Davide Gerosa (University of Birmingham)
Title: Growing up. The next generation of black holes moves out of the cluster.
Abstract: Pair instabilities in supernovae might prevent the formation of black holes with masses between ∼50 and ∼130 solar masses. Multiple generations of black-hole mergers provide a possible way to populate this "mass gap" from below. However, this requires an astrophysical environment with a sufficiently large escape speed to retain merger remnants and prevent them from being ejected by gravitational-wave recoils. We show that a single LIGO/Virgo observation of a black hole in the pair-instability mass gap implies that its progenitors grew in an environment with escape speed >50 km/s. This is larger than the escape speeds of most globular clusters, requiring denser and heavier environments such as nuclear star clusters or disks-assisted migration in galactic nuclei. A single detection in the mass gap would also hint at the existence of a much larger population of first-generation events from the same environment, thus providing a tool to disentangle the contribution of different formation channels to the observed merger rate.