Divergence in mass ratio distributions between low-mass and high-mass coalescing binary black holes

TL;DR

This study analyzes how the distribution of mass ratios in binary black hole systems varies with the primary mass, revealing a divergence that supports different formation channels for low- and high-mass systems.

Contribution

It provides the first evidence of divergence in mass ratio distributions between low- and high-mass binary black holes using Bayesian analysis.

Findings

High-mass BBHs tend to be more symmetric in mass ratio.

Significant divergence in mass ratio distribution between low- and high-mass ranges.

Mild evidence of different mass ratio distributions for BBHs in the Gaussian-like peak.

Abstract

Coalescing binary black hole (BBH) systems are likely formed via several channels, and it is challenging to understand their formation / evolutionary processes. Some features in the mass function of the primary components ($m_1$), such as the distinct Gaussian-like peak located at $\sim 34M_\odot$, have been previously found. In this work, we investigate the possible dependence of the mass ratio ($q=m_2/m_1$) distribution on the primary mass. We find a Bayesian odds ratio of 18.1 in favor of divergence in the mass ratio distributions between the low- and high-mass ranges over an invariable mass ratio distribution. The BBHs with $m_1\gtrsim29M_{\odot}$ have a stronger preference to be symmetric compared to those with $m_1\lesssim29M_{\odot}$ at a 97.6\% credible level. Additionally, we find mild evidence that the BBHs with $m_1$ located in the Gaussian-like peak have a mass ratio distribution different from that of other BBHs. Our findings may be in favor of some formation channels, such as the chemically homogeneous evolution and the dynamical assembly in globular clusters/nuclear star clusters, which are more likely to provide symmetric BBHs in the high-mass range.