Princeton Gravity Initiative | PCTS Workshop, “Exploring Supermassive Black Holes”

Wed, Oct 14, 2020, 11:00 am to Fri, Oct 16, 2020, 2:00 pm
Princeton Gravity Initiative | PCTS

October 14, 11:00 am-2:30 pm EDT (GMT-4) time | Day 1: SMBH-galaxy co-evolution (observations)

  • Marta Volonteri 11:00-11:30am
  • Xin Liu 11:30-12:00pm

Break 15 min

  • Chung Pei-Ma  12:15-12:45pm 
  •  Brian Metzger 12:45-1:15pm 

Break 15 min

  • Karina Voggel  1:30-2:00pm
  • Michael Tremmel 2:00-2:30pm

October 15,  11:00 am-2:30 pm EDT (GMT-4) time | Day 2: SMBH-galaxy co-evolution (theory)

  • Smadar Naoz 11:00-11:30am 
  • Fabio Antonini 11:30-12:00pm 

Break 15 min

  • Panel discussion with Suvi Gezari, Jenny Greene, Marta Volonteri and Brian Metzger  12:15-1:15 pm 

Break 15 min

  • Diego Munoz 1:30-2:00pm 
  • Stefan Gillessen  2:00-2:30pm

October 16, 11:00 am-2:00 pm EDT (GMT-4) time |  Day 3:  TDEs and SMBH growth

  • Julie Comerford 11:00-11:30am 
  • Yuri Levin  11:30-12:00pm 

Break 15 min

  • Fani Dosopoulou  12:15-12:45pm 
  • Suvi Gezari 12:45-1:15pm

Break 15 min

  • Manuela Campanelli  1:30-2:00pm

Talks Titles and Abstract

Marta Volonteri
Massive black hole dynamics in evolving galaxies
Massive black holes in their cosmic evolution interact with diverse environments, starting from messy and rapidly evolving galaxies at high redshift to quiescent galaxies today. I’ll discuss how these changing environments affect the orbital decay of massive black holes following galaxy mergers as well as tidal disruption events. 


Xin Liu
The Hunt for Binary Supermassive Black Holes
When galaxies frequently merge in the hierarchical universe, their central black holes (BHs) pair and form binary supermassive BHs. The merger of binary BHs provides a means of growth; the abundance on sub-galactic scales may be used to constrain the nature of dark matter particles; the final coalescence should make the loudest sirens in the low-frequency gravitational wave sky. Despite significant merits, however, direct observational evidence for binary supermassive BHs has remained elusive. The primary hurdle to detecting binary supermassive BHs on sub-galactic scales is spatial resolution. I will talk about our ongoing efforts with Gaia and the Dark Energy Survey using temporal information to break the angular resolution limit, analogous to detecting stellar binaries and exoplanets.


Chung-Pei Ma
Measuring Supermassive Black Hole Masses via Stellar Dynamics
Over 30 years of efforts by multiple research groups have discovered and measured masses of about 100 supermassive black holes (SMBHs)in local galaxies via direct dynamical modeling. The size of the M87 photon ring imaged by the Event Horizon Telescope in 2019 provided a critical independent method for inferring SMBH masses.  I will discuss some of the issues and recent developments in stellar dynamical measurements of SMBH masses.


Brian Metzger 
Colliding EMRIs as TDE Imposters and Sources of Periodic AGN Activity
When a main sequence star undergoes Roche lobe overflow onto a supermassive black hole (SMBH) in a circular extreme mass ratio inspiral (EMRI), a phase of steady mass transfer ensues. Over millions of years, the binary evolves to a period minimum before reversing course and migrating outwards. Because the time interval between consecutive EMRIs is comparable to the mass-transfer timescale, the semi-major axes of two consecutive mass-transferring EMRIs will cross on a radial scale < few AU. We show that such EMRI crossing events are inevitably accompanied by a series of mildly relativistic, grazing physical collisions between the stars. Each collision strips a small quantity of mass, primarily from the more massive star, which
generally increases their radial separation to set up the next collision after a delay of decades to centuries (or longer) set by further gravitational radiation. Depending on the mass of the SMBH,
this interaction can result in N ~ 1-1e4 gas production events of mass Msun/N, thus powering a quasi-periodic sequence of SMBH accretion-powered flares over a total duration of thousands of years or longer. Although the EMRI rate is 2-3 orders of magnitude lower than the rate of tidal disruption events (TDE), the ability of a single interacting EMRI pair to produce a large number of luminous flares - and to make more judicious use of the available stellar fuel - could make their observed rate competitive with the TDE rate, enabling them to masquerade as "TDE Imposters" or otherwise appear as periodic AGN.  We predict flares with luminosities that decay both as
power laws shallower than t^(-5/3) or as decaying exponentials. Viscous spreading of the gas disks produced by the accumulation of previous mass-stripping events places substantial mass on radial scales > 10-100 AU, providing a reprocessing source required to explain the unexpectedly high optical luminosities of some flares.


Karina Voggel
A review on Black Holes in surviving Nuclear Star Clusters of Stripped Galaxies
In my talk I will review what we currently know about how many massive black holes exist in the surviving nuclear star clusters of former galaxies. Many Nuclear Star Clusters (NSCs) end up in the halos of massive galaxies after they were stripped of their surrounding stars by tidal forces. These stripped NSCs are a fossil record of a galaxy's accretion history, and are crucial for obtaining a census of massive black holes. The existence of these surviving Nuclei with SMBHs are a direct consequence of hierarchical galaxy formation, but until now their impact on the SMBH density had not been quantified. In the last 5 years we have discovered several such SMBHs in massive former stripped nuclei.

These massive black holes have been hidden from typical black hole searches for accretion signatures, as the old NSCs that contain them have no gas and thus they are quiet and have few to no accretion activity. Only through direct dynamical detection of the BHs influence on its nearby stars, could we unveil their presence. A first estimate of their relative importance has shown that SMBHs in surviving nuclear star clusters can increase the number of SMBHS in the local Universe up to 30%.


Michael Tremmel
Dynamic Duos: Supermassive Black Hole Pairs in Merging Galaxies
The kpc-scale dynamical evolution of SMBHs in merging galaxies is a critical aspect of the formation of SMBH binaries that is often overlooked, or greatly simplified, in predictions for SMBH mergers. I will present results from the Romulus25 simulation predicting that the evolution of SMBH pairs depends on the properties of their merging host galaxies and that SMBHs commonly spend several Gyrs at separations of several kpc before forming a binary and merging. I discuss the importance of this galaxy-scale dynamical evolution on SMBH merger rate predictions, as well as how they may be affected by dark matter physics. Finally, I’ll examine our simulations’ predicted population of “wandering” SMBHs that will orbit around their host galaxy and never form binaries, including results from very high resolution simulations of dwarf galaxies.


Smadar Naoz 
Title: Hidden Friends of the Galactic Center Black Hole, Sgr A* 

Almost every galaxy, including our Milky Way, has a supermassive black hole at its heart, with masses of millions to billions of times the mass of the Sun. The hierarchical nature of galaxy formation suggests that a supermassive black hole binary should be common, and may exist even in our own galactic center. We propose a new approach to constraining the possible orbital configuration of such a binary companion to the galactic center black hole Sagittarius A* (Sgr A*) through the measurement of stellar orbits. Focusing on the star S0-2, we show that requiring its orbital stability in the presence of a companion to Sgr A* yields stringent constraints on such a companion's possible configurations. Using existing data on S0-2 we derive upper limits on the binary black hole separation as a function of the companion mass. If such a companion exists, it will emit gravitational wave radiation, potentially detectable with the Laser Interferometer Space Antenna (LISA).  If time allows, I will mention some other possible detectable, tinnier, hidden friends of Sgr A*. 


Fabio Antonini
A stellar-dynamical solution to the final-parsec problem
A binary supermassive black hole loses energy via ejection of stars in a galactic nucleus, until emission of gravitational waves becomes strong enough to induce rapid coalescence. Evolution requires that stars be continuously supplied to the binary, and it is known that in spherical galaxies the reservoir of such stars is quickly depleted, leading to the stalling of the binary at parsec-scale separations. I will review the dynamical evolution of supermassive black hole binaries and present  the results of N-body and Monte Carlo simulations of galaxy mergers showing that coalescence times are less than ~1Gyr. These results constitute a fully stellar-dynamical solution to the ''final-parsec problem'' and imply a high rate of detectable events for LISA. I will then consider the evolution of the binary orbit orientation and eccentricity. During the early stages of binary formation, the binaries often flip their orbital plane, enabling them to achieve large eccentricities, which has significant implications for the merger timescale and waveform models. The origin of the orbital plane flip lies in the triaxiality of the merger remnant and it is affected by the rotation of the host system.


Diego Munoz
Hydrodynamic Simulations of Circumbinary Disks
Circumbinary gas disks are expected to assemble around the supermassive binary black holes that form in galactic nuclei after galactic mergers. These disks are thought to play a fundamental role in the process of binary migration and, in particular, in overcoming the "last parsec problem." Despite the long-recognized importance of circumbinary disks, the gravitational and accretional coupling between the gas and the binary is still not fully understood. Theory and numerical simulations indicate that circumbinary disks can modify the orbital elements of the binary; however, whether the gas reduces or increases the angular momentum of the binary is not a settled issue. In this talk, I will discuss how non-linear circumbinary hydrodynamics still hold many surprises for the long-term evolution of super-massive black hole  binaries. In particular, I will provide an overview on the state-of-the art of long-term hydrodynamical simulations of circumbinary accretion, highlighting recent developments on viscous steady-state simulations and their impact on the secular evolution of binaries. I will describe a newly emerging consensus on the migration rate of accreting binaries. I will explain how binaries can gain angular momentum from the gas and thus increase in separation, and how this surprising outcome was previously overlooked. I will discuss the implications that these new findings have for low-frequency gravitational waves sources, and for the gravitational wave background.


Fani Dosopoulou
The Merger History of Elliptical Galaxies: Mass Deficit
The structure and kinematics of galactic nuclei are fossil relics of the merger histories of galaxies and of the interaction between stars and SMBH binaries. Stars ejection during the SMBH binary evolution should reduce a galaxy’s central density and expand its core. This has been suggested as a plausible explanation for why large elliptical galaxies have lower central densities and weaker density cusps than small ellipticals. An important element that determines the effect of a massive binary on a galactic core is the time it takes for the binary separation to decay to the regime where GW emission will take over. But whether or not a massive binary manages to coalesce, it will leave behind an imprint on both the mass density and kinematics of the galaxy: (i) a mass deficit - a lowered density of stars near the centre of the galaxy; (ii) a tangentially anisotropic velocity distribution - an excess of stars in the core moving along tangential directions. I use N-body simulations to simulate the merger of two galaxies and study the destruction of density cusps around SMBHs in multiple mergers of elliptical galaxies. I follow the details of the merger from its earliest stages, when the two galaxies are distinct, to its late stages, when the SMBHs have formed a hard binary and the binary has decayed via energy exchange with stars to the GW dominated regime. I present the morphological and kinematical structure of the merged galaxy as a function of the merger mass-ratio, the galaxy density profile slope and the number of mergers and discuss observational signatures associated with the formation picture. 


Julie Comerford
AGN Are Preferentially Triggered in Late-Stage, Major Mergers
Galaxy mergers play an important role in driving the coevolution of galaxies and their supermassive black holes, as mergers can trigger both star formation and AGN activity. However, it has been a persistent observational challenge to build up a large sample of AGN in mergers to address the complex interplay between mergers, star formation, and AGN.  In this talk, I will present a catalog of 220 AGN in galaxy mergers with stellar bulge separations <20 kpc.  We built this sample from HST observations of galaxies at 0.2 < z <2.5, and we identified the AGN with Chandra and Spitzer.  We find that AGN are preferentially triggered in major mergers, and in late-stage mergers with bulge separations <4 kpc.  Interestingly, we also find that the star formation rate in galaxy mergers with AGN does not depend on merger mass ratio or bulge separation.


Yuri Levin
Title: Thermodynamics of rotating black hole clusters
Abstract: I will discuss secular-dynamical equilibria of stellar clusters near supermassive black holes using the methods of statistical physics.

New results include 

1. New and simple numerical algorithm for finding such equilibria. Proofs of phase transitions to lopsided equilibria.

2. Lopsided rotating clusters such as eccentric discs in M31

3. Special orbits of stellar mass black holes: clustering in eccentricity, discs, secular-dynamical friction.


Stefan Gillessen
Testing general relativity with the massive black hole in the Galactic Center
Stellar orbits have proven the existence of a massive black hole in the Galactic Center - a fact that now has been honored by the Nobel prize. But the field has moved much beyond that, using the Galactic Center as a laboratory for relativstic astrophysics. Key to the most recent results is the advent of near-infrared interferometry at very large telescopes. I will report on the suite of GRAVITY results in the Galactic Center.


Suvi Gezari
Entering A New Era of Population Studies of Tidal Disruption Events

The tidal disruption of a star by a central supermassive black hole originated as a theoretical concept, but is now a routine observational reality.  Nuclear transients are being discovered by a rich landscape of optical time domain surveys (in alphabetical order: ASAS-SN, ATLAS, Gaia, PanSTARRS, TESS, ZTF) and are being followed-up with space and ground-based facilities across the electromagnetic spectrum.  I will highlight our recent sample of over 25 systematically selected, uniformly characterized tidal disruption events (TDEs) from the Zwicky Transient Facility (ZTF).  The growing number of TDE discoveries from optical surveys, with hundreds more on the horizon with the start of the Vera Rubin Observatory Legacy Survey of Space and Time, are enabling us to do population studies of TDEs for the first time.  I will present exciting new developments in our understanding of the physical conditions driving the light curves, broadband spectral energy distributions, and spectroscopic sub-classes in TDEs, and how they relate to the properties of their host galaxies and the masses of their central supermassive black holes.


Manuela Campanelli
Simulations of Supermassive Black Hole Binaries on their way to Merger 
Supermassive black hole mergers are one of the most dramatic phenomena in the Universe.  For a few hours, they can emit as much power in gravitational waves as all the stars in the Universe produce in light.  Moreover, they are an important element in determining the mass distribution of the entire population of supermassive black holes.  However, none has yet been caught in the act, in large part because they are rare, and no one knows what sort of light they should emit along with the gravitational waves.  In this talk, I will present new simulations aimed at providing detailed astrophysical knowledge about the environments close to supermassive black hole binaries on their way to merger. I will show how gas flows in the immediate neighborhoods of these binaries, especially when both black holes are spinning, and present calculations of jet launching and light signals that observers should search for in order to find examples.

  • Fabio Antonini, Cardiff University
  • Manuela Campanelli, RIT
  • Julie Comerford, University of Colorado-Boulder
  • Suvi Gezari, University of Maryland
  • Steffan Gillessen, MAX PLANCK INSTITUTE (MPE)
  • Yuri Levin, Columbia University and Flat Iron Institute, CCA
  • Xin Liu, University of Illinois
  • Chung-Pei Ma, UC Berkeley
  • Brian Metzger, Columbia university
  • Diego Munoz, Northwestern University (CIERA)
  • Smadar Naoz, UCLA
  • Michael Tremmel, Yale
  • Karina Voggel, Strasbourg Observatory, France
  • Marta Volonteri, Institut d’Astrophysique de Paris (IAP)

Organizing committee:
Fani Dosopoulou, Jeremy Goodman, Jenny Greene, and James Stone