A closer look at dwarf galaxies exhibiting MIR variability: AGN confirmation and comparison with non-variable dwarf galaxies

TL;DR

This study demonstrates that mid-infrared variability is an effective method for detecting active low-mass black holes in dwarf galaxies, revealing AGN activity and related features in a sample of low-mass, low-metallicity galaxies.

Contribution

It provides the first large-scale analysis linking MIR variability to AGN activity in dwarf galaxies, confirming AGN signatures with multi-wavelength diagnostics.

Findings

68% of MIR-variable candidates confirmed as AGN via optical/NIR diagnostics

Detection of near-infrared coronal line [S IX] in the lowest mass galaxy in the sample

Broad Paα emission suggests presence of broad-line region in some targets

Abstract

Detecting active black holes in dwarf galaxies has proven to be a challenge due to their small size and weak electromagnetic signatures. Mid-infrared variability has emerged as a promising tool that can be used to detect active low-mass black holes in dwarf galaxies. We analyzed 10.4 years of photometry from the ALL$WISE$/NEO$WISE$ multi-epoch catalogs, identifying 25 objects with AGN-like variability. Independent confirmation of AGN activity was found in 68% of these objects using optical and near-infrared diagnostics. Notably, we discovered a near-infrared coronal line [S IX] $\lambda$ 1.252 $\mu$m in J1205, the galaxy with the lowest stellar mass (log M$_{*}$ = 7.5 M$_{\odot}$) and low metallicity (12 + log(O/H) = 7.46) in our sample. Additionally, we found broad Pa$\alpha$ potentially from the BLR in two targets, and their implied black hole masses are consistent with black hole-stellar mass relations. Comparing non-variable galaxies with similar stellar masses and $WISE$ $W1-W2$ colors, we found no clear trends between variability and large-scale galaxy properties. However, we found that AGN activity likely causes redder $W1-W2$ colors in variable targets, while for the non-variable galaxies, the contribution stems from strong star formation activity. A high incidence of optical broad lines was also observed in variable targets. Our results suggest that mid-infrared variability is an effective method for detecting AGN activity in low-mass galaxies and can help uncover a larger sample of active low-mass ($<$ 10$^{6}$ M$_{\odot}$) black holes in the universe.