Multiple stellar-mass black holes in globular clusters: theoretical confirmation

TL;DR

This paper uses N-body simulations to confirm that multiple stellar-mass black holes can be retained in globular clusters like M22, aligning with recent observations and challenging previous assumptions about black hole ejection.

Contribution

It provides the first direct N-body simulation demonstrating the retention of multiple black holes in a globular cluster similar to M22, supporting recent observational findings.

Findings

16 black holes retained at 12 Gyr in the model

Presence of black hole binaries with main sequence stars

Agreement with observed black hole population in M22

Abstract

While tens or hundreds of stellar-remnant black holes are expected to form in globular star clusters, it is still unclear how many of those will be retained upon formation, and how many will be ejected through subsequent dynamical interactions. No such black holes have been found in any Milky Way globular cluster until the recent discovery of stellar-mass black holes in the globular cluster M22 (NGC 6656) with now an estimated population of 5-100 black holes. We present a direct N-body model of a star cluster of the same absolute and dynamical age as M22. Imposing an initial retention fraction of approx. 10% for black holes, 16 stellar-remnant black holes are retained at a cluster age of 12 Gyr, in agreement with the estimate for M22. Of those 16 BHs, two are in a binary system with a main sequence star each while also one pure black hole binary is present. We argue that multiple black holes can be present in any Milky Way cluster with an extended core radius, such as M22 or the model presented here.