High-Spin BBH Subpopulation from AGN Accretion

Abstract: The formation environments of merging binary black holes remain uncertain. While hierarchical assembly in dense stellar clusters has been widely explored as an explanation for black holes exceeding the stellar-mass limit, growth through gas accretion in active galactic nucleus (AGN) disks is an alternative that has received less observational scrutiny. Here we search for an accretion-origin subpopulation using only spin magnitudes, fitting a three-component mixture model to 166 binary black hole mergers from LIGO--Virgo--KAGRA with component shapes fixed from theoretical predictions and only the mixing fractions inferred from the data. We find strong evidence ($ln B = 5.7$) that $\sim 10\%$ (90% credible interval $[1\%, 14\%]$) of detected mergers belong to a subpopulation with primary spins clustered near $a_1 \approx 0.9$, consistent with the theoretical prediction for accretion spin-up. The hierarchical-merger prediction of $a_1 \approx 0.7$ is decisively disfavored as the location of the high-spin subpopulation ($ln B = 5.7$). Post hoc validation reveals that the accretion candidates have systematically higher masses (median $m_1 = 58\,M_\odot$) and aligned spins (median $χ<em>{\rm eff} = 0.33$, vs. $0.04$ for standard-dominated events). The accretion subpopulation is not limited to systems above the pair-instability mass gap: GW190517 ($m_1 \approx 39 M</em>\odot$) is among the top candidates, demonstrating that accretion spin-up operates across a range of masses. GW190521, previously interpreted as a hierarchical merger, shows comparable support for an accretion origin. These results provide the first population-level observational evidence for an accretion-origin subpopulation in black hole mergers.