The Impact of High-Mass Clusters on the Binary Black Hole Merger Rate over Cosmic Time

Abstract

We present a cosmological calculation of the binary black hole merger rate from dense star clusters, using a $\texttt{Cluster Monte Carlo}$ ($\texttt{CMC}$) model grid spanning the full observed parameter space of cluster mass, metallicity, and virial radius. For the first time, we incorporate $N=10^{7}$-particle $\texttt{CMC}$ models into this calculation, including a new solar-metallicity model, extending the $\texttt{CMC Cluster Catalog}$ to the most massive clusters observed in the local Universe. Using an analytic cluster formation model which convolves the cluster formation rate with delay-time distributions from our model suite, we find that the local cluster BBH merger rate is dominated by low-mass ($M_1 \lesssim 20\,M_\odot$) mergers originating from recently formed, high-metallicity clusters. While high-mass clusters are the primary sites of hierarchical (second-generation and higher) mergers, their contribution to the overall merger rate is suppressed by the exponential cutoff of the cluster initial mass function. The hierarchical merger rate is therefore highly sensitive to the exact value of the Schechter mass, $M_S$, suggesting that gravitational wave constraints on the hierarchical merger fraction can independently constrain the high-mass end of the cluster initial mass function. Our cluster formation model reproduces the observed present-day GC number density without fine tuning, and we find $M_S = 10^7\,M_\odot$ provides the best match to the observed Virgo GC mass distribution. For this value, $27\%$ of local dynamical mergers are hierarchical, compared to $18\%$ for the canonical value of $M_S = 10^{6.26}\,M_\odot$. Our merger weights, which we make publicly available, can be used for direct comparison with LVK distributions and other formation models.