The maximum mass ratio of hierarchical binary black hole mergers may cause the $q$-$\chi_{\rm eff}$ correlation

TL;DR

This paper investigates the correlation between mass ratio and effective spin in binary black hole mergers, suggesting hierarchical mergers influence observed spin distributions and identifying a subpopulation consistent with second-generation black holes.

Contribution

It introduces a method to identify hierarchical merger signatures in LIGO-Virgo-KAGRA data and quantifies their prevalence in the observed black hole merger population.

Findings

A subpopulation with primary spins consistent with hierarchical mergers was identified.

19-88% of BBH systems below a certain mass ratio are consistent with hierarchical origins.

The results explain the observed narrowing of the effective spin distribution with mass ratio.

Abstract

Regardless of their initial spins, the merger of two roughly equal mass black holes (BHs) produces a remnant BH of dimensionless spin $0.69$. Such remnants can merge with other BHs in dense stellar environments and produce hierarchical mergers. Analyzing the latest catalog binary black hole (BBH) mergers from the LIGO-Virgo-KAGRA detectors, we identify a subpopulation with primary spins consistent with such hierarchical mergers. Consistent with astrophysical expectations for mergers of second-generation BHs with first-generation BHs, we find that this subpopulation has mass ratios below $0.59^{+0.18}_{-0.23}$. We also infer that $19$-$88\%$ of the BBH population below this mass ratio is consistent with belonging to the hierarchically merged population. Our results offer a natural explanation for the narrowing of the effective inspiral spin distribution with mass ratio observed in other studies.