AGN as potential factories for eccentric black hole mergers

There is some weak evidence that the black hole merger named GW190521 had a non-zero eccentricity^1,2. In addition, the masses of the component black holes exceeded the limit predicted by stellar evolution³. The large masses can be explained by successive mergers^4,5, 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.