The dragon-II simulations – II. Formation mechanisms, mass, and spin of intermediate-mass black holes in star clusters with up to 1 million stars

Abstract

ABSTRACT The processes that govern the formation of intermediate-mass black holes (IMBHs) in dense stellar clusters are still unclear. Here, we discuss the role of stellar mergers, star–BH interactions, and accretion, as well as BH binary (BBH) mergers in seeding and growing IMBHs in the Dragon-II simulation database, a suite of 19 direct N-body models representing dense clusters with up to 106 stars. Dragon-II IMBHs have typical masses of mIMBH = (100–380) M⊙ and relatively large spins χIMBH > 0.6. We find a link between the IMBH formation mechanism and the cluster structure. In clusters denser than 3 × 105 M⊙ pc−3, the collapse of massive star collision products represents the dominant IMBH formation process, leading to the formation of heavy IMBHs (mIMBH > 200 M⊙), possibly slowly rotating, that form over times <5 Myr and grow further via stellar accretion and mergers in just <30 Myr. BBH mergers are the dominant IMBH formation channel in less dense clusters, for which we find that the looser the cluster, the longer the formation time (10–300 Myr) and the larger the IMBH mass, although remaining within 200 M⊙. Strong dynamical scatterings and relativistic recoil efficiently eject all IMBHs in Dragon-II clusters, suggesting that IMBHs in this type of cluster are unlikely to grow beyond a few 102 M⊙.