MOCCA-Survey Database: extra galactic globular clusters - III. The population of black holes in Milky Way and Andromeda-like galaxies

This work investigates the black hole (BH) population of globular clusters (GCs) in Milky Way- and Andromeda-like galaxies. We combine the population synthesis code MASinGa and the MOCCA-Survey Database I to infer the properties of GCs harbouring a stellar-mass BH subsystem (BHS), an intermediate-mass BH (IMBH), or neither of those. We find that the typical number of GCs with a BHS, an IMBH, or none become comparable in the galactic outskirts, whilst the inner galactic regions are dominated by GCs without a significant dark component. We retrieve the properties of binary BHs (BBHs) that have either merged in the last 3 Gyr or survived in their parent cluster until present-day. We find that around 80 per cent of the merging BBHs form due to dynamical interactions while the remaining originate from evolution of primordial binaries. The inferred merger rate for both in-cluster and ejected mergers is $1.0\!-\!23\, \, \rm {yr^{-1}\, Gpc^{-3}}$ in the local Universe, depending on the adopted assumptions. We find around 100-240 BBHs survive in GCs until present-day and are mostly concentrated in the inner few kpc of the galaxy. When compared with the field, GCs are at least two times more efficient in the formation of BHs and binaries containing at least one BH. Around 1000-3000 single BHs and 100-200 BBHs are transported into the galactic nucleus from infalling clusters over a time span of 12 Gyr. We estimate that the number of BHs and BBHs lurking in the star cluster to be about 1.4-2.2 × 10⁴ and 700-1100, respectively.