A subpopulation of low-mass, spinning black holes: signatures of dynamical assembly

TL;DR

This paper identifies a new subpopulation of low-mass, rapidly spinning black holes likely formed through hierarchical mergers in dense stellar environments, revealing insights into black hole formation and evolution.

Contribution

It provides evidence for a low-mass second-generation black hole population and clarifies the primary mass distribution of first-generation black holes.

Findings

Evidence for a low-mass, rapidly spinning black hole subpopulation.

Mass distribution of second-generation black holes peaks at ~16 M_sun.

Primary mass distribution of first-generation black holes shows a dip between 12-20 M_sun.

Abstract

Gravitational-wave observations of massive, rapidly spinning binary black holes mergers provide increasing evidence for the dynamical origin of some mergers. Previous studies have interpreted the mergers with primary mass $\gtrsim45\,M_\odot$ 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 reveal confident evidence of another subpopulation with rapid and isotropic spins at low mass containing the two exceptional events GW241011 and GW241110, consistent with a hierarchical merger hypothesis. Our result suggests the mass distribution of the second-generation black holes is peaked at low primary masses of $\sim16\,M_\odot$ rather than $\gtrsim45\,M_\odot$ 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 higher-generation black holes, our result reveals the primary mass distribution of first-generation black holes formed from stellar collapse, which shows a significant dip between $\sim12\,M_\odot$ to $\sim20\,M_\odot$. This may indicate a dearth of black holes due to variation in the core compactness of the progenitor.