Aligned Hierarchical Black Hole Mergers in AGN disks revealed by GWTC-4
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| Format: | Preprint |
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2025
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| _version_ | 1866908563550502912 |
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| author | Li, Yin-Jie Wang, Yuan-Zhu Tang, Shao-Peng Fan, Yi-Zhong |
| author_facet | Li, Yin-Jie Wang, Yuan-Zhu Tang, Shao-Peng Fan, Yi-Zhong |
| contents | 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 $χ\sim 0.8$ as well as the broad mass distribution up to $\gtrsim 150M_\odot$, 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 $\sim 0.57^{+0.23}_{-0.31}$ of the second subpopulation, corresponding to a local rate of $\sim 0.25^{+0.38}_{-0.16} ~ {\rm Gpc}^{-3} {\rm yr}^{-1}$ (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. |
| format | Preprint |
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arxiv_https___arxiv_org_abs_2509_23897 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | Aligned Hierarchical Black Hole Mergers in AGN disks revealed by GWTC-4 Li, Yin-Jie Wang, Yuan-Zhu Tang, Shao-Peng Fan, Yi-Zhong High Energy Astrophysical Phenomena Cosmology and Nongalactic Astrophysics General Relativity and Quantum Cosmology 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 $χ\sim 0.8$ as well as the broad mass distribution up to $\gtrsim 150M_\odot$, 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 $\sim 0.57^{+0.23}_{-0.31}$ of the second subpopulation, corresponding to a local rate of $\sim 0.25^{+0.38}_{-0.16} ~ {\rm Gpc}^{-3} {\rm yr}^{-1}$ (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. |
| title | Aligned Hierarchical Black Hole Mergers in AGN disks revealed by GWTC-4 |
| topic | High Energy Astrophysical Phenomena Cosmology and Nongalactic Astrophysics General Relativity and Quantum Cosmology |
| url | https://arxiv.org/abs/2509.23897 |