Massive black hole and gas dynamics in mergers of galaxy nuclei - ii. Black hole sinking in star-forming nuclear discs

Mergers of gas-rich galaxies are key events in the hierarchical built-up of cosmic structures,and can lead to the formation of massive black hole binaries. By means of high-resolutionhydrodynamical simulations we consider the late stages of a gas-rich major merger, detailingthe dynamics of two circumnuclear discs, and of the hosted massive black holes during theirpairing phase. During the merger gas clumps with masses of a fraction of the black hole massform because of fragmentation. Such high-density gas is very effective in forming stars, andthe most massive clumps can substantially perturb the black hole orbits. After ∼10 Myr fromthe start of the merger a gravitationally bound black hole binary forms at a separation of a fewparsecs, and soon after, the separation falls below our resolution limit of 0.39 pc. At the time ofbinary formation the original discs are almost completely disrupted because of SNa feedback,while on pc scales the residual gas settles in a circumbinary disc with mass ∼105 M⊙. Wealso test that binary dynamics is robust against the details of the SNa feedback employed inthe simulations, while gas dynamics is not. We finally highlight the importance of the SNatime-scale on our results.