Aligned Hierarchical Black Hole Mergers in AGN disks revealed by GWTC-4

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+0.23-0.31 of the second subpopulation, corresponding to a local rate of ~ 0.25+0.38-0.16 ~ 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 can effectively align spins. Our results identify AGN-disk hierarchical assembly as an important channel in the present gravitational-wave sample and provide concrete, testable predictions for future detection.