Gravitational Recoil: Signatures on the Massive Black Hole Population

In the last stages of a black hole merger, the binary can experience a recoil due to asymmetric emission of gravitational radiation. Recent numerical relativity simulations suggest that the recoil velocity can be as high as a few thousands kilometers per second for particular configurations. We consider here the effect of a worst case scenario for orbital and phase configurations on the hierarchical evolution of the massive black hole (MBH) population. The orbital configuration and spin orientation in the plane is chosen to be the one yielding the highest possible kick. Masses and spin magnitudes are instead derived self-consistently from the MBH evolutionary models. If seeds form early, for example, as remnants of the first stars, almost the totality of the first few generation of binaries are ejected. The fraction of lost binaries decreases at later times due to a combination of the binary mass ratio distribution becoming shallower, and the deepening of the hosts' potential wells. If seeds form at later times, in more massive halos, then the retention rate is much higher. We show that the gravitational recoil does not pose a threat to the evolution of the MBH population that we observe locally in either case, although high-mass seeds seem to be favored. The gravitational recoil is instead a real hazard for (1) MBHs in biased halos at high redshift, where mergers are more common and the potential wells still relatively shallow. Similarly, it is very challenging to retain (2) MBHs merging in star clusters.