Binary black holes mergers from hierarchical triples in open clusters

TL;DR

This paper investigates how hierarchical triple systems in open clusters contribute to binary black hole mergers, estimating their rates and timing, and highlighting their occurrence during the star-forming phase in galaxies.

Contribution

It combines semi-analytic and Monte Carlo methods to quantify merger rates and delay times for black holes in hierarchical triples within open clusters, a novel focus in the field.

Findings

Merger rate density of 1-10 Gpc$^{-3}$ yr$^{-1}$ in the local universe.

Significant fraction (20-50%) of mergers occur during the open cluster phase.

Many mergers from hierarchical triples happen within star-forming regions.

Abstract

A promising channel for producing binary black hole mergers is the Lidov-Kozai orbital resonance in hierarchical triple systems. While this mechanism has been studied in isolation, the distribution of such mergers in time and across star-forming environments is not well characterized. In this work, we explore Lidov-Kozai-induced black hole mergers in open clusters, combining semi-analytic and Monte Carlo methods to calculate merger rates and delay times for eight different population models. We predict a merger rate density of $\sim$1--10\,Gpc$^{-3}$\,yr$^{-1}$ for the Lidov-Kozai channel in the local universe, and all models yield delay-time distributions in which a significant fraction of binary black hole mergers (e.g., $\sim$20\%--50\% in our baseline model) occur during the open cluster phase. Our findings suggest that a substantial fraction of mergers from hierarchical triples occur within star-forming regions in spiral galaxies.