Mass Distribution of Binary Black Hole Mergers from Young and Old Dense Star 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 ≈ 30 M ⊙ . 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 ⊙ to above 100 M ⊙ , with a peak near 8 M ⊙ , 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 ≲ 20 M ⊙ ) originate in metal-rich ( Z ∼ Z ⊙ ) 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.