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.