Massive black hole assembly in nuclear star clusters

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(circle dot). 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(circle dot) pc(-3). A million solar mass black hole can form within 100 Myr in the heaviest (> 10(7)M(circle dot)) and most compact (< 0.5 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.