Tidal Disruption Events Prefer Unusual Host Galaxies

TL;DR

Tidal Disruption Events are predominantly found in rare, quiescent Balmer-strong galaxies, which are over-represented by a factor of 33-190 compared to their occurrence in the general galaxy population.

Contribution

This study quantifies the over-representation of TDE hosts in quiescent Balmer-strong galaxies using SDSS data, revealing a significant preference for these unusual host galaxies.

Findings

TDE hosts are over-represented by a factor of 33-190 in quiescent Balmer-strong galaxies.

Most optical/UV-selected TDE candidates occur in galaxies with recent starburst signatures.

High-energy TDE Swift J1644 is also in a Balmer-strong galaxy, linking optical/UV and high-energy TDE classes.

Abstract

Tidal Disruption Events (TDEs) are transient events observed when a star passes close enough to a supermassive black hole to be tidally destroyed. Many TDE candidates have been discovered in host galaxies whose spectra have weak or no line emission yet strong Balmer line absorption, indicating a period of intense star formation that has recently ended. As such, TDE host galaxies fall into the rare class of quiescent Balmer-strong galaxies. Here, we quantify the fraction of galaxies in the Sloan Digital Sky Survey (SDSS) with spectral properties like those of TDE hosts, determining the extent to which TDEs are over-represented in such galaxies. Galaxies whose spectra have Balmer absorption H$\delta_{\rm A}$ $-$ $\sigma$(H$\delta_{\rm A}$) $>$ 4 \AA\ (where $\sigma$(H$\delta_{\rm A}$) is the error in the Lick H$\delta_{\rm A}$ index) and H$\alpha$ emission EW $<$ $3$ \AA\ have had a strong starburst in the last $\sim$Gyr. They represent 0.2% of the local galaxy population, yet host 3 of 8 (37.5%) optical/UV-selected TDE candidates. A broader cut, H$\delta_{\rm A} >$ 1.31 \AA\ and H$\alpha$ EW $<$ $3$ \AA, nets only 2.3% of SDSS galaxies, but 6 of 8 (75%) optical/UV TDE hosts. Thus, quiescent Balmer-strong galaxies are over-represented among the TDE hosts by a factor of 33-190. The high-energy-selected TDE Swift J1644 also lies in a galaxy with strong Balmer lines and weak H$\alpha$ emission, implying a $>80\times$ enhancement in such hosts and providing an observational link between the $\gamma$/X-ray-bright and optical/UV-bright TDE classes.