A Star Cluster Population of High Mass Black Hole Mergers in Gravitational Wave Data

TL;DR

This paper investigates the presence of high-mass black hole mergers in gravitational wave data, supporting the idea that dense star clusters produce black holes that fill the expected mass gap through hierarchical mergers.

Contribution

It provides evidence for a high-mass, isotropically spinning black hole population consistent with dense star cluster formation, and estimates the lower edge of the pair-instability mass gap.

Findings

Detection of a high-mass black hole population with isotropic spins

Mass gap lower edge estimated at 44^{+6}_{-4} solar masses

Bayes factor >10^4 supporting the hierarchical formation model

Abstract

Stellar evolution theories predict a gap in the black hole birth mass spectrum as the result of pair instability processes in the cores of massive stars.This gap, however, is not seen in the binary black hole masses inferred from gravitational wave data. One explanation is that black holes form dynamically in dense star clusters where smaller black holes merge to form more massive black holes, populating the mass gap. We show that this model predicts a distribution of the effective and precessing spin parameters, $\chi_{\rm eff}$ and $\chi_{\rm p}$, within the mass gap that is insensitive to assumptions about black hole natal spins and other astrophysical parameters. We analyze the distribution of $\chi_{\rm eff}$ as a function of primary mass for the black hole binaries in the third gravitational wave transient catalog. We infer the presence of a high-mass and isotropically spinning population of black holes that is consistent with hierarchical formation in dense star clusters and a pair-instability mass gap with a lower edge at $44^{+6}_{-4} M_\odot$. We compute a Bayes factor $\mathcal{B}>10^4$ relative to models that do not allow for a high-mass population with a distinct $\chi_{\rm eff}$ distribution.