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Using high-resolution SPH numerical simulations, we investigate the effectsof gas on the inspiral and merger of a massive black hole binary. This study ismotivated by both observational and theoretical work that indicate the presenceof large amounts of gas in the central regions of merging galaxies. N-bodysimulations have shown that the coalescence of a massive black hole binaryeventually stalls in a stellar background. However, our simulations suggestthat the massive black hole binary will finally merge if it is embedded in agaseous background. Here we present results in which the gas is assumed to beinitially spherical with a relatively smooth distribution. In the earlyevolution of the binary, the separation dimishes due to the gravitational dragexerted by the background gas. In the later stages, when the binary dominatesthe gravitational potential in its vicinity, the medium responds by forming anellipsoidal density enhancement whose axis lags behind the binary axis, andthis offset produces a torque on the binary that causes continuing loss ofangular momentum and is able to reduce the binary separation to distances wheregravitational radiation is efficient. Assuming typical parameters fromobservations of Ultra Luminous Infrared Galaxies, we predict that a black holebinary will merge within $10^{7}$yrs; therefore these results imply that in amerger of gas-rich galaxies, any massive central black holes will coalescencesoon after the galaxies merge. Our work thus supports scenarios of massiveblack hole evolution and growth where hierarchical merging plays an importantrole. The final coalescence of the black holes leads to gravitational radiationemission that would be detectable up to high redshift by LISA. We show thatsimilar physical effects are important for the formation of close binary stars.Comment: 38 pages, 14 figures, submitted to Ap

The Role of Gas in the Merging of Massive Black Holes in Galactic Nuclei. I. Black Hole Merging in a Spherical Gas Cloud