Supermassive black hole binaries in gaseous and stellar circumnuclear discs: orbital dynamics and gas accretion

The dynamics of two massive black holes in a rotationally supported nuclear disc of mass Mdisc = 10 8M⊙ is explored using N-Body/SPH simulations. Gas and star particles are co–present in the disc. Described with a Mestel profile, the disc has a vertical support provided by turbulence of the gas, and by stellar velocity dispersion. A primary black hole of mass 4 × 106M⊙ is placed at the centre of the disc, while a secondary black hole is set initially on an eccentric co-rotating orbit in the disc plane. Its mass is in a 1 to 1, 1 to 4, and to 10 ratio, relative to the primary. With this choice, we mimic the dynamics of black hole pairs released in the nuclear region at the end of a gas-rich galaxy merger. It is found that, under the action of dynamical friction, the two black holes form a close binary in ∼ 10 Myrs. The inspiral process is insensitive to the mass fraction in stars and gas present in the disc and is accompanied by the circularization of the orbit. We have revealed the occurrence of radial inflows around each individual black hole that can create small gaseous Keplerian disc weighing ∼ 2% of the black hole mass, and of size ∼ 0.01 pc. The mass of the tightly (loosely) bound particles increases (decreases) with time as the black holes spiral into closer and closer orbits. Double AGN activity is expected to occur on an estimated timescale of ∼ < 10 Myrs, comparable to the inspiral timescale. The double nuclear point–like sources that may appear during dynamical evolution have typical separations of ∼ < 10 pc.