Long-term follow-up observations of extreme coronal line emitting galaxies

TL;DR

This study provides two decades of follow-up observations of extreme coronal line emitting galaxies, confirming some as tidal disruption event echoes and others as active galactic nuclei, revealing diverse long-term behaviors.

Contribution

It offers the first long-term spectroscopic and photometric follow-up of ECLEs, distinguishing TDE-related echoes from persistent AGN activity, and constructs spectral templates for classification.

Findings

Five ECLEs show no coronal line recurrence, supporting TDE origin.

Two ECLEs exhibit persistent coronal lines, indicating AGN activity.

Mid-infrared declines suggest ongoing long-term evolution.

Abstract

We present new spectroscopic and photometric follow-up observations of the known sample of extreme coronal line emitting galaxies (ECLEs) identified in the Sloan Digital Sky Survey (SDSS). With these new data, observations of the ECLE sample now span a period of two decades following their initial SDSS detections. We confirm the nonrecurrence of the iron coronal line signatures in five of the seven objects, further supporting their identification as the transient light echoes of tidal disruption events (TDEs). Photometric observations of these objects in optical bands show little overall evolution. In contrast, mid-infrared (MIR) observations show ongoing long-term declines. The remaining two objects had been classified as active galactic nuclei (AGN) with unusually strong coronal lines rather than being TDE related, given the persistence of the coronal lines in earlier follow-up spectra. We confirm this classification, with our spectra continuing to show the presence of strong, unchanged coronal-line features and AGN-like MIR colours and behaviour. We have constructed spectral templates of both subtypes of ECLE to aid in distinguishing the likely origin of newly discovered ECLEs. We highlight the need for higher cadence, and more rapid, follow-up observations of such objects to better constrain their properties and evolution. We also discuss the relationships between ECLEs, TDEs, and other identified transients having significant MIR variability.