Binary black hole mergers from hierarchical triples in open clusters

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 starforming 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 ∼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., ∼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.