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

TL;DR

This study uses the Dragon-II simulation database to investigate how intermediate-mass black holes form, grow, and are ejected in dense star clusters, revealing different dominant formation channels based on cluster density.

Contribution

It provides the first detailed analysis of IMBH formation mechanisms, masses, and spins in large-scale N-body simulations of star clusters with up to one million stars.

Findings

Dense clusters (>3x10^5 M_sun/pc^3) form heavy IMBHs via stellar collisions within 5 Myr.

Less dense clusters produce IMBHs mainly through binary black hole mergers over 10-300 Myr.

IMBHs are often ejected due to dynamical interactions, limiting their growth beyond a few hundred solar masses.

Abstract

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. In clusters denser than $3\times 10^5$ M$_\odot$ pc$^{-3}$, the collapse of massive star collision products represents the dominant IMBH formation process, leading to the formation of heavy IMBHs ($m_{\rm IMBH} > 200$ M$_\odot$), 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$_\odot$. Strong dynamical scatterings and relativistic recoil efficiently eject all IMBHs in \textsc{Dragon-II} clusters, suggesting that IMBHs in this type of cluster are unlikely to grow beyond a few $10^2$ M$_\odot$.