Formation and growth of intermediate-mass black holes in dense star clusters: Lessons from N-body and MOCCA Monte Carlo Simulations

Abstract

Dense star clusters are promising nurseries for the formation and growth of intermediate-mass black holes (IMBHs; $\sim 10^2-10^5{\mathrm{M}}_{\odot }$), with increasing observational evidence pointing to their presence in massive star clusters and stripped dwarf-galaxy nuclei. During the early evolution of compact clusters, massive stars can rapidly segregate to the center, where frequent collisions may trigger the runaway growth of a very massive star (VMS). This object can subsequently collapse to form an IMBH or merge with a stellar-mass black hole. We carried out direct $N$-body and Monte Carlo simulations of star clusters with initial core densities between $10^6$ to $4\times 10^8{\mathrm{M}}_{\odot }{\mathrm{pc}}^{-3}$ and total masses of $5.9\times 10^5$ and $1.3\times 10^6{\mathrm{M}}_{\odot }$. These models show that IMBHs of $10^3-10^4{\mathrm{M}}_{\odot }$ can form within $\le 5$ Myr through the runaway collision channel. At later times, the IMBHs continue to grow through mergers with black holes, stars, and compact remnants, providing predictions testable with future gravitational-wave and transient surveys.

4 pages, 2 figures, 1 table. To appear in the Proceedings of IAU Symposium 398 and MODEST-25: "Compact Objects and Binaries in Dense Stellar Systems" (Seoul, South Korea, June 2025)