Seeds to success: growing heavy black holes in dense star clusters

TL;DR

This study investigates the formation of intermediate-mass black holes in star clusters, emphasizing stellar collisions as a key process, and uses advanced simulations to explore different formation scenarios and their observational implications.

Contribution

It introduces a new version of the B-POP code to model IMBH formation via multiple mechanisms and assesses their efficiencies across various star cluster types.

Findings

Stellar collisions are identified as the dominant IMBH formation channel.

The simulations suggest wandering IMBHs may exist in Milky Way-like galaxies.

Correlations between cluster and IMBH masses can help determine their origins.

Abstract

The observational dearth of black holes (BHs) with masses between $\sim$100 and 100,000 $M_\odot$ raises questions about the nature of intermediate-mass black holes (IMBHs). Proposed formation channels for IMBHs include runaway stellar collisions and repeated binary BH (BBH) mergers driven by dynamical interactions in stellar clusters, but the formation efficiency of these processes and the associated IMBH occupation fraction are largely unconstrained. In this work, we study IMBH formation via both mechanisms in young, globular, and nuclear star clusters. We carry out a comprehensive investigation of IMBH formation efficiency by exploring the impact of different seeding models and star cluster formation histories. We employ a new version of the B-POP population synthesis code, able to model several seeding mechanisms as well as hierarchical BBH mergers. We quantify the efficiency of IMBH production across different cluster families, and estimate the fraction of BBH mergers involving an IMBH primary. Comparison with low-redshift IMBH candidates suggests that, depending on the seeding mechanism, stellar collisions can play a pivotal role in explaining potential IMBHs in local globular clusters. Our simulations highlight stellar collisions as the primary IMBH formation channel across a wide range of cluster types. They further suggest that wandering IMBHs may populate Milky Way-like galaxies and that correlations between cluster and IMBH masses can help distinguish the origins of Galactic globular clusters.