Mid-Infrared Colors of Dwarf Galaxies: Young Starbursts Mimicking Active Galactic Nuclei

TL;DR

This study evaluates infrared color methods for identifying active galactic nuclei in dwarf galaxies, revealing that star-forming dwarf galaxies can mimic AGN infrared signatures, thus complicating selection criteria.

Contribution

It demonstrates that common infrared color diagnostics are unreliable for dwarf galaxy AGN detection and provides a refined sample of candidate dwarf AGNs for follow-up.

Findings

Most optically-selected AGNs are not identified by WISE infrared colors.

Star-forming dwarf galaxies can mimic AGN infrared signatures.

A simple W1-W2 color cut is insufficient for reliable AGN selection in dwarfs.

Abstract

Searching for active galactic nuclei (AGN) in dwarf galaxies is important for our understanding of the seed black holes that formed in the early Universe. Here, we test infrared selection methods for AGN activity at low galaxy masses. Our parent sample consists of ~18,000 nearby dwarf galaxies (M*< 3 x 10^9 Msun, $z<0.055$) in the Sloan Digital Sky Survey with significant detections in the first three bands of the AllWISE data release from the Wide-field Infrared Survey Explorer (WISE). First, we demonstrate that the majority of optically-selected AGNs in dwarf galaxies are not selected as AGNs using WISE infrared color diagnostics and that the infrared emission is dominated by the host galaxies. We then investigate the infrared properties of optically-selected star-forming dwarf galaxies, finding that the galaxies with the reddest infrared colors are the most compact, with blue optical colors, young stellar ages and large specific star formation rates. These results indicate that great care must be taken when selecting AGNs in dwarf galaxies using infrared colors, as star-forming dwarf galaxies are capable of heating dust in such a way that mimics the infrared colors of more luminous AGNs. In particular, a simple $\mathrm{W1}-\mathrm{W2}$ color cut alone should not be used to select AGNs in dwarf galaxies. With these complications in mind, we present a sample of 41 dwarf galaxies worthy of follow-up observations that fall in WISE infrared color space typically occupied by more luminous AGNs.