Black Hole Binary Formation in AGN Discs: From Isolation to Merger

Motivated by the increasing number of gravitational wave detections of merging black holes (BHs) by LIGO-VIRGO-KAGRA, BH binary mergers in the discs of active galactic nuclei (AGN) is investigated as a possible merger channel. In this pathway, BHs in the large gas disc are expected to encounter one another, become mutually bound as a BH binary system through interaction with the gas in the disc and subsequently inspiral through gravitational torques induced by the local gas until merger. To more precisely determine the feasibility of this merger pathway, we present the first 3D global hydrodynamic simulations of both the formation and evolution of a stellar-mass BH binaries AGN discs with three different AGN disc masses and five different initial radial separations. These 15 different simulations show that binary capture can be successful in a range of local gas densities including cases well below that of a standard radiatively efficient alpha disc, and identify that the majority of these captured binaries are then subsequently hardened by the surrounding gas. Both prograde and retrograde binaries are formed in our simulations, which tend to be slowly hardened by the gas. The eccentricity evolution is found to depend strongly on the orbital rotation of the binary where prograde binaries are governed by gravitational torques form their circumbinary mini-disc, with eccentricities being damped over time, while for retrograde binaries the eccentricities are excited to >0.9 by accretion torques. In two cases, retrograde binaries ultimately undergo a close periapsis passage which results in a merger via gravitational waves after only a few thousand binary orbits. Thus, the merger timescale for retrograde binaries can be far shorter than the lifetime of an AGN disc. These simulations support the case that the AGN merger pathway could be highly efficient for merging BHs.