Three subpopulations of binary black holes in mass, spin magnitudes and tilts following GWTC-5

Abstract

The mass and spin distributions of merging binary black holes offer clues to their formation pathways, which remain uncertain. In this work, we model the population of binary black holes using generic subpopulations with distinct mass-ratio, spin-magnitude, and spin-tilt distributions, whose relative contributions vary with primary mass. Using the 259 binary black holes in the fifth LIGO-Virgo-KAGRA catalog (GWTC-5), we resolve three subpopulations with low, medium, and high spins. The low-spin population dominates for primary masses near $35 M_\odot$, contributes roughly half of the binaries for primary masses of $10 M_\odot$, and favors nearly equal mass ratios overall; its spins are consistent with both black holes having negligible spin, with a log Bayes factor of $\log_{10}B =6.1^{+0.2}_{-0.3}$ in favor of vanishing spins. The medium-spin population accounts for the other half of the population around primary masses of $10 M_\odot$ and favors spin magnitudes $χ_i \approx 0.2$ for both components, preferentially aligned orientations, and comparable mass ratios. The high-spin population dominates above primary masses of $60 M_\odot$ and also contributes at lower masses $\sim 20 M_\odot$; it favors a rapidly spinning primary ($χ_1 \approx 0.7$), a broad range of secondary spins, widely distributed spin orientations, and a broad mass-ratio distribution. Allowing for a fourth subpopulation yields no distinct additional mass-ratio or spin distribution, as it becomes degenerate with existing features. To resolve highly concentrated features in spin, we control the variance of standard Monte Carlo likelihood estimators using truncated Gaussian mixture models. Taken together, our results add to a growing body of evidence for substructure in the population of LIGO-Virgo-KAGRA black holes, providing further clues to uncover their formation histories.