Stellar-mass black holes in young massive and open stellar clusters -- VII. Comparisons with gravitational-wave events until LVK-O4a and Gaia compact binaries

TL;DR

This study uses N-body simulations of young massive clusters to evaluate their contribution to gravitational-wave events, finding they can explain many observed features but not all, such as low-mass mergers.

Contribution

It provides a comprehensive set of N-body models of star clusters evolving into old open clusters, assessing their role in GW merger populations with detailed physics included.

Findings

Reproduces key features of observed GW events, including asymmetric low-mass mergers.

Merger rate density from models accounts for 25-33% of observed rates.

Models produce Gaia-like BH-NS binaries consistent with observations.

Abstract

Gravitational-wave (GW) detections by the LIGO-Virgo-KAGRA (LVK) observatories suggest multiple formation channels for GW compact binary mergers. Here I assess the role of young massive clusters (YMC) evolving into old open clusters (OC) -- the YMC/OC channel -- to the GW merger population. A homogeneous grid of 90 N-body evolutionary model star clusters, spanning initial masses of $10^4M_\odot\leq M_{cl}(0)\leq10^5M_\odot$, half-mass radii of 1-3 pc, and metallicities between 0.0002-0.02 is computed with the direct, post-Newtonian N-body code NBODY7. The N-body simulations include primordial binaries, delayed stellar-remnant model forming black holes (BH) and neutron stars (NS), BH spin prescriptions, and GW recoil kicks, and they are evolved until BH depletion. Most GW mergers from the cluster models are dynamically assembled binary black holes (BBH) that merge within their host clusters. Merger mass ratios reach down to 0.1-0.2 despite an overall bias toward nearly symmetric pairs. The GW merger efficiency varies non-monotonically with cluster mass, peaking around $M_{cl}(0)=7.5\times10^4M_\odot$ and also for $M_{cl}(0)\leq3.0\times10^4M_\odot$. The computed mergers reproduce some of the key features of the latest observed GW event catalogue, including asymmetric low-mass mergers, misaligned events among highly spinning, massive BHs, and an excess of $30M_\odot$ primaries, though they under-produce $10M_\odot$ primaries, hinting at contributions from other channels. The model merger rate density accounts for 25%-33% of the observed rate; it increases with redshift somewhat faster than the cosmic star formation, consistently with LVK's inferences. The model effective spin distribution is positively asymmetric at zero redshift and broadens with redshift. The models yield field BH- and NS-main sequence star binaries with parameters consistent with the Gaia-discovered candidates. [Abgd.]