Motivated by observational searches for massive black hole (MBH) pairs at kiloparsec separations we develop a semi-analytic model to describe their orbital evolution under the influence of stellar and gaseous dynamical friction (DF). The goal of this study is to determine how the properties of the merger remnant galaxy and the MBHs affect the likelihood and timescale for formation of a close MBH pair with separation of < 1 pc. We compute approximately 40,000 configurations that cover a wide range of host galaxy properties and investigate their impact on the orbital evolution of unequal mass MBH pairs. We find that the percentage for MBH pairing within a Hubble time is larger than 80% in remnant galaxies with a gas fraction < 20% and in galaxies hosting MBH pairs with total mass > 10^6 solar mass and mass ratios > 1/4. Among these, the remnant galaxies characterized by the fastest formation of close, gravitationally bound MBHs have one or more of the following properties: (1) large stellar bulge, (2) comparable mass MBHs and (3) a galactic gas disk rotating close to the circular speed. In such galaxies, the MBHs with the shortest inspiral times, which are likely progenitors of coalescing MBHs, are either on circular prograde orbits or on very eccentric retrograde orbits. Our model also indicates that remnant galaxies with opposite properties, that host slowly evolving MBH pairs, are the most likely hosts of dual AGNs at kiloparsec separations.
Dynamical friction (DF) against stars and gas is thought to be an important mechanism for orbital evolution of massive black holes (MBHs) in merger remnant galaxies. Recent theoretical investigations however show that DF does not always lead to MBH inspiral. For MBHs evolving in gas-rich backgrounds, the ionizing radiation that emerges from the innermost parts of their accretion flow can affect the surrounding gas in such a way to cause the MBHs to accelerate and gain orbital energy. This effect was dubbed "negative DF". We use a semi-analytic model to study the impact of negative DF on pairs …
We study the infall of a subhalo in its parent halo due to dynamical friction. Using expected mass and spatial distributions of subhaloes, in unison with the derived infall time-scale, we calculate the expected merger stellar mass growth rate (SMGR) of the central galaxy in two different models (instantaneous and continuous merging). We find that our continuous merging SMGR (which corresponds to smooth accretion from satellites) agrees with the results of the Millennium-II simulation, predicting low growth rates for low stellar masses ($10^{8}$ M$_{\odot}-10^{10}$ M$_{\odot}$) and accelerating…
In the near future, projects like LISA and Pulsar Timing Arrays are expected to detect gravitational waves from mergers between supermassive black holes, and it is crucial to precisely model the underlying merger populations now to maximize what we can learn from this new data. Here we characterize expected high-redshift (z > 2) black hole mergers using the very large volume Astrid cosmological simulation, which uses a range of seed masses to probe down to low-mass BHs, and directly incorporates dynamical friction so as to accurately model the dynamical processes which bring black holes to the…
In this work we establish and test methods for implementing dynamical friction for massive black hole pairs that form in large volume cosmological hydrodynamical simulations which include galaxy formation and black hole growth. We verify our models and parameters both for individual black hole dynamics and for the black hole population in cosmological volumes. Using our model of dynamical friction (DF) from collisionless particles, black holes can effectively sink close to the galaxy center, provided that the black hole's dynamical mass is at least twice that of the lowest mass resolution part…
Dynamics of low-mass black hole seeds in the BRAHMA simulations using subgrid-dynamical friction: Impact on merger-driven black hole growth in the high redshift Universe
The ultra-diffuse galaxy NGC 1052-DF2 has an overabundance of luminous globular clusters (GCs), and its kinematics is consistent with the presence of little to no dark matter. As the velocity dispersion among the GCs is comparable to the expected internal dispersions of the individual GCs, the galaxy might be highly conducive to GC-GC merging. If true, this could explain the puzzling luminosity function of its GCs. Here, we examine this possibility by re-simulating three of our earlier simulations of the GC system (Dutta Chowdhury et al. 2019), where the GCs were modeled as single particles, w…
Stellar shells observed in many giant elliptical and lenticular as well as a few spiral and dwarf galaxies presumably result from radial minor mergers of galaxies. We show that the line-of-sight velocity distribution of the shells has a quadruple-peaked shape. We found simple analytical expressions that connect the positions of the four peaks of the line profile with the mass distribution of the galaxy, namely, the circular velocity at the given shell radius and the propagation velocity of the shell. The analytical expressions were applied to a test-particle simulation of a radial minor merger…
The forthcoming Laser Interferometer Space Antenna (LISA) will probe the population of coalescing massive black hole (MBH) binaries up to the onset of structure formation. Here we simulate the galactic-scale pairing of $\sim10^6 M_\odot$ MBHs in a typical, non-clumpy main-sequence galaxy embedded in a cosmological environment at $z = 7-6$. In order to increase our statistical sample, we adopt a strategy that allows us to follow the evolution of six secondary MBHs concomitantly. We find that the magnitude of the dynamical-friction induced torques is significantly smaller than that of the large-…
The launch of space based gravitational wave (GW) detectors (e.g. Laser Interferometry Space Antenna; LISA) and current and upcoming Pulsar Timing Arrays (PTAs) will extend the GW window to low frequencies, opening new investigations into dynamical processes involving massive black hole binaries (MBHBs) and their mergers across cosmic time. MBHBs are expected to be among the primary sources for the upcoming low frequency ($10^{-4}-10^{-1}$ Hz) window probed by LISA. It is important to investigate the expected MBH merger rates and associated signals, to determine how potential LISA events are a…
Dual active galaxy nuclei (dAGNs) trace the population of post-merger galaxies and are the precursors to massive black hole (MBH) mergers, an important source of gravitational waves that may be observed by LISA. In Paper I of this series, we used the population of nearly 2000 galaxy mergers predicted by the TNG50-3 simulation to seed semi-analytic models of the orbital evolution and coalescence of MBH pairs with initial separations of about 1 kpc. Here, we calculate the dAGN luminosities and separation of these pairs as they evolve in post-merger galaxies, and show how the coalescence fraction…
The five globular clusters (GCs) of the Fornax dSph are puzzling for two reasons; the mass in GCs is high with respect to the galaxy's old stellar mass, and their survival and large distance (> 1 kpc) is at odds with naive expectations of dynamical friction. We present here a semi-analytic model, simultaneously addressing both problems in a comprehensive evolutionary framework for Fornax. Key to the model is inclusion of: 1) hydrodynamical constraints on the GC formation locations, 2) self-consistent velocity distribution functions in the dynamical friction calculations and 3) expansion of GC …
We present a model for the dynamical evolution of subhaloes based on an approach combining numerical and analytical methods. Our method is based on tracking subhaloes in an N-body simulation up to the last point that it can be resolved, and applying an analytic prescription for its merger timescale that takes dynamical friction and tidal disruption into account. When applied to cosmological N-body simulations with mass resolutions that differ by two orders of magnitude, the technique produces halo occupation distributions that agree to within 3%.
Blue straggler stars are more massive than the average star in globular clusters, as they originate from the merger of two stars. Consequently, they experience dynamical friction, progressively sinking to the cluster center. Recently, several indicators of the degree of dynamical relaxation of a globular cluster have been proposed, based on the observed radial distribution of blue straggler stars. The most successful is the Alessandrini indicator, or A+ for short, which is the integral of the cumulative distribution of the blue straggler stars minus that of a lighter reference population. A+ c…
Direct observational evidence for the creation of nuclear star clusters (NSCs) is needed to support the proposed scenarios for their formation. We analysed the dwarf galaxy UGC 7346, located in the peripheral regions of the Virgo Cluster, to highlight a series of properties that indicate the formation of a NSC caught in its earlier stages. First, we report on remnants of a past interaction in the form of diffuse streams or shells, suggesting a recent merging of two dwarf galaxies with a 1:5 stellar mass ratio. Second, we identify a number of globular cluster (GC) candidates that are broadly co…
The accreted component of stellar halos is composed of the contributions of several satellites, falling onto their host with their different masses, at different times, on different orbits. This work uses a suite of idealised, collisionless N-body simulations of minor mergers and a particle tagging technique to understand how these different ingredients shape each contribution to the accreted halo, in both density and kinematics. I find that more massive satellites deposit their stars deeper into the gravitational potential of the host, with a clear segregation enforced by dynamical friction. …
We present post-Newtonian $N$-body simulations on mergers of accreting stellar-mass black holes (BHs), where such general relativistic effects as the pericenter shift and gravitational wave (GW) emission are taken into consideration. The attention is concentrated on the effects of the dynamical friction and the Hoyle-Lyttleton mass accretion by ambient gas. We consider a system composed of ten BHs with initial mass of $30~M_\odot$. As a result, we show that mergers of accreting stellar-mass BHs are classified into four types: a gas drag-driven, an interplay-driven, a three body-driven, or an a…
Massive black hole (MBH) coalescences are powerful sources of low-frequency gravitational waves. To study these events in the cosmological context we need to trace the large-scale structure and cosmic evolution of a statistical population of galaxies, from dim dwarfs to bright galaxies. To cover such a large range of galaxy masses, we analyse two complementary simulations: Horizon-AGN with a large volume and low resolution which tracks the high-mass (> 1e7 Msun) MBH population, and NewHorizon with a smaller volume but higher resolution that traces the low-mass (< 1e7 Msun) MBH population. Whil…
Recently, advanced Laser Interferometer Gravitational-Wave Observatory (aLIGO) has detected black hole (BH) merger events, most of which are sourced by BHs more massive than $30~M_\odot$. Especially, the observation of GW170104 suggests dynamically assembled binaries favoring a distribution of misaligned spins. It has been argued that mergers of unassociated BHs can be engendered through a "chance meeting" in a multiple BH system under gas-rich environments. In this paper, we consider the merger of unassociated BHs, concordant with the massive BH merger events. To that end, we simulate a multi…
(Abridged) Dynamical friction can be used to distinguish Newtonian gravity and modified Newtonian dynamics (MOND) because it works differently in these frameworks. This concept, however, has yet to be explored very much with MOND. Previous simulations showed weaker dynamical friction during major mergers for MOND than for Newtonian gravity with dark matter. Analytic arguments suggest the opposite for minor mergers. In this work, we verify the analytic predictions for MOND by high-resolution $N$-body simulations of globular clusters (GCs) moving in isolated ultra-diffuse galaxies (UDGs). We tes…
Globular clusters (GCs) may harbour intermediate-mass black holes (IMBHs) at their centres. In these dynamically active environments stellar-mass black holes (SBHs) sink to the center soon after formation, due to dynamical friction and start interacting among themselves and with the central IMBH. Likely, some of the SBHs will form bound systems with the IMBH. A fraction of those will be triple systems composed of binary SBHs and the IMBH acting as a third distant perturber. If the SBH binary orbit is sufficiently inclined it can develop Lidov-Kozai (LK) oscillations, which can drive the system…
Dynamical friction is thought to be a principal mechanism responsible for orbital evolution of massive black holes (MBHs) in the aftermath of galactic mergers and an important channel for formation of gravitationally bound MBH binaries. We use 2D radiative hydrodynamic simulations to investigate the efficiency of dynamical friction in the presence of radiative feedback from an MBH moving through a uniform density gas. We find that ionizing radiation that emerges from the innermost parts of the MBH's accretion flow strongly affects the dynamical friction wake and renders dynamical friction inef…
We present the evolution of black holes (BHs) and their relationship with their host galaxies in Astrid, a large-volume cosmological hydrodynamical simulation with box size 250 $h^{-1} \rm Mpc$ containing $2\times5500^3$ particles evolved to z=3. Astrid statistically models BH gas accretion and AGN feedback to their environments, applies a power-law distribution for BH seed mass $M_{\rm sd}$, uses a dynamical friction model for BH dynamics and executes a physical treatment of BH mergers. The BH population is broadly consistent with empirical constraints on the BH mass function, the bright end …
We consider how dynamical friction acts on black holes that receive a velocity kick while located at the center of a gravitational potential, analogous to a star cluster, due to either a natal kick or the anisotropic emission of gravitational waves during a black hole-black hole merger. Our investigation specifically focuses on how well various Chandrasekhar-based dynamical friction models can predict the orbital decay of kicked black holes with $m_{bh} \lesssim 100 M_\odot$ due to an inhomogeneous background stellar field. In general, the orbital evolution of a kicked black hole follows that …
We study the dynamical response of extended systems, hosts, to smaller systems, satellites, orbiting around the hosts using extremely high-resolution N-body simulations with up to one billion particles. This situation corresponds to minor mergers which are ubiquitous in the scenario of hierarchical structure formation in the universe. According to Chandrasekhar (1943), satellites create density wakes along the orbit and the wakes cause a deceleration force on satellites, i.e. dynamical friction. This study proposes an analytical model to predict the dynamical response of hosts as reflected in …
In this paper, we study the secular dynamical evolution of binaries composed of intermediate-mass and stellar-mass black holes (IMBHs and SBHs, respectively) in orbit about a central super-massive black hole (SMBH) in galactic nuclei. Such BH triplets could form via the inspiral of globular clusters toward galactic nuclei due to dynamical friction, or even major/minor galaxy mergers. We perform, for reasonable initial conditions that we justify, sophisticated $N$-body simulations that include both regularization and Post-Newtonian corrections. We find that mass segregation combined with Kozai-…
We study the orbital decay of a pair of massive black holes (BHs) with masses 5 * 10^5 and 10^7 M_sun, using hydrodynamical simulations of circumnuclear disks (CNDs) with the alternating presence of sub-grid physics such as radiative cooling, star formation, supernova feedback, BH accretion and feedback. In the absence of such processes, the orbit of the secondary BH decays over timescales of ~10 Myr to the center of the CND, where the primary BH resides. When strong dissipation operates in CNDs, fragmentation into massive objects the size of giant molecular clouds and with densities in the ra…
GW231123: Likely a product of successive mergers from $\sim 10 $ stellar-mass black holes
Key Laboratory of Dark Matter and Space Astronomy, Purple Mountain Observatory, Chinese Academy of Sciences（暗物质和空间天文重点实验室，中国科学院紫金山天文台）；Department of Physics, University of Florida（佛罗里达大学物理系）；School of Astronomy and Space Science, University of Science and Technology of China（中国科学技术大学天文与空间科学学院）
Galaxy mergers are known to host abundant young massive cluster (YMC) populations, whose formation mechanism is still not well-understood. Here, we present a high-resolution galaxy merger simulation with explicit star formation and stellar feedback prescriptions to investigate how mergers affect the properties of the interstellar medium and YMCs. Compared with a controlled simulation of an isolated galaxy, the mass fraction of dense and high-pressure gas is much higher in mergers. Consequently, the mass function of both molecular clouds and YMCs becomes shallower and extends to higher masses. …
We investigate the dynamical evolution of galaxies in groups with different formation epochs. Galaxy groups have been selected to be in different dynamical states, namely dynamically old and dynamically young, which reflect their early and late formation times, respectively, based on their halo mass assembly. Brightest galaxies in dynamically young groups have suffered their last major galaxy merger typically $\sim 2$ Gyr more recently than their counterparts in dynamically old groups. Furthermore, we study the evolution of velocity dispersion in these two classes and compare them with the ana…
We model here the merger histories of the supermassive black hole (SMBH) population in the late stages of a cosmological simulation of a $\sim 2 \times 10^{13} M_\odot$ galaxy group. The gravitational dynamics around the several tens of SMBHs ($M_{\bullet} \gtrsim 7.5\times 10^7 M_\odot$) hosted by the galaxies in the group is computed at high accuracy using regularized integration with the KETJU code. The 11 SMBHs which form binaries and hierarchical triplets eventually merge after hardening through dynamical friction, stellar scattering, and gravitational wave (GW) emission. The binaries for…
Intermediate mass black holes (IMBHs) in the mass range $10^2-10^5\,\mathrm{M_{\odot}}$ bridge the gap between stellar black holes (BHs) and supermassive BHs. Here, we investigate the possibility that IMBHs form in young star clusters via runaway collisions and BH mergers. We analyze $10^4$ simulations of dense young star clusters, featuring up-to-date stellar wind models and prescriptions for core collapse and (pulsational) pair instability. In our simulations, only 9 IMBHs out of 218 form via binary BH mergers, with a mass $\sim{}100-140$ M$_\odot$. This channel is strongly suppressed by the…
The supermassive black holes originally in the nuclei of two merging galaxies will form a binary in the remnant core. The early evolution of the massive binary is driven by dynamical friction before the binary becomes "hard" and eventually reaches coalescence through gravitational wave emission. { We consider the dynamical friction evolution of massive binaries consisting of a secondary hole orbiting inside a stellar cusp dominated by a more massive central black hole.} In our treatment we include the frictional force from stars moving faster than the inspiralling object which is neglected in …
We examine massive black hole (MBH) mergers and their associated gravitational wave signals from the large-volume cosmological simulation Astrid. Astrid includes galaxy formation and black hole models recently updated with a MBH seed population between $3\times 10^4M_{\odot}/h$ and $3\times 10^5M_{\odot}/h$ and a sub-grid dynamical friction (DF) model to follow the MBH dynamics down to $1.5\;\text{ckpc}/h$. We calculate initial eccentricities of MBH orbits directly from the simulation at kpc-scales, and find orbital eccentricities above $0.7$ for most MBH pairs before the numerical merger. Aft…
