Abstract

We report on key studies on the dynamics of black holes (BHs) in gas-rich galaxy mergers that underscore the vital role played by gas dissipation in promoting BH inspirals down to the smallest scales ever probed with the use of high-resolution numerical simulations. In major mergers, the BHs sink rapidly under the action of gas-dynamical friction while orbiting inside the massive nuclear disc resulting from the merger. The BHs then bind and form a Keplerian binary on a scale of * 5 pc. In minor mergers, BH pairing proceeds down to the minimum scale explored of 10–100 pc only when the gas fraction in the less massive galaxy is comparatively large to avoid its tidal and/or ram pressure disruption and the wandering of the light BH in the periphery of the main halo. Binary BHs enter the gravitational wave dominated inspiral only when their relative distance is typically of ~10−3 pc. If the gas preserves the degree of dissipation expected in a star-burst environment, binary decay continues down to 0.1 pc, the smallest length scale ever attained. Stalling versus hardening below * 0.1 pc is still matter of deep investigations, and there is no unique answer depending on the yet unexplored dynamics of gas in the vicinity of the binary.