Stellar-mass black holes in young massive and open stellar clusters -- VI. Role of external galactic field

TL;DR

This study uses detailed simulations to show that tidally dissolving star clusters can still produce black hole mergers and BH-star binaries, unaffected by external galactic tidal forces.

Contribution

It demonstrates that external tidal stripping does not significantly hinder black hole merger formation or the creation of BH-star binaries in young star clusters.

Findings

Tidal stripping does not prevent BBH merger formation.

BBH merger properties are unaffected by tidal stripping.

Tidally stripped clusters can produce BH-star binaries that escape into the galaxy.

Abstract

Young massive clusters (YMC) and open clusters (OC) are widely considered as potential environments for assembling merging binary stellar-remnant black holes (BBH) via dynamical interactions. However, such moderate mass systems are susceptible to being disrupted by the external tidal field of their host galaxies, potentially limiting their effectiveness as GW sources. In this study, I investigate the formation of BBH mergers in tidally dissolving star clusters. This is achieved with a newly computed grid consisting of 95 evolutionary model star clusters, where the clusters are subjected to a varying extent of tidal stripping. The cluster evolutions are computed with the direct N-body integrator NBODY7 that includes, among others, treatments for post-Newtonian (PN) effects in compact-binary members, mass loss due to stellar evolution, formation of stellar remnants, and tidal stripping. It is found that even strong tidal stripping does not quench the formation of a black hole (BH) core inside a cluster or the formation of dynamical BBH mergers in the system. The overall properties of BBH mergers, e.g., the form of the distribution of merger delay time, primary mass, and mass ratio, and the redshift evolution of merger rate are not significantly altered by the extent of tidal stripping of the parent cluster population. Furthermore, even strongly tidally stripped clusters are capable of dynamically forming Gaia-BH-like detached BH--main-sequence-star binaries that escape into the galactic field. Limitations of the present study and potential future improvements are discussed.