The DRAGON-II simulations - II. Formation mechanisms, mass, and spin of intermediate-mass black holes in star clusters with up to 1 million stars
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 DRAGON-II simulation database, a suite of 19 direct N-body models representing dense clusters with up to 106 stars. DRAGON-II IMBHs have typical masses of mIMBH = (100-380) M⊙ and relatively large spins χIMBH > 0.6. We find a link between the IMBH formation mechanism and the cluster structure. In clusters denser than 3 × 105 M⊙ pc-3, the collapse of massive star collision products represents the dominant IMBH formation process, leading to the formation of heavy IMBHs (mIMBH > 200 M⊙), 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⊙. Strong dynamical scatterings and relativistic recoil efficiently eject all IMBHs in DRAGON-II clusters, suggesting that IMBHs in this type of cluster are unlikely to grow beyond a few 102 M⊙.
- Publication:
-
Monthly Notices of the Royal Astronomical Society
- Pub Date:
- November 2023
- DOI:
- 10.1093/mnras/stad2292
- arXiv:
- arXiv:2307.04806
- Bibcode:
- 2023MNRAS.526..429A
- Keywords:
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- methods: numerical;
- galaxies: star clusters: general;
- stars: general;
- black holes;
- Astrophysics - Astrophysics of Galaxies
- E-Print:
- 15 pages, 6 figures, 2 tables, 1 appendix. Comments welcome. Submitted to MNRAS