Intermediate mass black holes in young star clusters

Intermediate mass black holes (IMBHs) in the mass range 10²-10⁵ Msun are thought to be the missing link between stellar black holes (BHs) and supermassive BHs. Theoretical models have predicted their formation via hierarchical binary black hole (BBH) mergers and via runaway collisions. Astronomers have sought them for decades, via radio, optical and X-ray observations, finding only a small bunch of strong candidates. The recent detection of GW190521, with a final remnant mass of 142 Msun (Abbott et al. 2019), has revived the hunt for IMBHs in the astrophysics community: future gravitational wave data from ground-based and space-borne detectors will give us the opportunity to unveil the properties of such elusive BHs. In this talk, I will discuss the possibility that IMBHs form in young star clusters via runaway collision and BBH mergers. I will present the results of 10⁴ simulations of young dense massive clusters. Only 9 IMBHs form in our simulations via hierarchical BBH mergers, with a mass ~100-140 Msun. This channel is strongly suppressed by the low escape velocity of our star clusters. In contrast, IMBHs with masses up to ~440 Msun efficiently form via runaway collisions, especially at low metallicity. Up to 0.4% of all simulated BHs are IMBHs, depending on progenitor's metallicity. The runaway formation channel is strongly suppressed in metal-rich (Z=0.02) star clusters, because of stellar winds. IMBHs are extremely efficient in pairing with other black holes: ~70% of them are members of a BBH at the end of the simulations.