The mass budget for intermediate-mass black holes in dense star clusters

Yanlong Shi; Michael Y. Grudić; Philip F. Hopkins

doi:10.1093/mnras/stab1470

paper

The mass budget for intermediate-mass black holes in dense star clusters

arXiv:2008.12290 · doi:10.1093/mnras/stab1470

Abstract

Intermediate-mass black holes (IMBHs) could form via runaway merging of massive stars in a young massive star cluster (YMC). We combine a suite of numerical simulations of YMC formation with a semi-analytic model for dynamical friction and merging of massive stars and evolution of a central quasi-star, to predict how final quasi-star and relic IMBH masses scale with cluster properties (and compare with observations). The simulations argue that inner YMC density profiles at formation are steep (approaching isothermal), producing some efficient merging even in clusters with relatively low effective densities, unlike models which assume flat central profiles resembling those of globular clusters (GCs) {\em after} central relaxation. Our results can be approximated by simple analytic scalings, with $M_{IMBH} \propto v_{cl}^{3/2}$ where $v_{cl}^{2} = G M_{cl} / r_{h}$ is the circular velocity in terms of initial cluster mass $M_{cl}$ and half-mass radius $r_{h}$ . While this suggests IMBH formation is {\em possible} even in typical clusters, we show that predicted IMBH masses for these systems are small, $\sim 100 - 1000 M_{⊙}$ or $\sim 0.0003 M_{cl}$ , below even the most conservative observational upper limits in all known cases. The IMBH mass could reach $≳ 1 0^{4} M_{⊙}$ in the centers nuclear star clusters, ultra-compact dwarfs, or compact ellipticals, but in all these cases the prediction remains far below the present observed supermassive BH masses in these systems.

arXiv comment10 pages, 6 figures. Comments welcomed!

The mass budget for intermediate-mass black holes in dense star clusters

Abstract

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