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