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. In addition, the component black holes' masses exceeded
the limit predicted by stellar evolution. The large masses can be explained by
successive mergers, which may be efficient in gas disks surrounding active
galactic nuclei (AGN), but it is difficult to maintain an eccentric orbit all
the way to the merger, as basic physics would argue for circularization. Here
we show that AGN-disk environments can lead to an excess of eccentric mergers,
if the interactions between single and binary black holes are frequent, 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 merger, referred to as the spin-orbit tilt, compared to the
remaining circular mergers.
