Mid-infrared Variability-based AGN Selection using the Multi-epoch Photometric Data from WISE

TL;DR

This study evaluates mid-infrared variability as a method to identify active galactic nuclei (AGNs) using WISE data, revealing its effectiveness in detecting AGNs in various host galaxies and its limitations in LINERs.

Contribution

The paper introduces a MIR variability-based AGN selection technique utilizing multi-epoch WISE data, demonstrating its ability to find AGNs in optically inactive hosts and analyzing its dependence on host galaxy properties.

Findings

Recovers ~28.2% of optically selected AGNs.

Identifies AGN candidates in 0.4-11.8% of inactive galaxies.

AGN fraction increases with star formation activity.

Abstract

We assess the systematics and efficiency of an AGN selection method based on mid-infrared (MIR) variability. To this end, we utilize various types of active and inactive galaxies from the Sloan Digital Sky Survey, matching them with multi-epoch photometric data from the NEOWISE mission. Using W1 and W2 band light curves with a $\sim10$-year baseline, we find that combining the likelihood of deviation from non-variability with the correlation coefficient between the W1 and W2 bands reliably identifies AGNs. Specifically, this MIR-based method recovers $\sim 28.2\%$ of optically selected AGNs. Applying the same technique to inactive galaxies, we identify AGN candidates at fractions ranging from $0.4$ to $11.8\%$, indicating that MIR variability allows us to detect AGN candidates even in optically inactive hosts. While some variable sources exhibit transient-like light curves, possibly originating from tidal disruption events or supernovae, their contribution to the total variable population is less than a few percent, indicating a minimal impact on our results. Across all subsamples, the AGN fraction marginally increases with star formation activity, implying coordinated evolution between central black hole growth and star formation. Finally, the AGN fraction inferred from our method drops dramatically in classical LINERs, consistent with their low accretion rates and absence of a dusty torus.