Missing black holes in brightest cluster galaxies as evidence for the occurrence of superkicks in nature

We investigate the consequences of superkicks on the population of supermassive black holes (SMBHs) in the Universe residing in brightest cluster galaxies (BCGs). There is strong observational evidence that BCGs grew prominently at late times (up to a factor 2-4 in mass from z = 1), mainly through mergers with satellite galaxies from the cluster, and they are known to host the most massive SMBHs ever observed. Those SMBHs are also expected to grow hierarchically, experiencing a series of mergers with other SMBHs brought in by merging satellites. Because of the net linear momentum taken away from the asymmetric gravitational wave emission, the remnant SMBH experiences a kick in the opposite direction. Kicks may be as large as 5000 km s-1 ('superkicks'), pushing the SMBHs out in the cluster outskirts for a time comparable to galaxy-evolution time-scales. We predict, under a number of plausible assumptions, that superkicks can efficiently eject SMBHs from BCGs, bringing their occupation fraction down to a likely range 0.9 < f < 0.99 in the local Universe. Future thirty-metre-class telescopes like ELT and TMT will be capable of measuring SMBHs in hundreds of BCGs up to z = 0.2, testing the occurrence of superkicks in nature and the strong-gravity regime of SMBH mergers.