Authors
Yin-Jie Li, Yuan-Zhu Wang, Shao-Peng Tang, Yi-Zhong Fan
Published in
Physical review letters. Volume 137. Issue 2. Pages 021407. Jul 10, 2026.
Abstract
The active galactic nucleus (AGN) accretion disks are ideal sites for hierarchical black hole (BH) mergers. To robustly probe such a possibility, we analyze binary black hole mergers in the GWTC-4 with a flexible mixture population model for component masses, spin magnitudes, and spin tilt angles, and identify two distinct subpopulations. In the second subpopulation characterized by high spin magnitudes χ∼0.8 as well as the broad mass distribution up to ≳150M_{⊙}, we find a pronounced preference for spins aligned with the orbital angular momentum: an isotropic tilt distribution is strongly disfavored (logarithmic Bayes factor =4.5). The aligned events account for ∼0.57_{-0.31}^{+0.23} of the second subpopulation, corresponding to a local rate of ∼0.25_{-0.16}^{+0.38} Gpc^{-3} yr^{-1} (all values reflect central 90% credible intervals). These notable features naturally arise from hierarchical mergers embedded in AGN disks, where gas torques may effectively align spins. Our results suggest that AGN-disk hierarchical assembly may be one important channel for the present gravitational-wave sample, and provide concrete, testable predictions for future detection.
PMID:
42503081
Bibliographic data and abstract were imported from PubMed on 26 Jul 2026.
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