Stellar-mass black holes in young massive and open stellar clusters and their role in gravitational-wave generation

TL;DR

This paper models the evolution of stellar clusters with black holes to understand their role in forming binary black holes that generate gravitational waves, revealing new insights into merger mechanisms and event rates.

Contribution

It presents the first use of the NBODY7 code to simulate black hole dynamics in young stellar clusters over 10 Gyr, highlighting the importance of triple-mediated mergers.

Findings

Triple-mediated mergers are prominent among bound BBHs.

Simulated BBH mergers resemble observed gravitational wave events.

Estimated BBH coalescence rates suggest detectable gravitational wave signals.

Abstract

The dynamical processes involving stellar-remnant black holes (BH) in stellar clusters has always drawn attention due to the BHs' potential in a number of astrophysical phenomena, especially the dynamical formation of binary black holes (BBH), which would potentially coalesce via radiation of gravitational waves (GW). This study presents a preliminary set of evolutionary models of compact stellar clusters with initial masses ranging over $1.0\times10^4M_\odot-5.0\times10^4M_\odot$, and half-mass radius of 2 pc or 1 pc, that is typical for young massive and starburst clusters. They have metallicities between $0.05Z_\odot-Z_\odot$. Including contemporary schemes for stellar wind and remnant-formation, such model clusters are evolved, for the first time, using the state-of-the-art direct N-body evolution program NBODY7, until their dissolution or at least for 10 Gyr. That way, a self-regulatory behaviour in the effects of dynamical interactions among the BHs, especially while heating and expanding the cluster and self-depleting the BHs, is demonstrated. The BBH mergers obtained here show a prominence in triple-mediated mergers while being bound to the clusters, compared to those occurring among the BBHs that are dynamically ejected from the clusters. This is in contrast with earlier N-body computations and also with recent Monte-Carlo method based ones. A broader mass spectrum of BHs and ejection of BBHs generally of wider orbits and in lower numbers, for the cluster masses explored here, might cause this which is yet to be fully understood. Among the BBH coalescences obtained here, there are ones that resemble the detected GW151226, LVT151012, and GW150914 events and also ones which are even more massive. A preliminary estimate suggests few 10s - 100s of BBH coalescences per year, originating due to dynamics in stellar clusters, that can be detected by the LIGO at its design sensitivity.