Divergence in Mass Ratio Distributions between Low-mass and High-mass Coalescing Binary Black Holes

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 (m1), such as the distinct Gaussian-like peak located at ∼34 M⊙, have been previously found. In this work, we investigate the possible dependence of the mass ratio (q = m2/m1) 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. BBHs with m1 ≳ 29 M⊙ have a stronger preference of being symmetric compared to those with m1 ≲ 29 M⊙ at a 97.6% credible level. Additionally, we find mild evidence that BBHs with m1 located in the Gaussian-like peak have a mass ratio distribution different from that of other BBHs. Our findings may favor some formation channels, such as chemically homogeneous evolution and dynamical assembly in globular clusters/nuclear star clusters, which are more likely to provide symmetric BBHs in the high-mass range.