Binary Black Hole Accretion Flows in Merged Galactic Nuclei

Abstract We consider accretion flows from circumbinary disks onto supermassive binary black holes on a subparsec scale of the galactic center based on a smoothed particles hydrodynamics (SPH) code. Simulation models are presented for four cases; that is, a circular binary and an eccentric one, each with equal and unequal masses. We find that the circumblack-hole disks are formed around each black hole regardless of the simulation parameters. There are two-step mechanisms that cause accretion flow. First, tidally induced elongation of the circumbinary disk triggers mass inflow toward two closest points on the circumbinary disk. Then, the gas is increasingly accumulated on these two points owing to the gravitational attraction of black holes. Second, when the gas can pass across the maximum loci of the effective binary potential, it starts to overflow via their two points, and freely infalls to each black hole. In circular binaries, the gas continues to be supplied from the circumbinary disk, (i.e., the gap between the circumbinary disk and the binary black hole is always closed). In eccentric cases, the mass supply undergoes periodic on/off transitions during one orbital period because of the variation of the periodic potential. The gap starts to close after the apoastron, and to open again after the next periastron passage. Due to the gap closing/opening cycles, the mass-capture rates are eventually strongly phase dependent. This could provide observable diagnosis for the presence of supermassive binary black holes in merged galactic nuclei.