Mass Distribution of Binary Black Hole Mergers from Young and Old Dense Star Clusters

TL;DR

This study models the mass distribution of binary black hole mergers from dense star clusters, revealing a wide mass range influenced by cluster formation history and metallicity, aligning with gravitational wave observations.

Contribution

It combines N-body simulations with formation histories to predict BBH merger mass distributions across different cluster environments and metallicities.

Findings

BBH mergers range from 6 to over 100 solar masses, peaking near 8 solar masses.

Most low-mass BBH mergers originate in metal-rich clusters.

Higher-mass BBH mergers mainly form in metal-poor globular clusters.

Abstract

Dense star clusters are thought to contribute significantly to the merger rates of stellar-mass binary black holes (BBHs) detected by the LIGO-Virgo-KAGRA collaboration. We combine $N$-body dynamic models of realistic dense star clusters with cluster formation histories to estimate the merger rate distribution as a function of primary mass for merging BBHs formed in these environments. It has been argued that dense star clusters -- most notably old globular clusters -- predominantly produce BBH mergers with primary masses $M_p\approx30\,M_{\odot}$. We show that dense star clusters forming at lower redshifts -- and thus having higher metallicities -- naturally produce lower-mass BBH mergers. We find that cluster BBH mergers span a wide range of primary mass, from about $6\,M_{\odot}$ to above $100\,M_{\odot}$, with a peak near $8\,M_{\odot}$, reproducing the overall merger rate distribution inferred from gravitational wave detections. Our results show that most low-mass BBH mergers (about $95\%$ with $M_p\lesssim 20\,M_{\odot}$) originate in metal-rich ($Z \sim Z_{\odot}$) dense star clusters, while more massive BBH mergers form predominately in metal-poor globular clusters. We also discuss the role of hierarchical mergers in shaping the BBH mass distribution. Gravitational wave detection of dynamically-formed low-mass BBH mergers -- potentially identifiable by features such as isotropic spin distributions -- may serve as probes of cluster formation histories in metal-rich environments at low redshifts.