Compactness Peaks and Subpopulations: Probing Stellar Physics and Formation Channels of Merging Binary Black Holes

Abstract

The growing catalog of gravitational-wave detections from the LIGO-Virgo-KAGRA (LVK) collaboration reveals structure in the binary black hole (BBH) mass distribution, including peaks near m1 = 10 solar masses and m1 = 35 solar masses, a high-mass suppression consistent with the pair-instability supernova gap, and a possible dearth of systems near chirp mass M = 10-12 solar masses. We apply Compactness Peaks + Channels, a stripped-star-motivated five-component population model, to 152 BBH mergers from GWTC-4.0. The model is decisively preferred over the LVK Broken Power Law + 2 Peaks baseline, with log10 Bayes factor = 7.69, and decomposes the population into isolated first-generation (1G), dynamical 1G, and hierarchical second-generation (2G) channels with fractions 0.75 (+0.11/-0.16), 0.22 (+0.16/-0.11), and 0.02 (+0.03/-0.01), respectively. The low-mass isolated component sharply localizes the 10 solar mass feature, with narrow masses, near-equal mass ratios, and low partially aligned spins, consistent with stripped-star binary evolution. The 35 solar mass feature is primarily captured by the high-mass dynamical 1G component. The hierarchical component is consistent with 2G+1G mergers, with elevated primary spins, mu_chi = 0.65 (+0.30/-0.22), and asymmetric mass ratios. We localize the compactness-peak edges at 12.3 (+2.4/-1.3) and 16.1 (+5.7/-5.3) solar masses, consistent with, but not yet requiring, a compactness-driven dearth between them. These results support a multi-component description of the BBH population and motivate further tests of compactness-driven isolated evolution.