Black hole binary mergers within gas discs
Black hole binaries form after major galaxy mergers, but their fate is unclear as hardening through stellar scattering gets very inefficient at sub-pc distances. We investigate numerically the evolution of binaries within thin gas discs. We find that binaries decay at a rate which is in good agreement with analytical estimates, and that the accretion rate onto the black holes varies with time. Scaling our
results, we find that decay due to gas discs can be faster than that due to stars for separations below 0.01--0.1 pc. The minimum merger time scale is shorter than the Hubble time for M < 10^7 Msun. This implies that gas discs could commonly attend low mass black hole mergers, and that a significant population of binaries might exist at separations of a few 0.01 pc. For more massive binaries, we suggest that scattering of stars formed within circumbinary gas discs could act as a significant sink of binary angular momentum, and show preliminary calculations of this process.