Post-Newtonian Dynamics in Dense Star Clusters: Highly-Eccentric, Highly-Spinning, and Repeated Binary Black Hole Mergers

TL;DR

This paper models post-Newtonian dynamics in globular clusters, revealing that many black hole mergers occur in-cluster with high eccentricities and spins, leading to second-generation black holes detectable by gravitational wave observatories.

Contribution

It introduces realistic models of globular clusters with relativistic effects, showing in-cluster mergers and formation of second-generation black holes with high spins and masses.

Findings

Nearly half of black hole mergers occur inside clusters.

About 10% of in-cluster mergers have high eccentricities (>0.1).

Clusters can produce second-generation black holes with detectable spins.

Abstract

We present models of realistic globular clusters with post-Newtonian dynamics for black holes. By modeling the relativistic accelerations and gravitational-wave emission in isolated binaries and during three- and four-body encounters, we find that nearly half of all binary black hole mergers occur inside the cluster, with about 10% of those mergers entering the LIGO/Virgo band with eccentricities greater than 0.1. In-cluster mergers lead to the birth of a second generation of black holes with larger masses and high spins, which, depending on the black hole natal spins, can sometimes be retained in the cluster and merge again. As a result, globular clusters can produce merging binaries with detectable spins regardless of the birth spins of black holes formed from massive stars. These second-generation black holes would also populate any upper mass gap created by pair-instability supernovae.