The Close Stellar Companions to Intermediate Mass Black Holes

Contrary to supermassive and stellar-mass black holes (SBHs), the existence of intermediate-mass black holes (IMBHs) with masses ranging between 10 2−4 M ⊙ has not yet been confirmed. The main problem in the detection is that the innermost stellar kinematics of globular clusters (GCs), the natural loci to IMBHs, are very difficult to resolve. However, if IMBHs reside in the center of GCs, a possibility is that they interact dynamically with their enviroment. A binary formed with the IMBH and a compact object of the GC would naturally lead to a prominent source of gravitational radiation, detectable with future observatories. We run for the first time direct-summation integrations of GCs with an IMBH including the dynamical evolution of the IMBH with the stellar system and relativistic effects, such as energy loss in gravitational waves (GWs) and periapsis shift, and gravitational recoil. We find in one of our models an intermediate-mass ratio inspiral (IMRI), which leads to a merger with a recoiling velocity higher than the escape velocity of the GC. The GWs emitted fall in the range of frequencies that a LISA-like observatory could detect, like the European eLISA or in mission options considered in the recent preliminary mission study conducted in China. The merger has an impact on the global dynamics of the cluster, as an important heating source is removed when the merged system leaves the GC. The detection of one IMRI would constitute a test of GR, as well as an irrefutable proof of the existence of IMBHs.

2014

In this paper, we address the question: What is the probability of stellar-mass black hole (BH) binaries co-existing in a globular cluster with an intermediate-mass black hole (IMBH)? Our results suggest that the detection of one or more BH binaries can strongly constrain the presence of an IMBH in most Galactic globular clusters. More specifically, the detection of one or more BH binaries could strongly indicate against the presence of an IMBH more massive than 10 3 M ⊙ in roughly 80% of the clusters in our sample. To illustrate this, we use a combination of N -body simulations and analytic methods to weigh the rate of formation of BH binaries against their ejection and/or disruption rate via strong gravitational interactions with the central (most) massive BH.

arXiv (Cornell University), 2022

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" (≈ 40-120 M). 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 in young, massive star clusters using state-of-the-art Cluster Monte Carlo (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 all of the IMBHs in our models are ejected from the host cluster within the first ∼ 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 R ≈ 2 Gpc −3 yr −1 at redshift z ≈ 2.

2015

The study of intermediate-mass black holes (IMBHs) is a young and promising field of research. Formed by runaway collisions of massive stars in young and dense stellar clusters, intermediate-mass black holes could still be present in the centers of globular clusters, today. Our group investigated the presence of intermediate-mass black holes for a sample of 10 Galactic globular clusters. We measured the inner kinematic profiles with integral-field spectroscopy and determined masses or upper limits of central black holes in each cluster. In combination with literature data we further studied the positions of our results on known black-hole scaling relations (such as M_bh - sigma) and found a similar but flatter correlation for IMBHs. Applying cluster evolution codes, the change in the slope could be explained with the stellar mass loss occurring in clusters in a tidal field over its life time. Furthermore, we present results from several numerical simulations on the topic of IMBHs and integral field units (IFUs). We ran N-body simulations of globular clusters containing IMBHs in a tidal field and studied their effects on mass-loss rates and remnant fractions and showed that an IMBH in the center prevents core collapse and ejects massive objects more rapidly. These simulations were further used to simulate IFU data cubes. For the specific case of NGC 6388 we simulated two different IFU techniques and found that velocity dispersion measurements from individual velocities are strongly biased towards lower values due to blends of neighbouring stars and background light. In addition, we use the Astrophysical Multipurpose Software Environment (AMUSE) to combine gravitational physics, stellar evolution and hydrodynamics to simulate the accretion of stellar winds onto a black hole.

Open Astronomy, 2018

In this paper, we address the question of how a central intermediate-mass black hole (IMBH) in a globular cluster (GC) affects dynamics, core collapse, and formation of the binary population. It is shown that the central IMBH forms a binary system that affects dynamics of stars in the cluster significantly. The presence of an intermediate-mass black hole with mass ≥ 1.0-1.7%of the total stellar mass in the cluster inhibits the formation of binary stars population.

Monthly Notices of the Royal Astronomical Society, 2010

We study the dynamics of stellar-mass black holes (BH) in star clusters with particular attention to the formation of BH-BH binaries, which are interesting as sources of gravitational waves (GW). In the present study, we examine the properties of these BH-BH binaries through direct N-body simulations of star clusters using the NBODY6 code on Graphical Processing Unit (GPU) platforms. We perform simulations for star clusters with 10 5 low-mass stars starting from Plummer models with an initial population of BHs, varying the cluster-mass and BH-retention fraction. Additionally, we do several calculations of star clusters confined within a reflective boundary mimicking only the core of a massive star cluster which can be performed much faster than the corresponding full cluster integration. We find that stellar-mass BHs with masses ∼ 10M ⊙ segregate rapidly (∼ 100 Myr timescale) into the cluster core and form a dense sub-cluster of BHs within typically 0.2 − 0.5 pc radius. In such a subcluster, BH-BH binaries can be formed through 3-body encounters, the rate of which can become substantial in dense enough BH-cores. While most BH binaries are finally ejected from the cluster by recoils received during super-elastic encounters with the single BHs, few of them harden sufficiently so that they can merge via GW emission within the cluster. We find that for clusters with N 5 × 10 4 , typically 1 -2 BH-BH mergers occur per cluster within the first ∼ 4 Gyr of cluster evolution. Also for each of these clusters, there are a few escaping BH binaries that can merge within a Hubble time, most of the merger times being within a few Gyr. These results indicate that intermediate-age massive clusters constitute the most important class of candidates for producing dynamical BH-BH mergers. Old globular clusters cannot contribute significantly to the present-day BH-BH merger rate since most of the mergers from them would have occurred much earlier. On the other hand, young massive clusters with ages less that 50 Myr are too young to produce significant number of BH-BH mergers. We finally discuss the detection rate of BH-BH inspirals by the "LIGO" and "Advanced LIGO" GW detectors. Our results indicate that dynamical BH-BH binaries constitute the dominant channel for BH-BH merger detection.

Monthly Notices of the Royal Astronomical Society, 2020

Young dense massive star clusters are promising environments for the formation of intermediate mass black holes (IMBHs) through collisions. We present a set of 80 simulations carried out with nbody6++gpu of 10 models of compact $\sim 7 \times 10^4 \, \mathrm{M}_{\odot }$ star clusters with half-mass radii Rh ≲ 1 pc, central densities $\rho _\mathrm{core} \gtrsim 10^5 \, \mathrm{M}_\odot \, \mathrm{pc}^{-3}$, and resolved stellar populations with 10 per cent primordial binaries. Very massive stars (VMSs) up to $\sim 400 \, \mathrm{M}_\odot$ grow rapidly by binary exchange and three-body scattering with stars in hard binaries. Assuming that in VMS–stellar black hole (BH) collisions all stellar material is accreted on to the BH, IMBHs with masses up to $M_\mathrm{BH} \sim 350 \, \mathrm{M}_\odot$ can form on time-scales of ≲15 Myr, as qualitatively predicted from Monte Carlo mocca simulations. One model forms an IMBH of 140 $\mathrm{M_{\odot }}$ by three BH mergers with masses of 17:28...

The Astrophysical Journal, 2018

There has been suggestive evidence of intermediate-mass black holes (IMBHs; 10 3−5 M e) existing in some globular clusters (GCs) and dwarf galaxies, but IMBHs as a population remain elusive. As a main-sequence star passes too close by an IMBH it might be tidally captured and disrupted. We study the long-term accretion and observational consequence of such tidal disruption events. The disruption radius is hundreds to thousands of the BH's Schwarzschild radius, so the circularization of the falling-back debris stream is very inefficient due to weak general relativity effects. Due to this and a high mass fallback rate, the bound debris initially goes through a ∼10 yr long super-Eddington accretion phase. The photospheric emission of the outflow ejected during this phase dominates the observable radiation and peaks in the UV/optical bands with a luminosity of 10 erg s 42 1-. After the accretion rate drops below the Eddington rate, the bolometric luminosity follows the conventional t −5/3 powerlaw decay, and X-rays from the inner accretion disk start to be seen. Modeling the newly reported IMBH tidal disruption event candidate 3XMM J2150-0551, we find a general consistency between the data and predictions. The search for these luminous, long-term events in GCs and nearby dwarf galaxies could unveil the IMBH population.

The Astrophysical Journal, 2008

In many theoretical scenarios it is expected that intermediate-mass black holes (IMBHs, with masses M ∼ 10 2−4 M ) reside at the centers of some globular clusters. However, observational evidence for their existence is limited. Several previous numerical investigations have focused on the impact of an IMBH on the cluster dynamics or brightness profile. Here we instead present results from a large set of direct N-body simulations including single and binary stars. These show that there is a potentially more detectable IMBH signature, namely on the variation of the average stellar mass between the center and the half-light radius. We find that the existence of an IMBH quenches mass segregation and causes the average mass to exhibit only modest radial variation in collisionally relaxed star clusters. This differs from when there is no IMBH. To measure this observationally requires high resolution imaging at the level of that already available from the Hubble Space Telescope (HST) for the cores of a large sample of galactic globular clusters. With a modest additional investment of HST time to acquire fields around the half-light radius, it will be possible to identify the best candidate clusters to harbor an IMBH. This test can be applied only to globulars with a half-light relaxation time 1 Gyr, which is required to guarantee efficient energy equipartition due to two-body relaxation.

Astronomy & Astrophysics

Context. We recently introduced a new method for simulating collisional gravitational N-body systems with approximately linear time scaling with N. Our method is based on the multi-particle collision (MPC) scheme, previously applied in fluid dynamics and plasma physics. We were able to simulate globular clusters with a realistic number of stellar particles (at least up to several times 106) on a standard workstation. Aims. We simulated clusters hosting an intermediate mass black hole (IMBH), probing a broad range of BH-cluster and BH–average-star mass ratios, unrestricted by the computational constraints that affect direct N-body codes. Methods. We set up a grid of hybrid particle-in-cell-MPC N-body simulations using our implementation of the MPC method, MPCDSS. We used either single mass models or models with a Salpeter mass function (a single power law with an exponent of −2.35), with the IMBH initially sitting at the centre. The force exerted by and on the IMBH was evaluated with...