Direct n-body simulations of tidal disruption rate evolution in unequal-mass galaxy mergers

TL;DR

This study uses direct N-body simulations to analyze how tidal disruption event rates evolve during unequal-mass galaxy mergers, revealing rate enhancements and dependencies on mass ratio.

Contribution

It extends previous equal-mass merger studies to unequal-mass mergers, providing new insights into TDE rate evolution and dependence on SMBH mass ratios.

Findings

TDE rates increase as SMBHs approach each other, peaking after they become bound.

The TDE rate enhancement can reach up to an order of magnitude compared to isolated galaxies.

TDE rate dependence on mass ratio follows a power law during the bound phase.

Abstract

The hierarchical galaxy formation model predicts supermassive black hole binaries (SMBHBs) in galactic nuclei. Due to the gas poor environment and the limited spatial resolution in observations they may hide in the center of many a galaxy. However, a close encounter of a star with one of the supermassive black holes (SMBHs) may tidally disrupt it to produce a tidal disruption event (TDE) and temporarily light up the SMBH. In a previous work, we investigated with direct N-BODY simulations the evolution of TDE rates of SMBHB systems in galaxy mergers of equal mass. In this work we extend to unequal mass mergers. Our results show that, when two SMBHs are far away from each other, the TDE rate of each host galaxy is similar as in an isolated galaxy. As the two galaxies and their SMBHs separation shrinks, the TDE rate is increasing gradually until it reaches a maximum shortly after the two SMBHs become bound. In this stage, the averaged TDE rate can be enhanced by several times to an order of magnitude relative to isolated single galaxies. Our simulations show that the dependence of the TDE accretion rate on the mass ratio in this stage can be well fitted by power law relations for both SMBHs. After the bound SMBHB forms, the TDE rate decreases with its further evolution. We also find that in minor mergers TDEs of the secondary SMBH during and after the bound binary formation are mainly contributed by stars from the other galaxy.