Enhanced off-center stellar tidal disruptions by supermassive black holes in merging galaxies

TL;DR

This paper models how merging galaxies with dual supermassive black holes significantly increase off-center stellar tidal disruption rates, complicating the identification of recoiling SMBHs and providing insights into galaxy merger histories.

Contribution

It introduces an analytical model to compute tidal flare rates in merging galaxies, highlighting the enhancement due to tidal perturbations and chaotic stellar orbits, and compares these with isolated and recoiling SMBHs.

Findings

Merging galaxies boost tidal disruption rates by up to 100 times compared to isolated SMBHs.

Off-center tidal flares from merging galaxies are comparable in frequency to those from single SMBHs.

Enhanced rates are mainly due to tidal perturbations from the companion galaxy.

Abstract

Off-center stellar tidal disruption flares have been suggested to be a powerful probe of recoiling supermassive black holes (SMBHs) out of galactic centers due to anisotropic gravitational wave radiations. However, off-center tidal flares can also be produced by SMBHs in merging galaxies. In this paper, we computed the tidal flare rates by dual SMBHs in two merging galaxies before the SMBHs become self-gravitationally bounded. We employ an analytical model to calculate the tidal loss-cone feeding rates for both SMBHs, taking into account two-body relaxation of stars, tidal perturbations by the companion galaxy, and chaotic stellar orbits in triaxial gravitational potential. We show that for typical SMBHs with mass 10^7 M_\sun, the loss-cone feeding rates are enhanced by mergers up to \Gamma ~ 10^{-2} yr^{-1}, about two order of magnitude higher than those by single SMBHs in isolated galaxies and about four orders of magnitude higher than those by recoiling SMBHs. The enhancements are mainly due to tidal perturbations by the companion galaxy. We suggest that off-center tidal flares are overwhelmed by those from merging galaxies, making the identification of recoiling SMBHs challenging. Based on the calculated rates, we estimate the relative contributions of tidal flare events by single, binary, and dual SMBH systems during cosmic time. Our calculations show that the off-center tidal disruption flares by un-bound SMBHs in merging galaxies contribute a fraction comparable to that by single SMBHs in isolated galaxies. We conclude that off-center tidal disruptions are powerful tracers of the merging history of galaxies and SMBHs.