On the inspiral of Massive Black Holes in gas-rich galaxy mergers

Abstract

We present a study on the dynamics of massive BHs in galaxy mergers, obtained from a series of high-resolution N-Body/SPH simulations. The presence of a gaseous component is essential for the rapid formation of an eccentric (Keplerian) BH binary, that resides at the center of a massive (~10^9 Msun) turbulent nuclear disc. Using physically and/or numerically motivated recipes, we follow the accretion history of the BHs during the merger. The mass of the BHs increases as large central inflows of gas occur inside each galaxy, and their mass ratio varies with time. Given the encountered strong degeneracy between numerical resolution and physical assumptions, we suggest here three possible paths followed by the galaxies and the BHs during a merger in order to fulfill the M-sigma relation : Adjustment, Symbiosis, and BH Dominance. In an extremely high resolution run, we resolved the turbulent gas pattern down to parsec scales, and found that BH feedback is expected to be effective near the end of the merger. We then trace the BH binary orbit down to a scale of 0.1 pc modeling the nuclear disc as an equilibrium Mestel disc composed either of gas, gas and stars, or just stars. Under the action of dynamical friction against the rotating gaseous and/or stellar background the orbit circularizes. When this occurs, each BH is endowed with its own small-size (~0.01 pc) accretion disc comprising a few percent of the BH mass. Double AGN activity is expected to occur on an estimated timescale of ~10 Myrs, comparable to the inspiral time. The double nuclear point--like sources that may appear have typical separations of ~10 pc, and are likely to be embedded in the still ongoing starburst.