A Closer Look at Dwarf Galaxies Exhibiting Mid-infrared Variability: Active Galactic Nuclei Confirmation and Comparison With Nonvariable Dwarf Galaxies

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 yr of photometry from the AllWISE/NEOWISE multiepoch catalogs, identifying 25 objects with active galactic nuclei (AGN)-like variability. Independent confirmation of AGN activity was found in 68% of these objects using optical and near-infrared diagnostics. Notably, we discover a near-infrared coronal line [S ix ] λ 1.252 μ m in J1205, the galaxy with the lowest stellar mass (log M _* = 7.5 M _⊙ ) and low metallicity (12 + log(O/H) = 7.46) in our sample. Additionally, we find broad Pa α potentially from the broad-line region in two targets, and their implied black hole masses are consistent with black hole-stellar mass relations. Comparing nonvariable galaxies with similar stellar masses and Wide-field Infrared Survey Explorer W 1 − W 2 colors, we find no clear trends between variability and large-scale galaxy properties. However, we find that AGN activity likely causes redder W 1 − W 2 colors in variable targets, while for the nonvariable 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 _⊙ ) black holes in the Universe.

Keywords

• Active galactic nuclei
• Dwarf galaxies
• Infrared galaxies