Aligned Hierarchical Black Hole Mergers in Active-Galactic-Nuclei Disks Revealed by GWTC-4
arXiv:2509.23897 · doi:10.1103/qyfv-wkv1
The authors analyze GWTC‑4 binary black hole events with a flexible population model and identify a subpopulation of high‑spin, massive black holes whose spins are preferentially aligned with the orbital angular momentum, a signature expected from hierarchical mergers in active‑galactic‑nucleus disks.
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 as well as the broad mass distribution up to , 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 of the second subpopulation, corresponding to a local rate of (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.
7 pages, 3 figures (main text) + 8 pages, 9 figures (Supplemental Material). Accepted for publication in PRL