Signatures of a subpopulation of hierarchical mergers in the GWTC-4 gravitational-wave dataset

TL;DR

This study identifies signatures of hierarchical black hole mergers in the GWTC-4 dataset, revealing a mass-dependent substructure in the black hole population consistent with dense stellar cluster origins.

Contribution

Introduces a model capturing spin dynamics of hierarchical mergers and provides evidence for a transition mass indicating the onset of the pair-instability gap.

Findings

Decisive evidence for a population transition at about 46 solar masses.

Peak hierarchical merger rate inferred at approximately 15.7 solar masses.

Supports the existence of subpopulations from different star cluster environments.

Abstract

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 supernova process. These hierarchical mergers carry distinct spin characteristics relative to first-generation black holes. 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 for a transition at $m_1 = 46.2_{-7.2}^{+12.6} M_\odot$, above which the population is nearly entirely hierarchical, a location consistent with the anticipated onset of the pair-instability gap. We also infer a global peak in the hierarchical merger rate at $m_1 = 15.7_{-1.1}^{+3.2} M_\odot$. 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.