Mass accretion toward black holes in the final phase of galaxy mergers

Abstract

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.