Discovering Changing-Look AGN in the 6dF Galaxy Survey using ATLAS Light curves

TL;DR

This study identifies new changing-look AGN by analyzing light curves and spectra from the 6dF Galaxy Survey and ATLAS, revealing rates of AGN changing states over 15-20 years.

Contribution

It presents the first systematic search for turn-on CLAGN in a complete galaxy sample using archival spectra and light curves, discovering new CLAGN candidates.

Findings

Identified 12 new turn-on CLAGN and 19 turn-off CLAGN.

Estimated lower limits for CLAGN rates: 1.7% for turn-on and 9.6% for turn-off.

Found similar numbers of turn-on and turn-off CLAGN, indicating survey completeness.

Abstract

Changing-Look Active Galactic Nuclei (CLAGN) are characterised by extreme variations in line emission over short timescales, mostly affecting broad H$\beta$ lines. While a few hundred CLAGN are known, a complete sample of turn-on CLAGN is still elusive. Here, we present a search for turn-on CLAGN in a complete sample of galaxies, using archival spectra and recent light curves. We obtained light curves from the Asteroid Terrestrial Impact Last Alert System (ATLAS) for 16,232 emission line galaxies, including both star-forming and active galaxies, at $z<0.1$ with spectra from the Six-degree Field Galaxy Survey (6dFGS). We first establish typical variability behaviour for different AGN types, as recorded between 2001 and 2009, and then select outliers from the bulk behaviour as CLAGN candidates. We obtain new spectra for the candidates and identify 12 new turn-on CLAGN (appearing broad H$\beta$ line) and 19 new turn-off CLAGN (disappearing broad H$\beta$ line). We may have missed AGN that changed and reverted their state over the 15$-$20 years since 6dFGS spectra were taken, and thus our CLAGN rates of 1.7$\%$ for turn-on and 9.6$\%$ for turn-off are lower limits. The turn-on rate is naturally much lower as the type 1.9/2 sample is dominated by obscured AGN due to orientation, which are not expected to change. However, the number of turn-on (27) and turn-off (24) CLAGN we find are similar, suggesting that our parent AGN sample is reasonably complete in our search volume at $z<0.1$.