Discovery of a Fast Iron Low-ionization Outflow in the Early Evolution of the Nearby Tidal Disruption Event AT2019qiz

TL;DR

This paper reports the discovery of a unique iron low-ionization outflow in the early evolution of the nearby TDE AT2019qiz, revealing new insights into TDE outflows and their connection to supernovae.

Contribution

It presents the first observation of an FeLoBAL system in a TDE and tracks its spectral evolution, linking TDE outflows to engine-powered supernovae.

Findings

Detection of FeLoBAL system in early TDE spectrum

Deceleration of outflow velocity from 15,000 km/s to 10,000 km/s

Spectral evolution from low to high ionization BALs over 50 days

Abstract

We report the results of ultraviolet (UV) and optical photometric and spectroscopic analysis of the tidal disruption event (TDE) AT2019qiz. Our follow-up observations started $<$10 days after the source began to brighten in the optical and lasted for a period of six months. Our late-time host-dominated spectrum indicates that the host galaxy likely harbors a weak active galactic nucleus. The initial {\it Hubble Space Telescope (HST)} spectrum of AT2019qiz exhibits an iron and low-ionization broad absorption line (FeLoBAL) system that is seen for the first time in a TDE. This spectrum also bears a striking resemblance to that of Gaia16apd, a superluminous supernova. Our observations provide insights into the outflow properties in TDEs and show evidence for a connection between TDEs and engine-powered supernovae at early phase, as originally suggested in Metzger & Stone (2016). In a time frame of 50 days, the UV spectra of AT2019qiz started to resemble previous TDEs with only high-ionization BALs. The change in UV spectral signatures is accompanied by a decrease in the outflow velocity, which began at $15,000$ km s$^{-1}$ and decelerated to $\sim10,000$ km s$^{-1}$. A similar evolution in the H$\alpha$ emission line width further supports the speculation that the broad Balmer emission lines are formed in TDE outflows. In addition, we detect narrow absorption features on top of the FeLoBAL signatures in the early HST UV spectrum of AT2019qiz. The measured HI column density corresponds to a Lyman-limit system whereas the metal absorption lines, such as NV, CIV, FeII, and MgII, are likely probing the circumnuclear gas and interstellar medium in the host galaxy.