Variability Selected Active Galactic Nuclei from ASAS-SN Survey: Constraining the Low Luminosity AGN Population

TL;DR

This study uses variability analysis of galaxy light curves from the ASAS-SN survey to identify low luminosity AGN, revealing their prevalence and characteristics in the galaxy population.

Contribution

It introduces a variability-based method to identify LLAGN and estimates their fraction in the galaxy population, including spectroscopic confirmation.

Findings

37 variable AGN candidates identified from 1218 galaxies.

Most are low Eddington ratio LLAGN, constituting about 2% of galaxies.

Up to 60% of candidates are confirmed as AGN spectroscopically.

Abstract

Low luminosity active galactic nuclei (LLAGN) probe accretion physics in the low Eddington regime and can provide additional clues about galaxy evolution. AGN variability is ubiquitous and thus provides a reliable tool for finding AGN. We analyze the All-Sky Automated Survey for SuperNovae light curves of 1218 galaxies with $g<14$ mag and Sloan Digital Sky Survey spectra in search of AGN. We find 37 objects that are both variable and have AGN-like structure functions, which is about 3% of the sample. The majority of the variability selected AGN are LLAGN with Eddington ratios ranging from $10^{-4}$ to $10^{-2}$. We thus estimate the fraction of LLAGN in the population of galaxies as 2% down to a median Eddington ratio of $2\times10^{-3}$. Combining the BPT line ratio diagnostics and the broad-line AGN, up to $\sim$60% of the AGN candidates are confirmed spectroscopically. The BPT diagnostics also classified 10-30% of the candidates as star forming galaxies rather than AGN.