Forming intermediate mass black holes in dense hierarchically assembling star clusters

by Dr Antti Rantala (Cantab/KICC, UK)

Wednesday 21 Jan 2026, 13:30 → 14:30 Europe/Stockholm

FC61 (AlbaNova Main Building)

We explore the formation of intermediate mass black holes (IMBHs), potential seeds for supermassive black holes (SMBHs), via runaway stellar collisions for a wide range of star cluster densities and metallicities. Our sample of isolated (>1400) and hierarchical (30) simulations of young, massive star clusters with up to 1.8 million stars includes collisional stellar dynamics, stellar evolution, and post-Newtonian equations of motion for black holes using the BIFROST code. High stellar wind rates suppress IMBH formation at high metallicities (above 0.2 of the solar value) and low collision rates prevent their formation at low densities. The assumptions about stellar wind loss rates strongly affect the final IMBH masses (6000 vs 25000 solar masses). We present fitting formulae for IMBH masses as a function of host star cluster properties, and formulate a simple model for the cosmic IMBH formation rate density. Depending on the cluster densities, the IMBH formation rates peak at z~2–4 at up to ∼1e-7 per yr per cMpc³. As more than 50% form below z < 1.5-3, the model challenges a view in which all local IMBHs are failed early Universe SMBH seeds.