Redshift Evolution of the Black Hole Merger Rate from Globular Clusters

TL;DR

This paper models the evolution of black hole merger rates over cosmic time, combining dynamical cluster models with cosmological data to predict how the rate changes with redshift and compare it to other formation channels.

Contribution

It introduces a combined dynamical and cosmological model to predict the redshift evolution of black hole merger rates from globular clusters.

Findings

Merger rate of 14 Gpc^{-3} yr^{-1} locally, rising to 6 times higher at z=2.7

Merger rate peaks at z=2.7 before declining at higher redshifts

Comparison of cluster-origin mergers with field and triple systems discussed

Abstract

As the sensitivity of current and future gravitational-wave detectors improves, it will become possible to measure the evolution of the binary black hole merger rate with redshift. Here, we combine detailed fits to state-of-the-art dynamical models of binary black hole formation in dense star clusters with a cosmological model of cluster formation across cosmic time. We find a typical merger rate of 14 $\rm{Gpc}^{-3} \rm{yr}^{-1}$ in the local universe, with a reasonable range of 4-18 $\rm{Gpc}^{-3} \rm{yr}^{-1}$, depending on the rate of cluster disruption and the cluster initial mass function. This rate increases by a factor of 6 to redshift $z=2.7$ before declining at higher redshifts. We compare the merger rate from binaries produced in clusters to similar estimates from isolated binaries and triples in galactic fields, and discuss various ways that these different formation channels could add up to the current merger rate observed by LIGO/Virgo.