Resolving the Nuclear Environments of Tidal Disruption Event Host Galaxies within 45 pc

TL;DR

This study uses high-resolution spectroscopy to analyze the nuclear environments of TDE host galaxies, revealing young starbursts, high stellar densities, and ionization signatures that suggest recent gas inflows and possible prior accretion events.

Contribution

It provides the highest spatial resolution spectroscopic analysis of TDE hosts, uncovering detailed nuclear stellar populations, gas properties, and ionization features within 45 pc.

Findings

Younger starburst populations within 44 pc of the nucleus.

High stellar densities exceeding typical nuclear star cluster densities.

Detection of high-ionization coronal lines indicating recent accretion activity.

Abstract

Using HST/STIS observations, we present the highest-spatial-resolution spectroscopic study to date of four tidal disruption event (TDE) host galaxies, with the best observed being the post-starburst (PSB) host of ASASSN-14li. The stellar population of ASASSN-14li's host, within 44 pc of the nucleus, reveals a younger recent starburst ($\sim$340 Myr) compared to the population at an offset radius of 88 pc that excludes the nucleus ($\sim$550 Myr), a radial age gradient suggesting gas inflows from a minor merger. We estimate a stellar density of $\sim5900 \pm 800 \, M_\odot / \mathrm{pc}^3$ within 30 pc of the nucleus of ASASSN-14li's host, exceeding densities expected for nuclear star clusters. High-ionization ``coronal" emission lines, [Fe VI] $\lambda 5677$, [Fe VII] $\lambda 6087$, and [Fe X] $\lambda 6375$, are also detected within the nuclear spectra of the hosts of ASASSN-14li and PTF09ge, importantly alongside the non-detection of [O III] at the same scale. We similarly do not detect [O III] in the nuclear region of ASASSN-14ae's host despite its presence in the SDSS spectrum. The different ionization radiation levels detected at various radii from TDE host nuclei may indicate echoes of earlier accretion episodes, including, potentially, a prior TDE. We posit that a minor merger driving gas inflow to the nucleus could drive the enhanced TDE rates in post-starburst galaxies, inducing variation in nuclear gas properties and star formation history on $<$150 pc scales in TDE hosts.