Mass accretion toward black holes in the final phase of galaxy mergers
  We studied the final phases of galactic mergers, focusing on interactions
between supermassive black holes (SMBHs) and the interstellar medium in a
central sub-kpc region, using an N-body/hydrodynamics code. This numerical
experiment aims to understand the fate of the gas supplied by mergers of two or
more galaxies with SMBHs, whose masses are $10^7 M_\odot$. We observed that the
mass accretion rate to one SMBH exceeds the Eddington accretion rate when the
distance between two black holes (BHs) rapidly decreases. However, this rapid
accretion phase does not last for more than $10^7$ yrs, and it drops to $\sim$
10% of the Eddington rate in the quasi-steady accretion phase. The rapid
accretion is caused by the angular momentum transfer from the gas to the
stellar component, and the moderate accretion in the quasi-steady phase is
caused {by turbulent viscosity and gravitational torque in the disk. The second
merger event enhances the mass accretion to the BHs; however, this phase takes
place on a similar timescale to the first merger event. We also found that the
AGN feedback and the mass accretion to BHs can coexist in the central region of
merged galaxies, if the amount of feedback energy is given as $(2 \times
10^{-4} - 2 \times 10^{-3} )\dot{M} c^2$, where $\dot{M}$ is the accretion rate
to $r= 1$ pc. The accretion rate is suppressed by $\sim$ 1/50 in the
quasi-steady accretion phase for $0.02 \dot{M} c^2$. The fraction of the gas
that finally falls to each BH is approximately 5-7% of the supplied total gas
mass ($10^8 M_\odot$), and 15-20% of the gas forms a circumnuclear gas inside
100 pc. This remnant gas heavily obscures the luminous phase of the active
galactic nuclei (AGN) during merger events, and the moderate AGN feedback does
not alter this property.
