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

Abstract Using Hubble Space Telescope/Space Telescope Imaging Spectrograph 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 host of ASASSN-14li. The stellar population of ASASSN-14li’s host, within 44 pc of the nucleus, reveals a younger recent starburst (∼340 Myr) compared to the population at an offset radius of 88 pc that excludes the nucleus (∼550 Myr), a radial age gradient suggesting gas inflows from a minor merger. We estimate a stellar density of ∼5900 ± 800 M ⊙ 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 ] λ 5677, [Fe vii ] λ 6087, and [Fe x ] λ 6375, are also detected within the nuclear spectra of the hosts of ASASSN-14li and PTF09ge, importantly alongside the nondetection 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 Sloan Digital Sky Survey 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.