  Using SPH numerical simulations, we investigate the effects of gas on the
inspiral and merger of a massive black hole binary. This study is motivated by
the very massive nuclear gas disks observed in the central regions of merging
galaxies. Here we present results that expand on the treatment in a previous
work (Escala, Larson, Coppi & Mardones 2004; henceforth Paper I), by studying
more realistic models. We run a variety of models, ranging from simulations
with a relatively smooth gas disk to cases in which the gas has a more clumpy
spatial distribution. We also vary the inclination angle between the plane of
the binary and the plane of the disk, and the mass ratio between the MBHs and
the gaseous disk. We find that as in Paper I, in the early evolution of the
system the binary separation diminishes due to gravitational drag, and in the
later stages the medium responds by forming an ellipsoidal density enhancement
whose axis lags behind the binary axis; this offset produces a torque on the
binary that causes continuing loss of angular momentum and is able to reduce
the separation to distances where gravitational radiation is efficient. In the
simulations that we perform, we find that the coalescence timescale for the MBH
binary varies between 5 10^6 yr and 2.5 10^7 yr. For MBHs that satisfy the
observed `m-sigma_c' relation, we predict that in a merger of galaxies that
have at least 1% of their total mass in gas, the MBHs will coalesce soon after
the galaxies merge. We also predict that if the MBHs depart from the
`m-sigma_c' relation, the MBH binary can create a circumbinary gap in the disk
that stalls the coalescence, but this gap formation can act as a
self-regulatory mechanism on MBH growth that can help to explain the existence
of the `m-sigma_c' relation.
