Binary Black Hole Accretion Flows in Merged Galactic Nuclei

We study the accretion flows from the circumbinary disks onto the supermassive binary black holes in a subparsec scale of the galactic center, using a smoothed particles hydrodynamics (SPH) code. Simulation models are presented in four cases of a circular binary with equal and unequal masses, and of an eccentric binary with equal and unequal masses. We find that the circumblack-hole disks are formed around each black holes regardless of simulation parameters. There are two-step mechanisms to cause an accretion flow from the circumbinary disk onto supermassive binary black holes: First, the tidally induced elongation of the circumbinary disk triggers mass inflow towards two closest points on the circumbinary disk from the black holes. 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 binaries, the mass supply undergoes the periodic on/off transitions during one orbital period because of the variation of periodic potential. The gap starts to close after the apastron and to open again after the next periastron passage. Due to this 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.