very weak dependence on the supply rate of gas (V∝Mdot1/8). Therefore, as binaries enter their GW-dominated inspiral, they inevitably induce large periodic shifts in the broad emission lines of any associated quasar(s). The probability of finding a binary in tighter configurations scales as V-8 owing to their much shorter lifetimes. Systematic monitoring of the broad emission lines of quasars on timescales of months to decades can set a lower limit on the expected rate of GW sources for LISA. | ||
| Speaker: Ann-Marie Madigan (Leiden) | Title: Resonant relaxation and the angular momentum distribution of stars around massive black holes | Session: Tue 8/12/09 11:00-11:30 |
| Abstract: The energy evolution of stars around massive black holes, arising from the two-body diffusion process, is a well-studied problem. In contrast, the angular momentum diffusion of such systems, dominated by resonant relaxation (RR), is poorly understood. In this talk I will present a new strategy to investigate the evolution of stellar angular momenta around black holes. First I will present results of extensive N-body simulations, performed with the intention of quantifying the statistical properties of RR. I will then introduce an “autoregressive moving average” (ARMA) model which reproduces the essential properties, and hence gives a good description, of RR. Finally I will use this model, calibrated using our N-body simulations, to explore the angular momentum evolution of a stellar cusp around a massive black hole and apply the results to the evolution of the S-stars orbits in the Galactic Centre. | ||
| Speaker: Yohai Meiron (Technion) | Title: Kinematical signature of binary MBHs | Session: Tue 8/12/09 11:30-12:00 |
| Abstract: The stalling radii of massive binary black holes (BBHs) are expected to be below 0.1 arcsec even in nearby galaxies (Yu 2002), and thus BBHs will not be directly detectable in the near future. However, a BBH may be detectable through its influence on the stellar kinematics on scales 5-10 times larger than the binary separation. We explored the phase space of stable orbits, by solving the restricted three body problem of a BBH embedded in a bulge potential. We present high resolution maps of moments of the line of sight velocity distributions, based on 108 points in phase space. We find that the kinematic signature of a BBH, in the average-velocity maps, is a counter rotating torus outside the BBH Hill spheres. The velocity dispersion maps reveals a dip in the inner region. Similar effects are seen in the third and fourth Gauss-Hermite velocity moments maps. If these signatures are short lived, their detection may indicate the presence of a BBH currently, or in the recent past. | ||
| Speaker: Kristen Menou (Columbia) | Title: The electromagnetic signature of white dwarf inspiral into a MBH | Session: Thu 10/12/09 14:45-15:15 |
| Abstract: Observing an astrophysical phenomenon simultaneously in the electromagnetic and the gravitational skies can provide great insight into the physics of the event. I describe a promising scenario for such dual detections, whereby a massive black hole tidally strips mass from a white dwarf on an eccentric orbit, leading to repeated mass transfer episodes. | ||
| Speaker: David Merritt (RIT) | Title: Dynamical models of the Galactic center | Session: Tue 8/12/09 9:15-10:00 |
| Abstract: The center of the Milky Way galaxy is sometimes considered a prototype for relaxed galactic nuclei. But recent observations suggest that the distribution of stars at the Galactic center is very different than predicted by the collisionally relaxed models. I will summarize the observational constraints on the mass distribution and dynamics of the GC, then discuss possible explanations for its apparently unrelaxed state. | ||
| Speaker: David Merritt (RIT) | Title: Informal talk: Testing properties of the Galactic center black hole using stellar orbits | Session: Wed 9/12/09 Lunch 12:15-14:00 |
| Abstract: ... | ||
| Speaker: Cole Miller (U. Maryland) | Title: The astrophysics of Extreme Mass Ratio Inspirals | Session: Thu 10/12/09 9:00-9:45 |
| Abstract: Extreme mass ratio inspirals (EMRIs), in which a stellar-mass object spirals into a supermassive black hole, are considered to be a critical source for space-based gravitational wave detectors such as LISA because they will act as precise tests of the Kerr spacetime. There are currently several proposed pathways to EMRIs, including gradual inspiral via two-body or resonant relaxation, tidal separation of binaries, and capture or production of stellar-mass objects by a massive accretion disk. In each case, other effects such as mass segregation, rotation, and triaxiality may alter rates significantly. I will give an overview of these processes and their implications, and will explore what is needed to reduce uncertainties in rate estimates. | ||
| Speaker: Mark Morris (UCLA) | Title: On the tilt of gaseous and stellar disks around the Galactic black hole | Session: Fri 11/12/09 8:15-8:45 |
| Abstract: The well-defined, massive disk of young stars orbiting within 0.5 pc of the Galactic black hole (GBH) is strongly tilted with respect to the Galactic plane. In the context of in situ formation mechanisms, this orientation is inherited from that of the gas disk that formed these stars on the order of 6 million years ago. The tilt can be understood by positing that clouds fall toward the GBH on near-zero angular momentum orbits and form dispersion rings as they self-intersect after passing through periapse in their orbits around the GBH. If the angular momentum distribution of the approaching clouds is isotropic, then the resulting gaseous disks will be oriented at random. However, in a galaxy having m=2 symmetry, radial orbits are very unlikely for dense clouds massive enough to account for the central stellar cluster of young stars, since such clouds are constrained by their history to orbit in the Galactic gas layer, largely following streamlines in the Central Molecular Zone (CMZ) as they migrate toward the GBH under the action of dynamical friction. Encounters with other massive clouds that might be capable of placing a cloud on a near-radial orbit are more likely to lead to cloud mergers. We suggest – and illustrate with a toy model – that near-radial orbits, and thus strongly tilted disks, occur naturally if the Galaxy has a significant m=1 asymmetry, that is, if the massive constituents of the Galactic center undergo oscillatory motions about the Galactic center of mass. Indeed, the CMZ is clearly asymmetric in both position and velocity, consistent with the operation of an m=1 oscillatory mode. Not only can this give rise to strongly tilted disks, but it also enhances the time-averaged accretion rate onto the GBH. This mechanism can also have important consequences for the fueling of AGNs. | ||
| Speaker: Ryan O'Leary (CfA / Harvard) | Title: Searching for star clusters around recoiling black holes in the Milky Way halo | Session: Tue 8/12/09 10:00-10:30 |
| Abstract: Gravitational wave emission by coalescing black holes (BHs) kicks the remnant BH with a typical velocity of hundreds of km/s. This velocity is sufficiently large to remove the remnant BH from a low-mass galaxy but is below the escape velocity from the Milky Way (MW) galaxy. If central BHs were common in the galactic building blocks that merged to make the MW, then hundreds of BHs that were kicked out of low-mass galaxies should be freely floating in the MW halo today. I will describe the long-term N-body and Fokker-Planck simulations I have done to determine the present distribution of stars around these BHs. Then, I will discuss my ongoing search for candidate clusters in SDSS, as well as the prospects for detecting these objects around M31 and M33. | ||
| Speaker: Miguel Preto (ARI / U. Heidelberg) | Title: Stellar distributions around massive black hole: Implications for lower-frequency gravitational wave astrophysics | Session: Thu 10/12/09 15:45-16:15 |
| Abstract: I present clear N-body realizations of the strong mass segregation solution for the stellar distribution around a massive black hole. These N-body results are compared with those obtained by solving the orbit-averaged Fokker-Planck equation in energy space. The N-body segregation is slightly stronger than in the FP solution, but both confirm the robustness of the regime of strong segregation when the number fraction of heavy stars is a (realistically) small fraction of the total population. In view of recent observations revealing a dearth of giant stars in the sub-parsec region of the Milky Way, it is shown that the time scales asssociated with cusp re-growth are not longer than a Hubble time unless a very large core is postulated as initial condition. This conclusion is even stronger in case of MBHs with mass lower than that of SgrA*. Since EMRI detection rates by LISA are expected to peak for black hole masses between 4×105 and 106 solar masses, a good fraction of these events should originate from strongly segregated cusps. | ||
| Speaker: Luciano Rezzola (AEI, Potsdam) | Title: Astrophysics of binary black hole mergers: properties of the final black hole and EM counterparts | Session: Thu 10/12/09 14:00-14:45 |
| Abstract: Several attempts have been recently made to model the spin of the black hole resulting from the merger of a generic binary system of black holes via simple expressions exploiting the results of numerical-relativity simulations. While these expressions are in good agreement with the simulations, they are intrinsically imprecise when predicting the final spin direction, especially if applied to binaries with separations of hundred or thousands of gravitational radii. This is due to neglecting the precession of the orbital plane of the binary, and is a clear drawback if the formulas are employed in cosmological merger-trees or N-body simulations, which provide the spins and angular momentum of the two black holes when their separation is of thousands of gravitational radii. I will discuss an expression which is built on improved assumptions and that gives, for any separation, a very accurate prediction both for the norm of the final spin and for its direction. By comparing with the numerical data, we also show that the final spin direction is very accurately aligned with the total angular momentum of the binary at large separation. Hence, observations of the final spin direction (e.g. via a jet) can provide information on the orbital plane of the binary at large separations and could be relevant, for instance, to study X-shaped radio sources. A part of my talk will also be spent to discuss recent work aimed at defining the EM counterparts of the inspiral and merger of a binary system of black holes, either in terms of EM in vacuum or in terms of the shocks induced in a disk by a recoiling black hole. | ||
| Speaker: Elena Maria Rossi (HUJ) | Title: Hyper velocity stars from the Galactic Center | Session: Tue 8/12/09 14:30-15:00 |
| Abstract: The so-called hyper-velocity stars are stars observed travelling away from our Galactic Centre with a radial velocity of 400-700 km/s. They were first predicted as a manifestation of the presence of a supermassive black hole whose tidal forces would be sufficient to disrupt stellar binary systems and eject with “hype-velocity” one of the members. We adopt this scenario and we investigate it with a novel (but in fact ancient) semi-analytical method. I am going to show that important general results on the probability of disruption and on the energy distribution can be derived with this method. I will also show the predicted velocity distributions for HVS. | ||
| Speaker: Jeremy Schnittman (JHU) | Title: The three-body problem with radiation reaction: Observational implications for SMBH binaries | Session: Mon 14/12/09 9:45-10:15 |
| Abstract: The circular restricted three-body problem is one of the oldest and most-studied problem in celestial mechanics. Here we consider the case in which the two larger bodies are super-massive or intermediate-mass black holes (BHs) in a planar circular orbit, and the third object is a stellar-mass BH or main-sequence star. When including gravitational radiation losses, we find interesting changes in the location and stability of the Lagrange equilibrium points. In particular, we find that stellar objects captured into the stable L4 and L5 points may be shepherded in through the binary inspiral, eventually either merging with the secondary BH, getting tidally disrupted, or ejected entirely from the system. Each of these possibilities will lead to a unique observational signature that may be detectable in the near future. | ||
| Speaker: Alberto Sesana (AEI, Potsdam) | Title: Enhanced Tidal-Disruption Rates of Bound Stars from Massive Black Hole Binaries | Session: Tue 8/12/09 15:30-16:00 |
| Abstract: The observation of stellar tidal disruptions at galaxy centres is an important tool to probe massive black holes (MBHs) in quiescent galaxies. A handful of these events has been detected in the last years, and the inferred rates are consistent with theoretical predictions of disruption by a single MBH induced by two body driven relaxation (10-5–10-4) disruption per year per galactic nucleus). I will show how the formation of an unequal massive black hole binary (MBHB) in a dense power law stellar density profile (cusp) may increase the tidal disruption rate by order of magnitudes for a limited period of time. When compared to standard disruption rates caused by single MBHs, these results imply that, if on average every MBH formed an unequal binary at least once in its lifetime, the star disrupted during the binary shrinking in the bound cusp may account for up to 10% of the total tidal disruption rate in the Universe. Forthcoming all sky surveys like LSST and Pan STARRS are expected to catch hundreds of tidal disruption candidates, opening the appealing prospect of identifying MBHBs in galaxies hosting such tidal disruption flares. | ||
| Speaker: Nick Stone (CfA / Harvard) | Title: Stellar tidal disruption flares from bound recoiling SMBHs | Session: Sun 13/12/09 16:00-16:30 |
| Abstract: We analyze stellar tidal disruption events as a possible observational signature of gravitational wave induced recoil of supermassive black holes. Specifically, we examine the most common case - kicks below galactic escape velocity - and find up to an order of magnitude net enhancement to the tidal disruption rate, relative to galaxies with stationary supermassive black holes. This is balanced, however, against the relatively short recoil duty cycle. These events are largely due to interactions with unbound stars, though tidal disruption flares from stars gravitationally bound to the black hole offer different prospects for detection. We also discuss observability, and calculate that LSST is likely to find tidal disruption flares caused by recoiling supermassive black holes. | ||
| Speaker: Eugene Vasiliev (LPI, Moscow) | Title: Orbital structure of triaxial nuclear star clusters | Session: Tue 8/12/09 14:00-14:30 |
| Abstract: We consider the orbital structure of triaxial nuclear star clusters within sphere of influence of supermassive black hole. There exist four major types of regular orbits, most interesting being the pyramid orbits, which we describe in detail. Triaxial component of potential leads to non-conservation of angular momentum, and pyramid orbits constantly repopulate the low angular momentum region in phase space, where they are captured by black hole. Estimates show that the capture rate for pyramid orbits may well exceed that of two-body relaxation, and may contribute substantially to the black hole mass. | ||
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