Black hole mergers in compact star clusters and massive black hole formation beyond the mass-gap

Abstract

We present direct N-body simulations, carried out with Nbody6++GPU, of young and compact low metallicity star clusters with $1.1\times 10^5$ stars, a velocity dispersion of $\sim$ 10 $\mathrm{km}{\mathrm{s}}^{-1}$, a half mass radius $R_h=0.6$ pc, and a binary fraction of $10\%$ including updated evolution models for stellar winds and pair-instability supernovae (PISNe). Within the first tens of megayears of evolution, each cluster hosts several black hole (BH) merger events which nearly cover the complete mass range of primary and secondary BH masses for current LIGO/Virgo/Kagra gravitational wave detections. The importance of gravitational recoil is estimated statistically. We present several possible formation paths of massive BHs above the assumed lower PISNe mass-gap limit ($45M_{\odot }$) into the intermediate-mass BH (IMBH) regime ($>100M_{\odot }$) which include collisions of stars and BHs as well as the direct collapse of stellar merger remnants with low mass cores. The stellar evolution updates result in the early formation of higher mass stellar BHs than for the previous model. The resulting higher collision rates with massive stars support the rapid formation of massive BHs. For models assuming a high accretion efficiency for star-BH mergers, we present a first-generation formation scenario for GW190521-like events, a merger of two BHs in the PISN mass-gap, which is dominated by star-BH mergers. This IMBH formation path is independent of gravitational recoil and therefore conceivable in dense stellar systems with low escape velocities. One simulated cluster even forms an IMBH binary (153$M_{\odot }$,173$M_{\odot }$) which is expected to merge within a Hubble time.

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Cited by in corpus (12)

• The Formation of Intermediate Mass Black Holes in Galactic Nuclei
• The growth of intermediate mass black holes through tidal captures and tidal disruption events
• The impact of stellar evolution on rotating star clusters: the gravothermal-gravogyro catastrophe and the formation of a bar of black holes
• Isolated and dynamical black hole mergers with \texttt{B-POP}: the role of star formation and dynamics, star cluster evolution, natal kicks, mass and spins, and hierarchical mergers
• Formation of black holes in the pair-instability mass gap: Hydrodynamical simulations of a head-on massive star collision
• Global instability by runaway collisions in nuclear stellar clusters: Numerical tests of a route for massive black hole formation
• Binary black hole mergers from young massive clusters in the pair-instability supernova mass gap
• BIFROST: simulating compact subsystems in star clusters using a hierarchical fourth-order forward symplectic integrator code
• Study on the detectability of gravitational radiation from single-binary encounters between black holes in nuclear star cluster: the case of hyperbolic flybys
• Astrophysics with the Laser Interferometer Space Antenna
• MOCCA-Survey Database: Extra Galactic Globular Clusters. III. The population of black holes in Milky Way and Andromeda-like galaxies
• Hydrodynamics of Collisions and Close Encounters between Stellar Black Holes and Main-sequence Stars