GWTC-4 data supports two mass-dependent spin subpopulations: low-mass binaries mostly slow-spinning, high-mass ones dominated by moderate-to-rapid spins with transition from 35 to 70 solar masses.
High-Spin BBH Subpopulation from AGN Accretion
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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 $\chi_{\rm eff} = 0.33$, vs. $0.04$ for standard-dominated events). The accretion subpopulation is not limited to systems above the pair-instability mass gap: GW190517 ($m_1 \approx 39 M_\odot$) 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.
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UNVERDICTED 4representative citing papers
GWTC-5 population analysis finds high-spin black hole mass function traces low-spin remnant distribution with Bhattacharyya coefficient ~0.95, supporting hierarchical mergers and yielding a nuclear S-factor constraint.
BBH-Genesis applied to GWTC-4 finds strongest support for a two-channel model of binary black hole populations with possible mild evidence for an AGN-related third channel.
GWTC-5.0 analysis finds evidence for structure beyond a non-skewed Gaussian bulk in χ_eff, with suggestive mass-dependent excess of positive over negative spins outside the bulk at 13:1 odds in one mass bin.
citing papers explorer
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Evidence for mass-dependent spin subpopulations in GWTC-4
GWTC-4 data supports two mass-dependent spin subpopulations: low-mass binaries mostly slow-spinning, high-mass ones dominated by moderate-to-rapid spins with transition from 35 to 70 solar masses.
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Smoking-gun evidence for hierarchical black-hole mergers
GWTC-5 population analysis finds high-spin black hole mass function traces low-spin remnant distribution with Bhattacharyya coefficient ~0.95, supporting hierarchical mergers and yielding a nuclear S-factor constraint.
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BBH-Genesis: Disentangling Binary Black Hole Formation Channels with GWTC-4
BBH-Genesis applied to GWTC-4 finds strongest support for a two-channel model of binary black hole populations with possible mild evidence for an AGN-related third channel.
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Evidence for additional structure in the effective spin distribution hints at multiple formation pathways in GWTC-5.0
GWTC-5.0 analysis finds evidence for structure beyond a non-skewed Gaussian bulk in χ_eff, with suggestive mass-dependent excess of positive over negative spins outside the bulk at 13:1 odds in one mass bin.