Smoking-gun evidence for hierarchical black-hole mergers
Abstract
Abstract How stellar-mass black holes grow remains a fundamental question in astrophysics. Although the detection of high-mass, high-spin merging binary black holes (BBHs) via gravitational waves challenges conventional stellar-evolution theories, whether this phenomenon is driven by hierarchical mergers in dense environments or by accretion remains unresolved. Applying a flexible mixture model to 259 BBHs from GWTC-5, we identify a high-spin subpopulation, whose mass function peak-by-peak traces the remnant-mass distribution of a low-spin (stellar-collapse) subpopulation up to ∼ 80M⊙. This near-perfect morphological match (Bhattacharyya coefficient ∼ 0.95) cannot be explained without fine-tuning in alternative scenarios, providing smoking-gun evidence for hierarchical mergers. We also constrain the 12C(α, γ)16O reaction S-factor to S300 = 151+30 −26 keV b, based on the maximum mass of stellar-collapse BBHs of 54.2+7.7 −7.2M⊙, consistent with theoretical benchmarks. Our results indicate that gravitational-wave black holes are fully accounted for by stellar collapse and dynamical hierarchical assembly, with no primordial black holes required.
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