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1. Repeated Mergers, Mass-gap Black Holes, and Formation of Intermediate-mass Black Holes in Dense Massive Star Clusters

   https://doi.org/10.3847/1538-4357/ac5026  

   Fragione, Giacomo ; Kocsis, Bence ; Rasio, Frederic A. ; Silk, Joseph ( March 2022 , The Astrophysical Journal)

   Abstract Current theoretical models predict a mass gap with a dearth of stellar black holes (BHs) between roughly 50 M ⊙ and 100 M ⊙ , while above the range accessible through massive star evolution, intermediate-mass BHs (IMBHs) still remain elusive. Repeated mergers of binary BHs, detectable via gravitational-wave emission with the current LIGO/Virgo/Kagra interferometers and future detectors such as LISA or the Einstein Telescope, can form both mass-gap BHs and IMBHs. Here we explore the possibility that mass-gap BHs and IMBHs are born as a result of successive BH mergers in dense star clusters. In particular, nuclear star clusters at the centers of galaxies have deep enough potential wells to retain most of the BH merger products after they receive significant recoil kicks due to anisotropic emission of gravitational radiation. Using for the first time simulations that include full stellar evolution, we show that a massive stellar BH seed can easily grow to ∼10 3 –10 4 M ⊙ as a result of repeated mergers with other smaller BHs. We find that lowering the cluster metallicity leads to larger final BH masses. We also show that the growing BH spin tends to decrease in magnitude with the number ofmore »mergers so that a negative correlation exists between the final mass and spin of the resulting IMBHs. Assumptions about the birth spins of stellar BHs affect our results significantly, with low birth spins leading to the production of a larger population of massive BHs.« less
2. Evolutionary roads leading to low effective spins, high black hole masses, and O1/O2 rates for LIGO/Virgo binary black holes

   https://doi.org/10.1051/0004-6361/201936528  

   Belczynski, K. ; Klencki, J. ; Fields, C. E. ; Olejak, A. ; Berti, E. ; Meynet, G. ; Fryer, C. L. ; Holz, D. E. ; O’Shaughnessy, R. ; Brown, D. A. ; et al ( April 2020 , Astronomy & Astrophysics)

   All ten LIGO/Virgo binary black hole (BH-BH) coalescences reported following the O1/O2 runs have near-zero effective spins. There are only three potential explanations for this. If the BH spin magnitudes are large, then: (i) either both BH spin vectors must be nearly in the orbital plane or (ii) the spin angular momenta of the BHs must be oppositely directed and similar in magnitude. Then there is also the possibility that (iii) the BH spin magnitudes are small. We consider the third hypothesis within the framework of the classical isolated binary evolution scenario of the BH-BH merger formation. We test three models of angular momentum transport in massive stars: a mildly efficient transport by meridional currents (as employed in the Geneva code), an efficient transport by the Tayler-Spruit magnetic dynamo (as implemented in the MESA code), and a very-efficient transport (as proposed by Fuller et al.) to calculate natal BH spins. We allow for binary evolution to increase the BH spins through accretion and account for the potential spin-up of stars through tidal interactions. Additionally, we update the calculations of the stellar-origin BH masses, including revisions to the history of star formation and to the chemical evolution across cosmic time. Wemore »find that we can simultaneously match the observed BH-BH merger rate density and BH masses and BH-BH effective spins. Models with efficient angular momentum transport are favored. The updated stellar-mass weighted gas-phase metallicity evolution now used in our models appears to be key for obtaining an improved reproduction of the LIGO/Virgo merger rate estimate. Mass losses during the pair-instability pulsation supernova phase are likely to be overestimated if the merger GW170729 hosts a BH more massive than 50  M ⊙ . We also estimate rates of black hole-neutron star (BH-NS) mergers from recent LIGO/Virgo observations. If, in fact. angular momentum transport in massive stars is efficient, then any (electromagnetic or gravitational wave) observation of a rapidly spinning BH would indicate either a very effective tidal spin up of the progenitor star (homogeneous evolution, high-mass X-ray binary formation through case A mass transfer, or a spin- up of a Wolf-Rayet star in a close binary by a close companion), significant mass accretion by the hole, or a BH formation through the merger of two or more BHs (in a dense stellar cluster).« less
3. The Redshift Evolution of the Binary Black Hole Merger Rate: A Weighty Matter

   https://doi.org/10.3847/1538-4357/ac64a3  

   van Son, L. A. C. ; de Mink, S. E. ; Callister, T. ; Justham, S. ; Renzo, M. ; Wagg, T. ; Broekgaarden, F. S. ; Kummer, F. ; Pakmor, R. ; Mandel, I. ( May 2022 , The Astrophysical Journal)

   Gravitational-wave detectors are starting to reveal the redshift evolution of the binary black hole (BBH) merger rate,R_BBH(z). We make predictions forR_BBH(z) as a function of black hole mass for systems originating from isolated binaries. To this end, we investigate correlations between the delay time and black hole mass by means of the suite of binary population synthesis simulations,COMPAS. We distinguish two channels: the common envelope (CE), and the stable Roche-lobe overflow (RLOF) channel, characterized by whether the system has experienced a common envelope or not. We find that the CE channel preferentially produces BHs with masses below about 30M_⊙and short delay times (t_delay≲ 1 Gyr), while the stable RLOF channel primarily forms systems with BH masses above 30M_⊙and long delay times (t_delay≳ 1 Gyr). We provide a new fit for the metallicity-dependent specific star formation rate density based on the Illustris TNG simulations, and use this to convert the delay time distributions into a prediction ofR_BBH(z). This leads to a distinct redshift evolution ofR_BBH(z) for high and low primary BH masses. We furthermore find that, at high redshift,R_BBH(z) is dominated by the CE channel, while at low redshift, it contains a large contribution (∼40%) from the stable RLOF channel.more »Our results predict that, for increasing redshifts, BBHs with component masses above 30M_⊙will become increasingly scarce relative to less massive BBH systems. Evidence of this distinct evolution ofR_BBH(z) for different BH masses can be tested with future detectors.

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4. Massive black hole binary inspiral and spin evolution in a cosmological framework

   https://doi.org/10.1093/mnras/staa3826  

   Sayeb, Mohammad ; Blecha, Laura ; Kelley, Luke Zoltan ; Gerosa, Davide ; Kesden, Michael ; Thomas, July ( January 2021 , Monthly Notices of the Royal Astronomical Society)

   ABSTRACT Massive black hole (MBH) binary inspiral time-scales are uncertain, and their spins are even more poorly constrained. Spin misalignment introduces asymmetry in the gravitational radiation, which imparts a recoil kick to the merged MBH. Understanding how MBH binary spins evolve is crucial for determining their recoil velocities, their gravitational wave (GW) waveforms detectable with Laser Interferometer Space Antenna, and their retention rate in galaxies. Here, we introduce a sub-resolution model for gas- and gravitational wave (GW)-driven MBH binary spin evolution using accreting MBHs from the Illustris cosmological hydrodynamic simulations. We also model binary inspiral via dynamical friction, stellar scattering, viscous gas drag, and GW emission. Our model assumes that the circumbinary disc always removes angular momentum from the binary. It also assumes differential accretion, which causes greater alignment of the secondary MBH spin in unequal-mass mergers. We find that 47 per cent of the MBHs in our population merge by z = 0. Of these, 19 per cent have misaligned primaries and 10 per cent have misaligned secondaries at the time of merger in our fiducial model with initial eccentricity of 0.6 and accretion rates from Illustris. The MBH misalignment fraction depends strongly on the accretion disc parameters, however. Reducing accretion rates by a factor ofmore »100, in a thicker disc, yields 79 and 42 per cent misalignment for primaries and secondaries, respectively. Even in the more conservative fiducial model, more than 12 per cent of binaries experience recoils of >500 km s−1, which could displace them at least temporarily from galactic nuclei. We additionally find that a significant number of systems experience strong precession.« less
5. Merger Rates of Intermediate-mass Black Hole Binaries in Nuclear Star Clusters

   https://doi.org/10.3847/1538-4357/ac75d0  

   Fragione, Giacomo ; Loeb, Abraham ; Kocsis, Bence ; Rasio, Frederic A. ( July 2022 , The Astrophysical Journal)

   Abstract Repeated mergers of stellar-mass black holes in dense star clusters can produce intermediate-mass black holes (IMBHs). In particular, nuclear star clusters at the centers of galaxies have deep enough potential wells to retain most of the black hole (BH) merger products, in spite of the significant recoil kicks due to anisotropic emission of gravitational radiation. These events can be detected in gravitational waves, which represent an unprecedented opportunity to reveal IMBHs. In this paper, we analyze the statistical results of a wide range of numerical simulations, which encompass different cluster metallicities, initial BH seed masses, and initial BH spins, and we compute the merger rate of IMBH binaries. We find that merger rates are in the range 0.01–10 Gpc −3 yr −1 depending on IMBH masses. We also compute the number of multiband detections in ground-based and space-based observatories. Our model predicts that a few merger events per year should be detectable with LISA, DECIGO, Einstein Telescope (ET), and LIGO for IMBHs with masses ≲1000 M ⊙ , and a few tens of merger events per year with DECIGO, ET, and LIGO only.