There is some weak evidence that the black hole merger named GW190521 had a non-zero eccentricity1,2. In addition, the masses of the component black holes exceeded the limit predicted by stellar evolution3. The large masses can be explained by successive mergers4,5, which may be efficient in gas disks surrounding active galactic nuclei, but it is difficult to maintain an eccentric orbit all the way to the merger, as basic physics would argue for circularization6. Here we show that active galactic nuclei disk environments can lead to an excess of eccentric mergers, if the interactions between single and binary black holes are frequent5 and occur with mutual inclinations of less than a few degrees. We further illustrate that this eccentric population has a different distribution of the inclination between the spin vectors of the black holes and their orbital angular momentum at merger7, referred to as the spin–orbit tilt, compared with the remaining circular mergers.
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We are grateful to N. Bose, K. Holley-Bockelmann, A. Pai, M. Zevin and M. Safarzadeh for their useful suggestions. J.S. is supported by the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement no. 844629. D.J.D. received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement no. 101029157 and through Villum Fonden grant no. 29466. I.B. acknowledges the support of the Alfred P. Sloan Foundation and NSF grants PHY-1911796 and PHY-2110060. H.T. is financially supported by the Grants-in-Aid for Basic Research by the Ministry of Education, Science and Culture of Japan (17H01102, 17H06360) and the Japan Society for the Promotion of Science (JSPS) KAKENHI grant number JP21J00794. N.W.C.L. acknowledges the support of a Fondecyt Iniciación grant 11180005, the financial support from Millenium Nucleus NCN19-058 (TITANs) and the BASAL Centro de Excelencia en Astrofisica y Tecnologias Afines (CATA) grant CATA AFB170002 along with ANID BASAL projects ACE210002 and FB210003. Z.H. acknowledges support from NASA grant NNX15AB19G and NSF grants AST-1715661 and AST-2006176. B.L. gratefully acknowledges support from the European Union’s Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant agreement no. 847523 ‘INTERACTIONS’. This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme ERC-2014-STG under grant agreement no. 638435 (GalNUC) to B.K. M.E.P. gratefully acknowledges support from the Independent Research Fund Denmark (DFF) via grant no. DFF 8021-00400B.
The authors declare no competing interests.
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Samsing, J., Bartos, I., D’Orazio, D.J. et al. AGN as potential factories for eccentric black hole mergers. Nature 603, 237–240 (2022). https://doi.org/10.1038/s41586-021-04333-1