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

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 10(6) stars. DRAGON-II IMBHs have typical masses of m(IMBH) = (100-380) M-circle dot and relatively large spins.IMBH > 0.6. We find a link between the IMBH formation mechanism and the cluster structure. In clusters denser than 3 x 10(5) M-circle dot pc(-3), the collapse of massive star collision products represents the dominant IMBH formation process, leading to the formation of heavy IMBHs (m(IMBH) > 200 M-circle dot), 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-circle dot. 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 10(2) M-circle dot.

Automated summary

The processes that govern the formation of intermediate-mass black holes (IMBHs) in dense stellar clusters involve stellar mergers, star-BH interactions, accretion, and BH binary (BBH) mergers. The formation mechanism of IMBHs is related to the density of the cluster. In denser clusters, the collapse of massive star collision products is the dominant process, while in less dense clusters, BBH mergers are the main channel for IMBH formation.