Gravitational-wave observations of massive, rapidly spinning black holes provide increasing evidence for the dynamical origin of some mergers. Previous studies have interpreted the mergers with primary mass $\gtrsim 45M_{\odot }$ as being dominated by hierarchical, second-generation mergers, with rapidly spinning primaries being the products of previous black hole mergers assembled in dense stellar clusters. In this work, we present evidence of another subpopulation with rapid and isotropic spins at low mass containing the two exceptional events GW241011 and GW241110. Though other interpretations are possible, the properties of this subpopulation are consistent with a hierarchical-merger hypothesis. Interpreted in the hierarchical-merger framework, our result suggests the mass distribution of the candidate second-generation black holes may peak at low primary masses of $\sim 16M_{\odot }$ rather than $\gtrsim 45M_{\odot }$ in the pair-instability gap. Such low-mass second-generation black holes must be formed from the merger of even lighter first-generation black holes, implying that dense, metal-rich stellar environments contribute to the binary black hole population. By separating the contamination of the possible higher-generation black holes, our analysis provides an estimate of the primary-mass distribution of first-generation black holes formed from stellar collapse, which suggests a dip between $\sim 12M_{\odot }$ and $\sim 20M_{\odot }$. This may indicate a dearth of black holes due to variation in the core compactness of the progenitor.

Close

PrevNext

View figure in article

• Gravitational wave sources
• Gravitational waves
• Astronomical black holes