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

Clark, Peter and Graur, Or and Callow, Joseph and Aguilar, Jessica and Ahlen, S.P. and Anderson, Joseph and Berger, Edo and Müller-Bravo, Tomás E. and Brink, Thomas G. and Brooks, David and Chen, Ting-Wan and Claybaugh, Todd and Pettersson, Axel de la Macorra and Doel, Peter and Filippenko, Alexei and Forero-Romero, Jaime and Gomez, Sebastian and Gromadzki, Mariusz and Honscheid, Klaus and Inserra, Cosimo and Kisner, Theodore and Landriau, Martin and Makrygianni, Lydia and Manera, Marc and Meisner, Aaron and Miquel, Ramon and Moustakas, John and Nicholl, Matt and Nie, Jundan and Onori, Francesca and Palmese, Antonella and Poppett, Claire and Reynolds, Thomas and Rezaie, Mehdi and Rossi, Graziano and Sanchez, E. and Schubnell, Michael and Tarlé, Gregory and Weaver, Benjamin A and Wevers, Thomas and Young, David and Zheng, Weikang and Zhou, Zhimin (2024) Long-term follow-up observations of extreme coronal line emitting galaxies. Monthly Notices of the Royal Astronomical Society, 528 (4). pp. 7076-7102. ISSN 0035-8711

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 non-recurrence 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 consistent with power-law decay. The remaining two objects had been classified as active galactic nuclei (AGNs) 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.