Revealing the χeff–q Correlation among Coalescing Binary Black Holes and Tentative Evidence for AGN-driven Hierarchical Mergers
The origin of the correlation between the effective spins ( χ _eff ) and mass ratios ( q ) of LIGO–Virgo–KAGRA’s binary black holes (BBHs) is still an open question. Motivated by the recent identification of two subpopulations of the BBHs, in this work we investigate the potential χ _eff – q correla...
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| Main Authors: | , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
IOP Publishing
2025-01-01
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| Series: | The Astrophysical Journal |
| Subjects: | |
| Online Access: | https://doi.org/10.3847/1538-4357/add535 |
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| Summary: | The origin of the correlation between the effective spins ( χ _eff ) and mass ratios ( q ) of LIGO–Virgo–KAGRA’s binary black holes (BBHs) is still an open question. Motivated by the recent identification of two subpopulations of the BBHs, in this work we investigate the potential χ _eff – q correlation for each subpopulation. Surprisingly, the χ _eff – q correlation either significantly weakens or disappears for the low-mass subpopulation if we introduce a second χ _eff distribution for the high-mass subpopulation, which likely originates from hierarchical mergers. This suggests that the χ _eff – q correlation in the overall population can be explained by the superposition of two distinct subpopulations. We find Bayesian evidence strongly favoring two separate χ _eff distributions over a single mass ratio–dependent distribution, with Bayes factors $\mathrm{ln}{ \mathcal B }\gt 4.2$ . The first subpopulation has a narrow χ _eff distribution peaking at ∼0.05, whose primary mass function shows a rapid decline beyond ∼40 M _⊙ , in agreement with first-generation BBHs. The second χ _eff distribution is broad and peaks at μ _χ _,2 ∼ 0.4, aligning with predictions for hierarchical mergers in active galactic nucleus (AGN) disks. However, we cannot exclude negative χ _eff values in the second subpopulation, suggesting that hierarchical mergers might occur both in AGN disks and stellar clusters. Furthermore, the inferred second χ _eff distribution might alternatively arise from other formation channels, such as stable mass transfer or chemically homogeneous evolution, if not interpreted as hierarchical mergers. |
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| ISSN: | 1538-4357 |