Tidal disruption rates in non-spherical galactic nuclei formed by galaxy mergers

TL;DR

This paper investigates how non-spherical galactic nuclei formed by mergers influence tidal disruption event rates, finding that non-sphericity generally increases TDE rates beyond steady-state spherical estimates, especially in more massive black holes.

Contribution

It demonstrates that galaxy mergers and resulting non-spherical shapes significantly enhance TDE rates, challenging previous steady-state spherical models.

Findings

TDE rates are higher in non-spherical nuclei than in spherical models.

Non-sphericity effects dominate over initial anisotropy in most cases.

Observed low TDE rates are unlikely due to star depletion by black hole binaries.

Abstract

We explore the rates of tidal disruption events (TDEs) of stars by supermassive black holes (SBHs) in galactic nuclei formed in mergers followed by a formation and coalescence of a binary SBH. Such systems initially have a deficit of stars on low-angular-momentum orbits caused by the slingshot process during the binary SBH stage, which tends to reduce the flux of stars into the SBH compared to the steady-state value. On the other hand, a newly formed galactic nucleus has a non-spherical shape which enhances the mixing of stars in angular momentum and thus the TDE rate. In galaxies with relatively low SBH masses (<10^7 Msun), relaxation times are short enough to wash out the anisotropy in initial conditions, and for more massive SBH the enhancement of flux due to non-sphericity turns out to be more important than its suppression due to initial anisotropy. Therefore, the present-day TDE rates generally exceed conventional steady-state estimates based on a spherical isotropic approximation. We thus conjecture that the lower observationally inferred TDE rates compared to theoretical predictions cannot be attributed to the depletion of low-angular-momentum stars by SBH binaries.