Abstract

The processes that govern the formation of intermediate-mass black holes (IMBHs) in dense stellar clusters are still unclear. In this talk I will discuss the role of stellar mergers, star-BH interactions and accretion, as well as BH binary (BBH) mergers in seeding and growing IMB Hs in the Dragon-II simulation database, a suite of 19 direct N-body models representing dense clusters with up to 10^{6 stars. We will discuss mass and spin of IMB Hs in Dragon-II simulations, showing that there is a link between the route to IMBH formation and the host cluster structure. In clusters denser than 3 × 10}5 Msun pc^{-3, the collapse of massive star collision products represents the dominant IMBH formation process, leading to the formation of heavy IMB Hs (mIMBH > 200 Msun ), 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 Msun. Strong dynamical scatterings and relativistic recoil efficiently eject all IMB Hs in Dragon-II clusters, suggesting that IMB Hs in this type of clusters are unlikely to grow beyond a few 10}2 Msun.