The effect of gravitational recoil on black holes forming in a hierarchical universe

Galactic bulges are known to harbour central black holes whose mass is tightly correlated with the stellar mass and velocity dispersion of the bulge. In a hierarchical universe, galaxies are built up through successive mergers of subgalactic units, a process that is accompanied by the amalgamation of bulges and the likely coalescence of galactocentric black holes. In these mergers, the beaming of gravitational radiation during the plunge phase of the black hole collision can impart a linear momentum kick or ‘gravitational recoil’ to the remnant. If large enough, this kick will eject the remnant from the galaxy entirely, and populate intergalactic space with wandering black holes. Using a semi‐analytic model of galaxy formation, we investigate the effect of black hole ejections on the scatter of the relation between black hole and bulge mass. We find that while not being the dominant source of the measured scatter, they do provide a significant contribution and may be used to set a constraint, v kick ≲ 500 km s −1 , on the typical kick velocity, in agreement with values found from general relativistic calculations. Even for the more modest kick velocities implied by these calculations, we find that a substantial number of central black holes are ejected from the progenitors of present‐day galaxies, giving rise to a population of wandering intrahalo and intergalactic black holes whose distribution we investigate in high‐resolution N ‐body simulations of the Milk Way mass haloes. We find that intergalactic black holes make up only ∼2–3 per cent of the total galactic black hole mass but, within a halo, wandering black holes can contribute up to about half of the total black hole mass orbiting the central galaxy. Intrahalo black holes offer a natural explanation for the compact X‐ray sources often seen near the centres of galaxies and for the hyperluminous non‐central X‐ray source in M82.