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

TL;DR

This study investigates the final stages of galaxy mergers, revealing how SMBH accretion rates evolve, the impact of feedback, and the fate of gas, providing insights into AGN activity and obscuration during mergers.

Contribution

It presents detailed numerical simulations of SMBH accretion during galaxy mergers, highlighting rapid and moderate accretion phases, feedback effects, and gas dynamics in the central regions.

Findings

Rapid accretion exceeds Eddington rate during close BH approach.

Moderate accretion persists for over 10 million years.

Gas heavily obscures AGN activity during 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.