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

Understand

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 \textsc{Dragon-II} simulation database, a suite of 19 direct $N$-body models representing dense clusters with up to $10^6$ stars.
• \textsc{Dragon-II} IMBHs have typical masses of $m_{\rm IMBH} = (100-380)$ M$_\odot$ and relatively large spins $\chi_{\rm IMBH} > 0.6$.
• We find a link between the IMBH formation mechanism and the cluster structure.