How binary black holes form: evidence for the pair-instability gap and hierarchical mergers in the latest gravitational-wave detections

Uncovering the origins of binary black holes through gravitational-wave detections provides a powerful probe of the evolution, interactions, and environments of massive stars. Stellar theory predicts a forbidden range of black-hole masses between 50-130Msun due to pair-instability supernovae, but evidence for such a gap in the mass distribution in previous gravitational-wave studies has proved elusive. In this talk, I will present our latest observational evidence of the pair-instability gap in the distribution of black hole masses. While the gap is not present in the distribution of primary masses (the bigger of the two black holes in a binary system), it appears unambiguously in the distribution of secondary masses. The location of the gap lines up well with a previously identified transition in the binary black-hole spin distribution; binaries with primary components in the gap tend to spin more rapidly than those below the gap. We interpret these findings as evidence for a subpopulation of hierarchical mergers: binaries where the primary component is the product of a previous black-hole merger and thus populates the gap. If time permits, I will also highlight the signatures of hierarchical mergers that not only populate the mass gap but also present at lower mass.