Star cluster population of binary black hole mergers in the gravitational wave data

by Dr Fabio Antonini (Cardiff University)

Wednesday 27 May 2026, 14:30 → 16:00 Europe/Rome

I will show that gravitational-wave observations reveal a clear transition in the black-hole merger population at a primary mass of ~45 solar masses. Below this mass, the population is broadly consistent with low-spin, first-generation black holes formed through stellar collapse. Above it, mergers show broader and isotropic spin distributions, consistent with hierarchical mergers formed in dense star clusters. This transition is consistent with the existence of a pair-instability mass gap caused by electron–positron pair production in the cores of massive stars, which suppresses the direct formation of black holes above this scale. These results point to a connection between gravitational wave astronomy and nuclear astrophysics and highlight dense stellar clusters as key environments in the growth of black holes.