SkyMapper colours of Seyfert galaxies and Changing-Look AGN

TL;DR

This study demonstrates that SkyMapper broad-band colours, especially the $u-v$ index, effectively identify low-luminosity Seyfert galaxies and detect Changing-Look AGN through variability analysis over five years.

Contribution

It introduces a novel colour-based selection method using SkyMapper data to find Seyfert galaxies and Changing-Look AGN, including a new transient detection approach.

Findings

The $u-v$ index separates different galaxy types and detects AGN power-law continua.

A selection cut identifies type-1 AGN with high accuracy.

Detected 46 Changing-Look AGN candidates, including a transient flare lasting over five years.

Abstract

We study the utility of broad-band colours in the SkyMapper Southern Survey for selecting Seyfert galaxies at low luminosity. We find that the $u-v$ index, built from the ultraviolet $u$ and violet $v$ filters, separates normal galaxies, starburst galaxies and type-1 AGN. This $u-v$ index is not sensitive to age or metallicity in a stellar population but is instead a quenching-and-bursting indicator in galaxies and detects power-law continua in type-1 AGN. Using over 25,000 galaxies at $z<0.1$ from 6dFGS, we find a selection cut based on $u-v$ and central $u$ band brightness that identifies type-1 AGN. By eyeballing 6dFGS spectra we classify new Seyfert galaxies of type 1 to 1.8. Our sample includes eight known Changing-Look AGN, two of which show such strong variability that they move across the selection cut during the five years of SkyMapper observations in DR3, along mixing sequences of nuclear and host galaxy light. We identify 46 Changing-Look AGN candidates in our sample, one of which has been reported as a type-IIn supernova. We show that this transient persists for at least five years and marks a flare in a Seyfert-1 period of a new Changing-Look AGN.