Strong Scatterings Invalidate Proposed Models of Enhanced TDE Rates in Post-Starburst Galaxies

TL;DR

This study revisits models explaining the overrepresentation of Tidal Disruption Events in post-starburst galaxies, finding that common hypotheses like steep stellar densities and velocity anisotropies are insufficient when considering strong scatterings, indicating the need for new explanations.

Contribution

The paper introduces a revised loss cone theory that includes both weak and strong scatterings, challenging previous models of TDE enhancement in post-starburst galaxies.

Findings

Ultra-steep densities fail to explain TDE preference with strong scatterings.

Radial velocity anisotropies are insufficient unless combined with high SMBH mass and specific stellar profiles.

Existing models are inadequate, suggesting the need for new theoretical frameworks.

Abstract

Stars wandering too close to supermassive black holes (SMBHs) can be ripped apart by the tidal forces of the black hole. Recent optical surveys have revealed that E+A galaxies are overrepresented by a factor $\sim $ 30, while green galaxies are overrepresented in both optical and infrared surveys. Different stellar models have been proposed to explain this Tidal Disruption Event (TDE) preference: ultra-steep stellar densities in the nuclear cluster, radial velocity anisotropies, and top-heavy Initial Mass Function (IMF). Here we explore these hypotheses in the framework of our revised loss cone theory that accounts for both weak and strong scattering, i.e., a scattering strong enough to eject a star from the nuclear cluster. We find that, when accounting for weak and strong scatterings, both ultra-steep densities and radial velocity anisotropies fail to explain the post-starburst preference of TDEs except when considering a high anisotropy factor together with a high SMBH mass and a shallow density profile of stellar mass black holes $\gamma_{\rm bh} =7/4$. Our findings hold when combining either model with top-heavy IMFs. Hence, new models to explain the post-starburst preference of TDEs are needed.