AGN as Potential Factories for Eccentric Black Hole Mergers

TL;DR

This paper explores how active galactic nuclei (AGN) environments can produce eccentric black hole mergers, potentially explaining observed events like GW190521 and predicting distinct spin-orbit tilt distributions.

Contribution

It demonstrates that frequent interactions in AGN disks can generate an excess of eccentric mergers with specific spin-orbit tilt characteristics, a novel insight into black hole merger dynamics.

Findings

AGN disks can produce more eccentric black hole mergers.

Eccentric mergers have distinct spin-orbit tilt distributions.

Interactions with mutual inclinations below a few degrees are key.

Abstract

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.