Transitions in the Mass-ratio and Spin Properties of Binary Black Holes in GWTC-5

Abstract

We analyze the mass-ratio and effective-spin (${\chi }_{\mathrm{eff}}$) distributions of binary black hole mergers in the latest gravitational-wave catalog, GWTC-5, as a function of primary mass. Using hierarchical Bayesian inference with flexible Gaussian-process population models, we identify four distinct mass regions separated by sharp transitions in both mass-ratio and spin properties. Below $\sim 15M_{\odot }$, the population strongly favors equal-mass binaries and exhibits a narrow ${\chi }_{\mathrm{eff}}$ distribution peaked at positive values. In the range $18$-$30M_{\odot }$, the mass-ratio distribution becomes substantially flatter, while the ${\chi }_{\mathrm{eff}}$ distribution broadens, shifts to a peak consistent with zero, and shows tentative--but not statistically required--evidence for positive skewness. The region associated with the feature near $\simeq 35M_{\odot }$ returns to a narrow ${\chi }_{\mathrm{eff}}$ distribution consistent with symmetry at zero and strongly favors equal-mass binaries. Above $\simeq 45M_{\odot }$, both the mass-ratio and ${\chi }_{\mathrm{eff}}$ distributions broaden significantly. The inferred support of the spin distribution converges toward the range expected for binaries containing remnants of previous black hole mergers, making the highest-mass region fully consistent with a star cluster population of hierarchical mergers. The close correspondence between transitions in mass ratio and effective spin suggests that different primary-mass ranges trace distinct formation channels, with isolated binary or triple evolution likely dominating the lower-mass population and dynamical assembly becoming increasingly important at higher masses.