Growing the Intermediate-mass Black Hole in Omega Centauri

Abstract

The recent detection of fast-moving stars in the core of Omega Centauri (ω Cen), the most massive globular cluster (GC) in the Milky Way, has provided strong evidence for the presence of an intermediate-mass black hole (IMBH). As ω Cen is likely the accreted nucleus of a dwarf galaxy, this IMBH also represents a unique opportunity to study black hole (BH) seeding mechanisms and their potential role in the formation of supermassive BHs. We present Monte Carlo N-body models of ω Cen with detailed treatments for the loss-cone dynamics involving stars, binaries, and compact objects. Starting with BH seeds of 500–5000 M⊙ (consistent with runaway collisions of massive stars), our cluster models grow IMBHs with masses of ∼50,000 M⊙ after 12 Gyr, while successfully reproducing the present-day surface brightness and velocity dispersion profiles of ω Cen. We find a population of fast stars similar to those observed in the core of ω Cen, with the fastest stars originating from binaries that were tidally disrupted by the IMBH. The IMBH growth is primarily driven by mergers with 30–40 M⊙BHs, suggesting a present-day IMBH–BH merger rate of ∼(4–8) × 10−8 yr−1 in ω Cen–like GCs. Our models also predict a similar rate of tidal disruption events (TDEs; ∼5 × 10−8 yr−1), which, depending on the frequency of ω Cen–like GCs per galaxy, may represent anywhere from 0.1% to 10% of the observed TDE rate.