Evolution Of Supermassive Black Hole Binaries Research Articles

Modelling the accretion and feedback of supermassive black hole binaries in gas-rich galaxy mergers

ABSTRACTWe introduce a new model for the accretion and feedback of supermassive black hole (SMBH) binaries to the ketju code, which enables us to resolve the evolution of SMBH binaries down to separations of tens of Schwarzschild radii in gas-rich galaxy mergers. Our subgrid binary accretion model extends the widely used Bondi–Hoyle–Lyttleton accretion into the binary phase and incorporates preferential mass accretion on to the secondary SMBH, which is motivated by results from small-scale hydrodynamical circumbinary disc simulations. We perform idealized gas-rich disc galaxy merger simulations using pure thermal or pure kinetic active galactic nuclei (AGNs) feedback. Our binary accretion model provides more physically motivated SMBH mass ratios, which are one of the key parameters for computing gravitational wave (GW) induced recoil velocities. The merger time-scales of our simulated SMBH binaries are in the range tmerge ∼ 10–400 Myr. Prograde in-plane equal-mass galaxy mergers lead to the shortest merger time-scales, as they experience the strongest starbursts, with the ensuing high stellar density resulting in a rapid SMBH coalescence. Compared to the thermal AGN feedback, the kinetic AGN feedback predicts longer merger time-scales and results in more core-like stellar profiles, as it is more effective in removing gas from the galaxy centre and quenching star formation. This suggests that the AGN feedback implementation plays a critical role in modelling SMBH coalescences. Our model will be useful for improving the modelling of SMBH mergers in gas-rich galaxies, the prime targets for the upcoming LISA GW observatory.

Properties of loss cone stars in a cosmological galaxy merger remnant

Aims.We investigate the orbital and phase space properties of loss cone stars that interact strongly with a hard, high-redshift binary supermassive black hole (SMBH) system formed in a cosmological scenario.Methods.We use a novel hybrid integration approach that combines the directN-body codeφ-GRAPE with ETICS, a collisionless code that employs the self-consistent field method for force calculation. The hybrid approach shows considerable speed-up over direct summation for particle numbers > 106, while retaining accuracy of directN-body for a subset of particles. During the SMBH binary evolution we monitor individual stellar interactions with the binary in order to identify stars that noticeably contribute to the SMBH binary hardening.Results.We successfully identify and analyze in detail the properties of stars that extract energy from the binary. We find that the summed energy changes seen in these stars match very well with the overall binary energy change, demonstrating that stellar interactions are the primary drivers of SMBH binary hardening in triaxial, gas-poor systems. We find that 76% of these stars originate from centrophilic orbits, only possible in a triaxial system. As a result, even the slight triaxiality of our system results in efficient refilling of the loss cone, avoiding the final parsec problem. We distinguish three different populations of interactions based on their apocenter. We find a clear prevalence of interactions co-rotating with the binary. Nevertheless, retrograde interactions are the most energetic, contributing only slightly less than the prograde population to the overall energy exchange. The most energetic interactions are also likely to result in a change of sign in the angular momentum of the star. We estimate the merger timescale of the binary to be ≈20 Myr, a value larger by a factor of two than the timescale reported in a previous study.

The effect of AGN feedback on the migration time-scale of supermassive black holes binaries

The gravitational interaction at parsec to sub-parsec scales between a circumbinary gas disc and a super massive black hole binary (SMBHB) is a promising mechanism to drive the migration of SMBHBs toward coalescence. The typical dynamical evolution can be separated in two main regimes: I) Slow migration ($T_{\rm mig}$ $\sim$ $10^{3-4}\times T_{\rm orb}$), where viscous torques are not efficient enough to redistribute the extracted angular momentum from the binary, leading to the formation of a low density cavity around the binary. II) Fast migration ($T_{\rm mig}$ $\sim$ $10^{1-2} \times T_{\rm orb}$), in which the redistribution of angular momentum is efficient and no low density cavity is formed in the circumbinary disc. Using N-Body/SPH simulations we study the effect of AGN feedback in this phase of a SMBHB evolution. We implement an AGN feedback model in the SPH code Gadget-3 that includes momentum feedback from winds, X-ray heating/radial-momentum and Eddington force. Using this implementation we run a set of simulations of SMBHB+disc in the two main shrinking regimes. From these simulations we conclude that the effect of the AGN mechanical feedback is negligible for SMBHBs in the slowly shrinking regime. However, in the fast shrinking regime the AGN wind excavate a "feedback cavity" leaving the SMBHB almost naked, thus stalling the orbital decay of the binary.

Galaxy rotation and supermassive black hole binary evolution

Supermassive black hole (SMBH) binaries residing at the core of merging galaxies are recently found to be strongly affected by the rotation of their host galaxies. The highly eccentric orbits that form when the host is counterrotating emit strong bursts of gravitational waves that propel rapid SMBH binary coalescence. Most prior work, however, focused on planar orbits and a uniform rotation profile, an unlikely interaction configuration. However, the coupling between rotation and SMBH binary evolution appears to be such a strong dynamical process that it warrants further investigation. This study uses direct N-body simulations to isolate the effect of galaxy rotation in more realistic interactions. In particular, we systematically vary the SMBH orbital plane with respect to the galaxy rotation axis, the radial extent of the rotating component, and the initial eccentricity of the SMBH binary orbit. We find that the initial orbital plane orientation and eccentricity alone can change the inspiral time by an order of magnitude. Because SMBH binary inspiral and merger is such a loud gravitational wave source, these studies are critical for the future gravitational wave detector, LISA, an ESA/NASA mission currently set to launch by 2034.

Post-Newtonian Dynamical Modeling of Supermassive Black Holes in Galactic-scale Simulations

Abstract We present KETJU, a new extension of the widely used smoothed particle hydrodynamics simulation code GADGET-3. The key feature of the code is the inclusion of algorithmically regularized regions around every supermassive black hole (SMBH). This allows for simultaneously following global galactic-scale dynamical and astrophysical processes, while solving the dynamics of SMBHs, SMBH binaries, and surrounding stellar systems at subparsec scales. The KETJU code includes post-Newtonian terms in the equations of motions of the SMBHs, which enables a new SMBH merger criterion based on the gravitational wave coalescence timescale, pushing the merger separation of SMBHs down to ∼0.005 pc. We test the performance of our code by comparison to NBODY7 and rVINE. We set up dynamically stable multicomponent merger progenitor galaxies to study the SMBH binary evolution during galaxy mergers. In our simulation sample the SMBH binaries do not suffer from the final-parsec problem, which we attribute to the nonspherical shape of the merger remnants. For bulge-only models, the hardening rate decreases with increasing resolution, whereas for models that in addition include massive dark matter halos, the SMBH binary hardening rate becomes practically independent of the mass resolution of the stellar bulge. The SMBHs coalesce on average 200 Myr after the formation of the SMBH binary. However, small differences in the initial SMBH binary eccentricities can result in large differences in the SMBH coalescence times. Finally, we discuss the future prospects of KETJU, which allows for a straightforward inclusion of gas physics in the simulations.

THE QUEST FOR THE LARGEST DEPLETED GALAXY CORE: SUPERMASSIVE BLACK HOLE BINARIES AND STALLED INFALLING SATELLITES

ABSTRACT Partially depleted cores are practically ubiquitous in luminous early-type galaxies (M B ≲ −20.5 mag) and are typically smaller than 1 kpc. In one popular scenario, supermassive black hole (SMBH) binaries—established during dry (i.e., gas-poor) galaxy mergers—kick out the stars from a galaxy’s central region via three-body interactions. Here, this “binary black hole scouring scenario” is probed at its extremes by investigating the two galaxies reported to have the largest partially depleted cores found to date: 2MASX J09194427+5622012 and 2MASX J17222717+3207571 (the brightest galaxy in Abell 2261). We have fit these galaxy’s two-dimensional light distribution using the core-Sérsic model and found that the former galaxy has a core-Sérsic break radius , which is three times smaller than the published value. We use this galaxy to caution that other reportedly large break radii may too have been overestimated if they were derived using the “sharp-transition” (inner core)-to-(outer Sérsic) model. In the case of 2MASX J17222717+3207571, we obtain R b,cS = 3.6 kpc. While we confirm that this is the biggest known partially depleted core of any galaxy, we stress that it is larger than expected from the evolution of SMBH binaries—unless one invokes substantial gravitational-wave-induced (black hole-)recoil events. Given the presence of multiple nuclei located (in projection) within the core radius of this galaxy, we explored and found support for the alternative “stalled infalling perturber” core-formation scenario, in which this galaxy’s core could have been excavated by the action of an infalling massive perturber.

Infalling clouds on to supermassive black hole binaries – I. Formation of discs, accretion and gas dynamics

There is compelling evidence that most -if not all- galaxies harbour a super-massive black hole (SMBH) at their nucleus, hence binaries of these massive objects are an inevitable product of the hierarchical evolution of structures in the universe, and represent an important but thus-far elusive phase of galaxy evolution. Gas accretion via a circumbinary disc is thought to be important for the dynamical evolution of SMBH binaries, as well as in producing luminous emission that can be used to infer their properties. One plausible source of the gaseous fuel is clumps of gas formed due to turbulence and gravitational instabilities in the interstellar medium, that later fall toward and interact with the binary. In this context, we model numerically the evolution of turbulent clouds in near-radial infall onto equal-mass SMBH binaries, using a modified version of the SPH code GADGET-3. We present a total of 12 simulations that explore different possible pericentre distances and relative inclinations, and show that the formation of circumbinary discs and discs around each SMBH ('mini-discs') depend on those parameters. We also study the dynamics of the formed discs, and the variability of the feeding rate onto the SMBHs in the different configurations.

GALAXY ROTATION AND RAPID SUPERMASSIVE BINARY COALESCENCE

During a galaxy merger, the supermassive black hole (SMBH) in each galaxy is thought to sink to the center of the potential and form a supermassive black hole binary; this binary can eject stars via 3-body scattering, bringing the SMBHs ever closer. In a static spherical galaxy model, the binary stalls at a separation of about a parsec after ejecting all the stars in its loss cone -- this is the well-known final parsec problem. However it has been shown that SMBH binaries in non-spherical galactic nuclei harden at a nearly constant rate until reaching the gravitational wave regime. Here we use a suite of direct N-body simulations to follow SMBH binary evolution in both corotating and counterrotating flattened galaxy models. For N larger than 500K, we find that the evolution of the SMBH binary is convergent, and is independent of the particle number. Rotation in general increases the hardening rate of SMBH binaries even more effectively than galaxy geometry alone. SMBH binary hardening rates are similar for co- and counterrotating galaxies. In the corotating case, the center of mass of SMBH binary settles into an orbit that is in a corotation resonance with the background rotating model, and the coalescence time is roughly few hundred Myr faster than a non-rotating flattened model. We find that counterrotation drives SMBHs to coalesce on a nearly radial orbit promptly after forming a hard binary. We discuss the implications for gravitational wave astronomy, hypervelocity star production, and the effect on the structure of the host galaxy.

Effects of the Central Density Profile and the Stellar Mass Function of Merging Galaxies on Eccentricities and Coalescence Times of Supermassive Black Hole Binaries

AbstractIf able to coalesce in a Hubble time, Supermassive Black hole (SMBH) binaries are very promising sources of gravitational waves (GW). Our earlier studies have shown that SMBH binaries coalesce in post-merger galactic nuclei having triaxial or axisymmetric geometry in a few billion years. In this study, we model the complete evolution of SMBH binaries formed as a result of galaxy mergers having central density profiles that vary from shallow to very steep including a stellar mass function. Energy and angular momentum loss due to GW emission is taken into account using the post-Newtonian approximation. We carry out ten such simulations for each central density profile. The eccentricity of the SMBH binaries remain very high in shallow cusps and decrease systematically for steeper cusps. The coalescence times range from 0.6 to 1.5 Gyr with shorter times for steeper profiles. Typical coalescence times less than a Gyr strengthen our expectation that SMBH binaries should be very promising sources of GW radiation over a wide redshift range.

SEARCH FOR SUPERMASSIVE BLACK HOLE BINARIES IN THE SLOAN DIGITAL SKY SURVEY SPECTROSCOPIC SAMPLE

Supermassive black hole (SMBH) binaries are expected in a Lambda CDM cosmology given that most (if not all) massive galaxies contain a massive black hole at their center. So far, however, direct evidence for such binaries has been elusive. We use cross-correlation to search for temporal velocity shifts in the MgII broad emission lines of 0.36 < z < 2 quasars with multiple observations in the Sloan Digital Sky Survey. For ~ 10^9 Msun BHs in SMBH binaries, we are sensitive to velocity drifts for binary separations of ~ 0.1 pc with orbital periods of ~100 years. We find seven candidate sub-pc--scale binaries with velocity shifts > 3.4 sigma ~ 280 km/s, where sigma is our systematic error. Comparing the detectability of SMBH binaries with the number of candidates (N < 7), we can rule out that most 10^9 Msun BHs exist in ~ 0.03-0.2 pc scale binaries, in a scenario where binaries stall at sub-pc scales for a Hubble time. We further constrain that < one-third of quasars host SMBH binaries after considering gas-assisted sub-pc evolution of SMBH binaries, although this result is very sensitive to the assumed size of the broad line region. We estimate the detectability of SMBH binaries with ongoing or next-generation surveys (e.g., BOSS, Subaru Prime Focus Spectrograph), taking into account the evolution of the sub-parsec binary in circumbinary gas disks. These future observations will provide longer time baselines for searches similar to ours and may in turn constrain the evolutionary scenarios of SMBH binaries.

SUPERMASSIVE BLACK HOLE BINARY EVOLUTION IN AXISYMMETRIC GALAXIES: THE FINAL PARSEC PROBLEM IS NOT A PROBLEM

During a galaxy merger, the supermassive black hole (SMBH) in each galaxy is thought to sink to the center of the potential and form a supermassive black hole binary; this binary can eject stars via 3-body scattering, bringing the SMBHs ever closer. In a static spherical galaxy model, the binary stalls at a separation of about a parsec after ejecting all the stars in its loss cone -- this is the well-known final parsec problem. Earlier work has shown that the centrophilic orbits in triaxial galaxy models are key in refilling the loss cone at a high enough rate to prevent the black holes from stalling. However, the evolution of binary SMBHs has never been explored in axisymmetric galaxies, so it is not clear if the final parsec problem persists in these systems. Here we use a suite of direct N-body simulations to follow SMBH binary evolution in galaxy models with a range of ellipticity. For the first time, we show that mere axisymmetry can solve the final parsec problem; we find the the SMBH evolution is independent of N for an axis ratio of c/a=0.8, and that the SMBH binary separation reaches the gravitational radiation regime for c/a=0.75.

Post‐Newtonian simulations of super‐massive black hole binaries in galactic nuclei

AbstractWe present some preliminary results from recent numerical simulations that model the evolution of super‐massive black hole (SMBH) binaries in galactic nuclei. Including the post‐Newtonian terms for the binary system and adopting appropriate models for the galaxies allows us, for the.rst time, to follow the evolution of SMBH binaries from kpc scales down to the coalescence phase. We use our results to make predictions of the detectability of such events with the gravitational wave detector LISA. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Tidal Disruption of Stellar Objects by Hard Supermassive Black Hole Binaries

Supermassive black hole binaries (SMBHBs) are expected by the hierarchical galaxy formation model in ΛCDM cosmology. There is some evidence in the literature for SMBHBs in active galactic nuclei, but there are few observational constraints on the evolution of SMBHBs in inactive galaxies and gas-poor mergers. On the theoretical front, it is unclear how long is needed for a SMBHB in a typical galaxy to coalesce. In this paper we investigate the tidal interaction between stars and binary black holes (BHs) and calculate the tidal disruption rates of stellar objects by the BH components of the binary. We derive the interaction cross sections between SMBHBs and stars from intensive numerical scattering experiments with particle number ~107 and calculate the tidal disruption rates by both single and binary BHs for a sample of realistic galaxy models, taking into account the general relativistic effects and the loss-cone refilling because of two-body interaction. We estimate the frequency of tidal flares for different types of galaxies using the BH mass function in the literature. We find that because of the three-body slingshot effect, the tidal disruption rate in the SMBHB system is more than 1 order of magnitude smaller than that in a single supermassive black hole (SMBH) system. The difference is more significant in less massive galaxies and does not depend on detailed stellar dynamical processes. Our calculations suggest that comparisons of the calculated tidal disruption rates for both single and binary BHs and the surveys of X-ray or UV flares at galactic centers could tell us whether most SMBHs in nearby galaxies are single and whether the SMBHBs formed in gas-poor galaxy mergers coalesce rapidly.

Evolution of Supermassive Black Hole Binaries and Acceleration of Jet Precession in Galactic Nuclei

Supermassive black hole binaries (SMBHBs) are expected with the hierarchical galaxy formation model. Currently, physics processes dominating the evolution of a SMBHB are unclear. An interesting question is whether we could observationally determine the evolution of SMBHBs and give constraints on the physical processes. Jet precession has been observed in many active galactic nuclei (AGNs) and is generally attributed to disk precession. In this paper we calculate the time variation of jet precession and conclude that jet precession is accelerated in SMBHB systems but decelerated in others. The acceleration of jet precession, -->dPpr/dt, is related to the jet precession timescale, -->Ppr, and the SMBHB evolution timescale, -->?a, as -->dPpr/dt ? ? (Ppr/?a) . Our calculations based on the models for jet precession and SMBHB evolution show that -->dPpr/dt can be as high as about ?1.0, with a typical value of ?0.2, and can be easily detected. We discuss the differential jet precession for NGC 1275 that has been observed in the literature. If its observed rapid acceleration of jet precession is true, the jet precession is due to the orbital motion of an unbound SMBHB with a mass ratio of -->q ? 0.76. When jets precess from ancient bubbles to the currently active jets, the separation of the SMBHB decreases from about 1.46 kpc to 0.80 kpc, with an averaged decreasing velocity of -->da/dt ? 1.54 ? 106 cm s?1 and an evolution timescale of -->?a ? 7.5 ? 107 yr. However, if we assume steady jet precession for many cycles, the observations imply a hard SMBHB with a mass ratio of a -->q ? 0.21 and a separation of -->a ? 0.29 pc.