Massive black hole assembly in nuclear star clusters

TL;DR

This study models the formation of intermediate-mass black holes in nuclear star clusters, showing that gas-rich environments and high densities favor their rapid assembly through stellar mergers and accretion.

Contribution

It introduces a semianalytic model with new recipes for stellar collisions and gas processes, revealing conditions for black hole formation in nuclear clusters.

Findings

Black hole formation is almost inevitable in dense, gas-rich clusters.

A 10^6 solar mass black hole can form within 100 Myr in compact, massive clusters.

Potential gravitational-wave signatures are identified from the black hole formation process.

Abstract

Nuclear star clusters, which fragment into metal-poor stars in situ at the centers of protogalaxies, provide ideal environments for the formation of intermediate-mass black holes with masses $10^3-10^6M_\odot$. We utilize the semianalytic model implemented in Rapster, a public rapid cluster evolution code. We implement simple recipes for stellar collisions and gas accretion/expulsion into the code and identify the regimes where each channel contributes to the dynamical formation of intermediate-mass black holes via repeated mergers of stellar black hole seeds. We find that intermediate-mass black hole formation in gas-rich environments is almost inevitable if the initial mean density of the nuclear cluster is $>10^8M_\odot\,{\rm pc}^{-3}$. A million solar mass black hole can form within 100~Myr in the heaviest ($>10^7M_\odot$) and most compact ($<0.5~{\rm pc}$) nuclear clusters. We demonstrate that by today these resemble the observed range of nuclear clusters in dwarf galaxies and that there are potential gravitational-wave signatures of the massive black hole formation process.