Massive black hole assembly in nuclear star clusters
Konstantinos Kritos, Emanuele Berti, Joseph Silk
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 ${10}^{3}--{10}^{6}{M}_{\ensuremath{\bigodot}}$. 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 $>{10}^{8}{M}_{\ensuremath{\bigodot}}\text{ }\text{ }{\mathrm{pc}}^{\ensuremath{-}3}$. A million solar mass black hole can form within 100 Myr in the heaviest ($>{10}^{7}{M}_{\ensuremath{\bigodot}}$) and most compact ($<0.5\text{ }\text{ }\mathrm{pc}$) 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.