Formation of Nuclear Disks and Supermassive Black Hole Binaries in Multi-Scale Hydrodynamical Galaxy Mergers

TL;DR

This paper presents multi-scale hydrodynamical simulations of galaxy mergers, revealing rapid formation of nuclear disks and supermassive black hole binaries, and explores how gas dynamics influence black hole evolution.

Contribution

It provides the first high-resolution simulations showing parsec-scale gas inflows and SMBH binary formation, highlighting the impact of gas thermodynamics and feedback.

Findings

SMBH binaries form within a million years post-merger.

Nuclear disks' properties depend on gas thermodynamics.

Rapid central mass accumulation affects SMBH orbital decay.

Abstract

(Abridged) We review the results of the first multi-scale, hydrodynamical simulations of mergers between galaxies with central supermassive black holes (SMBHs) to investigate the formation of SMBH binaries in galactic nuclei. We demonstrate that strong gas inflows produce nuclear disks at the centers of merger remnants whose properties depend sensitively on the details of gas thermodynamics. In numerical simulations with parsec-scale spatial resolution in the gas component and an effective equation of state appropriate for a starburst galaxy, we show that a SMBH binary forms very rapidly, less than a million years after the merger of the two galaxies. Binary formation is significantly suppressed in the presence of a strong heating source such as radiative feedback by the accreting SMBHs. We also present preliminary results of numerical simulations with ultra-high spatial resolution of 0.1 pc in the gas component. These simulations resolve the internal structure of the resulting nuclear disk down to parsec scales and demonstrate the formation of a central massive object (~ 10^8 Mo) by efficient angular momentum transport. This is the first time that a radial gas inflow is shown to extend to parsec scales as a result of the dynamics and hydrodynamics involved in a galaxy merger, and has important implications for the fueling of SMBHs. Due to the rapid formation of the central clump, the density of the nuclear disk decreases significantly in its outer region, reducing dramatically the effect of dynamical friction and leading to the stalling of the two SMBHs at a separation of ~1 pc. We discuss how the orbital decay of the black holes might continue in a more realistic model which incorporates star formation and the multi-phase nature of the ISM.