Black Hole Mergers from Hierarchical Triples in Dense Star Clusters

TL;DR

This study investigates black hole mergers originating from hierarchical triple systems in dense star clusters, highlighting their role in eccentric mergers detectable by gravitational wave observatories and estimating their merger rates.

Contribution

It provides the first detailed analysis of black hole hierarchical triples formed via binary interactions in globular clusters, including their merger rates and observational signatures.

Findings

Triple systems can produce eccentric black hole mergers detectable by GW observatories.

Merger rate from triples is estimated at 0.35 Gpc$^{-3}$yr$^{-1}.

Up to 9% of mergers may have detectable eccentricity.

Abstract

Hierarchical triples are expected to be produced by the frequent binary-mediated interactions in the cores of globular clusters. In some of these triples, the tertiary companion can drive the inner binary to merger following large eccentricity oscillations, as a result of the eccentric Kozai-Lidov mechanism. In this paper, we study the dynamics and merger rates of black hole (BH) hierarchical triples, formed via binary--binary encounters in the CMC Cluster Catalog, a suite of cluster simulations with present-day properties representative of the Milky Way's globular clusters. We compare the properties of the mergers from triples to the other merger channels in dense star clusters, and show that triple systems do not produce significant differences in terms of mass and effective spin distribution. However, they represent an important pathway for forming eccentric mergers, which could be detected by LIGO--Virgo/KAGRA (LVK), and future missions such as LISA and DECIGO. We derive a conservative lower limit for the merger rate from this channel of $0.35$ Gpc$^{-3}$yr$^{-1}$ in the local Universe and up to $\sim9\%$ of these events may have a detectable eccentricity at LVK design sensitivity. Additionally, we find that triple systems could play an important role in retaining second-generation BHs, which can later merge again in the core of the host cluster.