The role of gas in the merging of massive black holes in galactic nuclei

A long-standing problem in astrophysics is whether galaxy mergers necessarily lead to massive black hole mergers in their centers. Over the past 20 years this problem has been studied in context of the long-term evolution of a black hole binary at the center of a dense stellar system. However, the fate of a binary in the stellar system is unclear and the coalescence possibly stalls unless some additional mechanism is able to extract angular momentum from the binary. The possible additional mechanism is very likely to be gas dynamical in origin, because observational and theoretical work both indicate that large amounts of gas can be present in the central regions of merging galaxies. Using high-resolution SPH numerical simulations, we investigate the effects of gas on the inspiral and merger of a massive black hole binary. We run a variety of models, ranging from simulations with a relatively smooth gas and nearly spherical cloud to cases in which the gas is in a disk and has a more clumpy spatial distribution.; We find that 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. Between these two regimes, we find a transition regime in which the evolution is temporarily slowed down when neither of these mechanisms is fully effective. In the variety of simulations that we perform, we find that a MBH binary will merge within few times 10 7yrs after the galaxies merge. For MBHs that satisfy the observed 'm - sigmac' 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 considerably from the 'm - sigma c' relation, strong tidal and/or resonant forces from 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.; Finally, we study accretion onto a MBH binary in a massive spherical gas cloud. We find that when the MBHs are close enough so that orbital effects become important, the Bondi-Hoyle theory overestimates the mass accretion rate by a factor of ten. In the cases that we study, we find accretion rates that are well above the Eddington accretion rate. The implications of accretion for the coalescence of the binary is that accretion tends to accelerate the process.