arXiv subjects

Kyle Kremer

Publications and source records attributed to Kyle Kremer.

At least 55 records · Page 3Linked to original sources

arXiv2024

From Seeds to Supermassive Black Holes: Capture, Growth, Migration, and Pairing in Dense Proto-Bulge Environments

The origins and mergers of supermassive black holes (BHs) remain a mystery. We describe a scenario from a novel multi-physics simulation featuring rapid ($\lesssim 1\,$Myr) hyper-Eddington gas capture by a $\sim 1000\,{\rm M}_{\odot}$ ``seed'' BH up to supermassive ($\gtrsim 10^{6}\,M_{\odot}$) masses, in a massive, dense molecular cloud complex typical of high-redshift starbursts. Due to the high cloud density, stellar feedback is inefficient and most of the gas turns into stars in star clusters which rapidly merge hierarchically, creating deep potential wells. Relatively low-mass BH seeds at random positions can be ``captured'' by merging sub-clusters and migrate to the center in $\sim1$ free-fall time (vastly faster than dynamical friction). This also efficiently produces a paired BH binary with $\sim 0.1$\,pc separation. The centrally-concentrated stellar density profile (akin to a ``proto-bulge'') allows the cluster as a whole to capture and retain gas and build up a large (pc-scale) circum-binary accretion disk with gas coherently funnelled to the central BH (even when the BH radius of influence is small). The disk is ``hyper-magnetized'' and ``flux-frozen'': dominated by a toroidal magnetic field with plasma $β\sim 10^{-3}$, with the fields amplified by flux-freezing. This drives hyper-Eddington inflow rates $\gtrsim 1\,\rm M_\odot yr^{-1}$, which also drive the two BHs to nearly-equal masses. The late-stage system appears remarkably similar to recently-observed high-redshift ``little red dots.'' This scenario can provide an explanation for rapid SMBH formation, growth and mergers in high-redshift galaxies.

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arXiv2024

Spin Doctors: How to diagnose a hierarchical merger origin

Gravitational-wave observations provide the unique opportunity of studying black hole formation channels and histories -- but only if we can identify their origin. One such formation mechanism is the dynamical synthesis of black hole binaries in dense stellar systems. Given the expected isotropic distribution of component spins of binary black hole in gas-free dynamical environments, the presence of anti-aligned or in-plane spins with respect to the orbital angular momentum is considered a tell-tale sign of a merger's dynamical origin. Even in the scenario where birth spins of black holes are low, hierarchical mergers attain large component spins due to the orbital angular momentum of the prior merger. However, measuring such spin configurations is difficult. Here, we quantify the efficacy of the spin parameters encoding aligned-spin ($χ_{\rm eff}$) and in-plane spin ($χ_{\rm p}$) at classifying such hierarchical systems. Using Monte Carlo cluster simulations to generate a realistic distribution of hierarchical merger parameters from globular clusters, we can infer mergers' $χ_{\rm eff}$ and $χ_{rm p}$. The cluster populations are simulated using Advanced LIGO-Virgo sensitivity during the detector network's third observing period and projections for design sensitivity. Using a "likelihood-ratio"-based statistic, we find that $\sim2\%$ of the recovered population by the current gravitational-wave detector network has a statistically significant $χ_{\rm p}$ measurement, whereas no $χ_{\rm eff}$ measurement was capable of confidently determining a system to be anti-aligned with the orbital angular momentum at current detector sensitivities. These results indicate that measuring spin-precession through $χ_{\rm p}$ is a more detectable signature of a hierarchical mergers and dynamical formation than anti-aligned spins.

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arXiv2024

IMBH Progenitors from Stellar Collisions in Dense Star Clusters

Very massive stars (VMSs) formed via a sequence of stellar collisions in dense star clusters have been proposed as the progenitors of massive black hole seeds. VMSs could indeed collapse to form intermediate-mass black holes (IMBHs), which would then grow by accretion to become the supermassive black holes observed at the centers of galaxies and powering high-redshift quasars. Previous studies have investigated how different cluster initial conditions affect the formation of a VMS, including mass segregation, stellar collisions, and binaries, among others. In this study, we investigate the growth of VMSs with a new grid of Cluster Monte Carlo (CMC) star cluster simulations -- the most expansive to date. The simulations span a wide range of initial conditions, varying the number of stars, cluster density, stellar initial mass function (IMF), and primordial binary fraction. We find a gradual shift in the mass of the most massive collision product across the parameter space; in particular, denser clusters born with top-heavy IMFs provide strong collisional regimes that form VMSs with masses easily exceeding 1000 solar masses. Our results are used to derive a fitting formula that can predict the typical mass of a VMS formed as a function of the star cluster properties. Additionally, we study the stochasticity of this process and derive a statistical distribution for the mass of the VMS formed in one of our models, recomputing the model 50 times with different initial random seeds.

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arXiv2024

Stellar Escape from Globular Clusters. II. Clusters May Eat Their Own Tails

We apply for the first time the Monte Carlo star cluster modeling method to study tidal tail and stellar stream formation from globular clusters, assuming a circular orbit in a smooth Galactic potential. Approximating energetically unbound bodies (potential escapers; PEs) as collisionless enables this fast but spherically symmetric method to capture asymmetric tidal phenomena with unprecedented detail. Beyond reproducing known stream features, including epicyclic overdensities, we show how 'returning tidal tails' may form after the stream fully circumnavigates the Galaxy back to the cluster, enhancing the stream's velocity dispersion. While a realistically clumpy, time-dependent Galactic potential may disrupt such tails, they warrant scrutiny as potentially excellent constraints on the Galactic potential's history and substructure. Re-examining the escape timescale $Δt$ of PEs, we find new behavior related to chaotic scattering in the three-body problem; the $Δt$ distribution features sharp plateaus corresponding to distinct locally smooth patches of the chaotic saddle separating the phase space basins of escape. We study for the first time $Δt$ in an evolving cluster, finding that $Δt\sim(E_{\rm J}^{-0.1},E_{\rm J}^{-0.4})$ for PEs with (low, high) Jacobi energy $E_{\rm J}$, flatter than for a static cluster ($E_{\rm J}^{-2}$). Accounting for cluster mass loss and internal evolution -- and (roughly) for ongoing relaxation among PEs -- lowers the median $Δt$ from ${\sim}10\,$Gyr to ${\lesssim}100\,$Myr. We finally outline future improvements to escape physics in the Monte Carlo method intended to enable both the first large-parameter-space studies of tidal tail/stellar stream formation from full globular cluster simulations and detailed comparisons to stream observations.

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arXiv2024

An Analytic Model For Magnetically-Dominated Accretion Disks

Recent numerical cosmological radiation-magnetohydrodynamic-thermochemical-star formation simulations have resolved the formation of quasar accretion disks with Eddington or super-Eddington accretion rates onto supermassive black holes (SMBHs) down to a few hundred gravitational radii. These 'flux-frozen' and hyper-magnetized disks appear to be qualitatively distinct from classical $α$ disks and magnetically-arrested disks: the midplane pressure is dominated by toroidal magnetic fields with plasma $β\ll 1$ powered by advection of magnetic flux from the interstellar medium (ISM), and they are super-sonically and trans-Alfvenically turbulent with cooling times short compared to dynamical times yet remain gravitationally stable owing to magnetic support. In this paper, we present a simple analytic similarity model for such disks. For reasonable assumptions, the model is entirely specified by the boundary conditions (inflow rate at the BH radius of influence [BHROI]). We show that the scalings from this model are robust to various detailed assumptions, agree remarkably well with the simulations (given their simplicity), and demonstrate the self-consistency and gravitational stability of such disks even in the outer accretion disk (approaching the BHROI) at hyper-Eddington accretion rates.

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arXiv2024

Single Millisecond Pulsars from Dynamical Interaction Processes in Dense Star Clusters

Globular clusters (GCs) are particularly efficient at forming millisecond pulsars. Among these pulsars, about half lack a companion star, a significantly higher fraction than in the Galactic field. This fraction increases further in some of the densest GCs, especially those that have undergone core collapse, suggesting that dynamical interaction processes play a key role. For the first time, we create N-body models that reproduce the ratio of single-to-binary pulsars in Milky-Way-like GCs. We focus especially on NGC 6752, a typical core-collapsed cluster with many observed millisecond pulsars. Previous studies suggested that an increased rate of neutron star binary disruption in the densest clusters could explain the overabundance of single pulsars in these systems. Here, we demonstrate that binary disruption is ineffective and instead we propose that two additional dynamical processes play the dominant role: (1) tidal disruption of main-sequence stars by neutron stars; and (2) gravitational collapse of heavy white-dwarf-binary merger remnants. Neutron stars formed through these processes may also be associated with fast radio bursts similar to those observed recently in an extragalactic GC.

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arXiv2024

FORGE'd in FIRE II: The Formation of Magnetically-Dominated Quasar Accretion Disks from Cosmological Initial Conditions

In a companion paper, we reported the self-consistent formation of quasar accretion disks with inflow rates $\sim 10\,{\rm M_{\odot}\,yr^{-1}}$ down to <300 Schwarzschild radii from cosmological radiation-magneto-thermochemical-hydrodynamical galaxy and star formation simulations. We see the formation of a well-defined, steady-state accretion disk which is stable against star formation at sub-pc scales. The disks are optically thick, with radiative cooling balancing accretion, but with properties that are distinct from those assumed in most previous accretion disk models. The pressure is strongly dominated by (primarily toroidal) magnetic fields, with a plasma $β\sim 10^{-4}$ even in the disk midplane. They are qualitatively distinct from magnetically elevated or arrested disks. The disks are strongly turbulent, with trans-Alfvenic and highly super-sonic turbulence, and balance this via a cooling time that is short compared to the disk dynamical time, and can sustain highly super-Eddington accretion rates. Their surface and 3D densities at $\sim 10^{3}-10^{5}$ gravitational radii are much lower than in a Shakura-Sunyaev disk, with important implications for their thermo-chemistry and stability. We show how the magnetic field strengths and geometries arise from rapid advection of flux with the inflow from much weaker galaxy-scale fields in these 'flux-frozen' disks, and how this stabilizes the disk and gives rise to efficient torques. Re-simulating without magnetic fields produces catastrophic fragmentation with a vastly smaller, lower-$\dot{M}$ Shakura-Sunyaev-like disk.

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arXiv2023

Growing Black Holes through Successive Mergers in Galactic Nuclei: I. Methods and First Results

We present a novel, few-body computational framework designed to shed light on the likelihood of forming intermediate-mass (IM) and supermassive (SM) black holes (BHs) in nuclear star clusters (NSCs) through successive BH mergers, initiated with a single BH seed. Using observationally motivated NSC profiles, we find that the probability of a ${\sim}100 \, M_\odot$ BH to grow beyond ${\sim}1000 \, M_\odot$ through successive mergers ranges from ${\sim}0.1\%$ in low-density, low-mass clusters to nearly $90\%$ in high-mass, high-density clusters. However, in the most massive NSCs, the growth timescale can be very long ($\gtrsim 1\,$Gyr); vice versa, while growth is least likely in less massive NSCs, it is faster there, requiring as little as ${\sim}0.1\,$Gyr. The increased gravitational focusing in systems with lower velocity dispersions is the primary contributor to this behavior. We find that there is a simple "7-strikes-and-you're-in" rule governing the growth of BHs: our results suggest that if the seed survives 7 to 10 successive mergers without being ejected (primarily through gravitational wave recoil kicks), the growing BH will most likely remain in the cluster and will then undergo runaway, continuous growth all the way to the formation of an SMBH (under the simplifying assumption adopted here of a fixed background NSC). Furthermore, we find that rapid mergers enforce a dynamically-mediated "mass gap" between about ${50-300 \, M_\odot}$ in an NSC.

astro-ph.GA↗

arXiv2023

Wind-Reprocessed Transients from Stellar-mass Black Hole Tidal Disruption Events

Tidal disruptions of stars by stellar-mass black holes are expected to occur frequently in dense star clusters. Building upon previous studies that performed hydrodynamic simulations of these encounters, we explore the formation and long-term evolution of the thick, super-Eddington accretion disks formed. We build a disk model that includes fallback of material from the tidal disruption, accretion onto the black hole, and disk mass losses through winds launched in association with the super-Eddington flow. We demonstrate that bright transients are expected when radiation from the central engine powered by accretion onto the black hole is reprocessed at large radii by the optically-thick disk wind. By combining hydrodynamic simulations of these disruption events with our disk+wind model, we compute light curves of these wind-reprocessed transients for a wide range of stellar masses and encounter penetration depths. We find typical peak bolometric luminosities of roughly $10^{41}-10^{44}\,$erg/s (depending mostly on accretion physics parameters) and temperatures of roughly $10^5-10^6\,$K, suggesting peak emission in the ultraviolet/blue bands. We predict all-sky surveys such as the Vera Rubin Observatory and ULTRASAT will detect up to thousands of these events per year in dense star clusters out to distances of several Gpc.

astro-ph.HE↗

arXiv2023

Connecting the Young Pulsars in Milky Way Globular Clusters with White Dwarf Mergers and the M81 Fast Radio Burst

The detections of four apparently young radio pulsars in the Milky Way globular clusters are difficult to reconcile with standard neutron star formation scenarios associated with massive star evolution. Here we discuss formation of these young pulsars through white dwarf mergers in dynamically-old clusters that have undergone core collapse. Based on observed properties of magnetic white dwarfs, we argue neutron stars formed via white dwarf merger are born with spin periods of roughly $10-100\,$ms and magnetic fields of roughly $10^{11}-10^{13}\,$G. As these neutron stars spin down via magnetic dipole radiation, they naturally reproduce the four observed young pulsars in the Milky Way clusters. Rates inferred from $N$-body cluster simulations as well as the binarity, host cluster properties, and cluster offsets observed for these young pulsars hint further at a white dwarf merger origin. These young pulsars may be descendants of neutron stars capable of powering fast radio bursts analogous to the bursts observed recently in a globular cluster in M81.

astro-ph.HE↗

arXiv2023

Astrophysics with the Laser Interferometer Space Antenna

The Laser Interferometer Space Antenna (LISA) will be a transformative experiment for gravitational wave astronomy, and, as such, it will offer unique opportunities to address many key astrophysical questions in a completely novel way. The synergy with ground-based and space-born instruments in the electromagnetic domain, by enabling multi-messenger observations, will add further to the discovery potential of LISA. The next decade is crucial to prepare the astrophysical community for LISA's first observations. This review outlines the extensive landscape of astrophysical theory, numerical simulations, and astronomical observations that are instrumental for modeling and interpreting the upcoming LISA datastream. To this aim, the current knowledge in three main source classes for LISA is reviewed; ultracompact stellar-mass binaries, massive black hole binaries, and extreme or intermediate mass ratio inspirals. The relevant astrophysical processes and the established modeling techniques are summarized. Likewise, open issues and gaps in our understanding of these sources are highlighted, along with an indication of how LISA could help making progress in the different areas. New research avenues that LISA itself, or its joint exploitation with upcoming studies in the electromagnetic domain, will enable, are also illustrated. Improvements in modeling and analysis approaches, such as the combination of numerical simulations and modern data science techniques, are discussed. This review is intended to be a starting point for using LISA as a new discovery tool for understanding our Universe.

gr-qc↗

arXiv2023

An elusive dark central mass in the globular cluster M4

Recent studies of nearby globular clusters have discovered excess dark mass in their cores, apparently in an extended distribution, and simulations indicate that this mass is composed mostly of white dwarfs (respectively stellar-mass black holes) in clusters that are core-collapsed (respectively with a flatter core). We perform mass-anisotropy modelling of the closest globular cluster, M4, with intermediate slope for the inner stellar density. We use proper-motion data from Gaia EDR3 and from observations by the Hubble Space Telescope. We extract the mass profile employing Bayesian Jeans modelling, and check our fits with realistic mock data. Our analyses return isotropic motions in the cluster core and tangential motions ($β\approx -0.4$$\pm$$0.1$) in the outskirts. We also robustly measure a dark central mass of roughly $800\pm300 \,$M$_{\odot}$, but it is not possible to distinguish between a point-like source, such as an intermediate-mass black hole (IMBH), or a dark population of stellar remnants of extent $\approx 0.016\,\rm pc \simeq 3300\,AU$. However, when removing a high-velocity star from the cluster centre, the same mass excess is found, but more extended ($\sim 0.034\, \rm{pc} \approx 7000\,\rm AU$). We use Monte Carlo $N$-body models of M4 to interpret the second outcome, and find that our excess mass is not sufficiently extended to be confidently associated with a dark population of remnants. Finally, we discuss the feasibility of these two scenarios (i.e., IMBH vs. remnants), and propose new observations that could help to better grasp the complex dynamics in M4's core.

astro-ph.GA↗

arXiv2023

Partial Tidal Disruption of Main-Sequence Stars by Intermediate-Mass Black Holes

We study close encounters of a $1\,M_{\odot}$ middle-age main-sequence star (modeled using MESA) with massive black holes through hydrodynamic simulations, and explore in particular the dependence of the outcomes on the black hole mass. We consider here black holes in the intermediate-mass range, $M_{\rm BH}= 100-10^4\,M_{\odot}$. Possible outcomes vary from a small tidal perturbation for weak encounters all the way to partial or full disruption for stronger encounters. We find that stronger encounters lead to increased mass loss at the first pericenter passage, in many cases ejecting the partially disrupted star on an unbound orbit. For encounters that initially produce a bound system, with only partial stripping of the star, the fraction of mass stripped from the star increases with each subsequent pericenter passage and a stellar remnant of finite mass is ultimately ejected in all cases. The critical penetration depth that separates bound and unbound remnants has a dependence on the black hole mass when $M_{\rm BH} \lesssim 10^3\,M_{\odot}$. We also find that the number of successive close passages before ejection decreases as we go from the stellar-mass black hole to the intermediate-mass black hole regime. For instance, after an initial encounter right at the classical tidal disruption limit, a $1\,M_{\odot}$ star undergoes 16 (5) pericenter passages before ejection from a $10\,M_{\odot}$ ($100\,M_{\odot}$) black hole. Observations of periodic flares from these repeated close passages could in principle indicate signatures of a partial tidal disruption event.

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arXiv2023

Probing the efficiency of tidal synchronization in outspiralling double white dwarf binaries with LISA

Compact-object binaries including a white dwarf component are unique among gravitational-wave sources because their evolution is governed not just by general relativity and tides, but also by mass transfer. While the black hole and neutron star binaries observed with ground-based gravitational-wave detectors are driven to inspiral due to the emission of gravitational radiation--manifesting as a "chirp-like" gravitational-wave signal--the astrophysical processes at work in double white dwarf (DWD) systems can cause the inspiral to stall and even reverse into an outspiral. The dynamics of the DWD outspiral thus encode information about tides, which tell us about the behaviour of electron-degenerate matter. We carry out a population study to determine the effect of the strength of tides on the distributions of the DWD binary parameters that the Laser Interferometer Space Antenna (LISA) will be able to constrain. We find that the strength of tidal coupling parameterized via the tidal synchronization timescale at the onset of mass transfer affects the distribution of gravitational-wave frequencies and frequency derivatives for detectably mass-transferring DWD systems. Using a hierarchical Bayesian framework informed by binary population synthesis simulations, we demonstrate how this parameter can be inferred using LISA observations. By measuring the population properties of DWDs, LISA will be able to probe the behavior of electron-degenerate matter.

astro-ph.HE↗

arXiv2023

Stellar Escape from Globular Clusters. I. Escape Mechanisms and Properties at Ejection

The theory of stellar escape from globular clusters (GCs) dates back nearly a century, especially the gradual evaporation of GCs via two-body relaxation coupled with external tides. More violent ejection can also occur via strong gravitational scattering, supernovae, gravitational wave-driven mergers, tidal disruption events, and physical collisions, but comprehensive study of the many escape mechanisms has been limited. Recent exquisite kinematic data from the Gaia space telescope has revealed numerous stellar streams in the Milky Way (MW) and traced the origin of many to specific MWGCs, highlighting the need for further examination of stellar escape from these clusters. In this study, the first of a series, we lay the groundwork for detailed follow-up comparisons between Cluster Monte Carlo (CMC) GC models and the latest Gaia data on the outskirts of MWGCs, their tidal tails, and associated streams. We thoroughly review escape mechanisms from GCs and examine their relative contributions to the escape rate, ejection velocities, and escaper demographics. We show for the first time that three-body binary formation may dominate high-speed ejection from typical MWGCs, potentially explaining some of the hypervelocity stars in the MW. Due to their mass, black holes strongly catalyze this process, and their loss at the onset of observable core collapse, characterized by a steep central brightness profile, dramatically curtails three-body binary formation, despite the increased post-collapse density. We also demonstrate that even when born from a thermal eccentricity distribution, escaping binaries have significantly nonthermal eccentricities consistent with the roughly uniform distribution observed in the Galactic field.

astro-ph.GA↗

arXiv2022

Prospects for Detecting Fast Radio Bursts in Globular Clusters of Nearby Galaxies

The recent detection of a repeating fast radio burst (FRB) in an old globular cluster in M81 challenges traditional FRB formation mechanisms based on magnetic activity in young neutron stars formed recently in core-collapse supernovae. Furthermore, the detection of this repeater in such a nearby galaxy implies a high local universe rate of similar events in globular clusters. Building off the properties inferred from the M81 FRB, we predict the number of FRB sources in nearby ($d \lesssim 20\,$Mpc) galaxies with large globular cluster systems known. Incorporating the uncertain burst energy distribution, we estimate the rate of bursts detectable in these galaxies by radio instruments such as FAST and MeerKat. Of all local galaxies, we find M87 is the best candidate for FRB detections. We predict M87's globular cluster system contains $\mathcal{O}(10)$ FRB sources at present and that a dedicated radio survey (by FAST or MeerKat) of $\mathcal{O}(10)\,$hr has a $90\%$ probability of detecting a globular cluster FRB in M87. The detection of even a handful of additional globular cluster FRBs would provide invaluable constraints on FRB mechanisms and population properties. Previous studies have demonstrated young neutron stars formed following collapse of dynamically-formed massive white dwarf binary mergers may provide the most natural mechanism for these bursts. We explore the white dwarf merger scenario using a suite of $N$-body cluster models, focusing in particular on such mergers in M87 clusters. We describe a number of outstanding features of this scenario that in principle may be testable with an ensemble of observed FRBs in nearby globular clusters.

astro-ph.HE↗

arXiv2022

Hyper-Eddington Black Hole Growth in Star-Forming Molecular Clouds and Galactic Nuclei: Can It Happen?

Formation of supermassive black holes (BHs) remains a theoretical challenge. In many models, especially beginning from stellar relic "seeds," this requires sustained super-Eddington accretion. While studies have shown BHs can violate the Eddington limit on accretion disk scales given sufficient "fueling" from larger scales, what remains unclear is whether or not BHs can actually capture sufficient gas from their surrounding ISM. We explore this in a suite of multi-physics high-resolution simulations of BH growth in magnetized, star-forming dense gas complexes including dynamical stellar feedback from radiation, stellar mass-loss, and supernovae, exploring populations of seeds with masses $\sim 1-10^{4}\,M_{\odot}$. In this initial study, we neglect feedback from the BHs: so this sets a strong upper limit to the accretion rates seeds can sustain. We show that stellar feedback plays a key role. Complexes with gravitational pressure/surface density below $\sim 10^{3}\,M_{\odot}\,{\rm pc^{-2}}$ are disrupted with low star formation efficiencies so provide poor environments for BH growth. But in denser cloud complexes, early stellar feedback does not rapidly destroy the clouds but does generate strong shocks and dense clumps, allowing $\sim 1\%$ of randomly-initialized seeds to encounter a dense clump with low relative velocity and produce runaway, hyper-Eddington accretion (growing by orders of magnitude). Remarkably, mass growth under these conditions is almost independent of initial BH mass, allowing rapid IMBH formation even for stellar-mass seeds. This defines a necessary (but perhaps not sufficient) set of criteria for runaway BH growth: we provide analytic estimates for the probability of runaway growth under different ISM conditions.

astro-ph.GA↗

arXiv2022

Intermediate-mass Black Holes on the Run from Young Star Clusters

The existence of black holes (BHs) with masses in the range between stellar remnants and supermassive BHs has only recently become unambiguously established. GW190521, a gravitational wave signal detected by the LIGO/Virgo Collaboration, provides the first direct evidence for the existence of such intermediate-mass BHs (IMBHs). This event sparked and continues to fuel discussion on the possible formation channels for such massive BHs. As the detection revealed, IMBHs can form via binary mergers of BHs in the "upper mass gap" ($\approx40 -120\,M_{\odot}$). Alternatively, IMBHs may form via the collapse of a very massive star formed through stellar collisions and mergers in dense star clusters. In this study, we explore the formation of IMBHs with masses between $120$ and $500\,M_{\odot}$ in young, massive star clusters using state-of-the-art Cluster Monte Carlo ($\texttt{CMC}$) models. We examine the evolution of IMBHs throughout their dynamical lifetimes, ending with their ejection from the parent cluster due to gravitational radiation recoil from BH mergers, or dynamical recoil kicks from few-body scattering encounters. We find that $ \textit{all}$ of the IMBHs in our models are ejected from the host cluster within the first $\sim 500$ Myr, indicating a low retention probability of IMBHs in this mass range for globular clusters today. We estimate the peak IMBH merger rate to be $\mathcal{R} \approx 2 \, \rm{Gpc}^{-3}\,\rm{yr}^{-1}$ at redshift $z \approx 2$.

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