Formation and Growth of IMBHs 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; ∼ 10^2–10^5 M⊙), with increasing observational evidence pointing to their presence in massive 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 use direct N-body and Monte Carlo simulations of clusters with initial core densities between 10^6 and 4 × 10^8 M⊙ pc−3 and total masses of 5.9 × 10^5 and 1.3 × 10^6 M⊙. These models show that IMBHs of 10^3–10^4 M⊙ can form within ≲ 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.