Kavli Affiliate: Salvatore Vitale
| Summary:
Repeated black-hole mergers in dense stellar clusters are a plausible mechanism to populate the predicted gap in black hole masses due to the pair-instability process. These hierarchical mergers carry distinct spin and tilt features relative to first-generation black holes, for which previous studies have found evidence at a population level by interpreting features in the effective inspiral spin domain. We introduce an astrophysically-motivated model in the joint space of effective inspiral and precessing spins, which captures the dominant spin dynamics expected for hierarchical mergers. We find decisive evidence both for a population transition above $sim 45M_odot$, consistent with the anticipated onset of the pair-instability gap, as well as a peak at $sim 15 M_odot$, which we interpret as the global peak in the hierarchical merger rate. The existence of low- and high-mass subpopulations of higher-generation black holes suggests the contribution of both near-solar-metallicity and metal-poor star clusters to the hierarchical merger population. Our results reinforce the growing evidence for detailed, mass-dependent substructure in the spin distribution of the binary black hole population.
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