Evidence for Black Holes in Green Peas from WISE colors and variability
  We explore the presence of active galactic nuclei (AGN)/black holes (BH) in
Green Pea galaxies (GPs), motivated by the presence of high ionization emission
lines such as HeII and [NeIII] in their optical spectra. In order to identify
AGN candidates, we used mid-infrared (MIR) photometric observations from the
all-sky Wide-field Infrared Survey Explorer (WISE) mission for a sample of 1004
GPs. Considering only $>5\sigma$ detections with no contamination from
neighboring sources in AllWISE, we select 31 GPs out of 134 as candidate AGN
based on a stringent 3-band WISE color diagnostic. Using multi-epoch photometry
in W1 and W2 bands based on time-resolved unWISE coadd images, we find two
sources exhibiting variability in both the WISE bands among 112 GPs with
W1$\leqslant16$ mag and no contamination from neighboring sources in unWISE.
These two variable sources were selected as AGN by the WISE 3-band color
diagnostic as well. Compared to variable AGN fractions observed among low-mass
galaxy samples in previous studies, we find a higher fraction ($\sim1.8\%$) of
MIR variable sources among GPs, which demonstrates the uniqueness and
importance of studying these extreme objects. Through this work, we demonstrate
that MIR diagnostics are promising tools to select AGN that may be missed by
other selection techniques (including optical emission-line ratios and X-ray
emission) in star-formation dominated, low-mass, low-metallicity galaxies.
