A Chandra and HST View of WISE-Selected AGN Candidates in Dwarf Galaxies
  Reliably identifying active galactic nuclei (AGNs) in dwarf galaxies is key
to understanding black hole demographics at low masses and constraining models
for black hole seed formation. Here we present Chandra X-ray Observatory
observations of eleven dwarf galaxies that were chosen as AGN candidates using
Wide-field Infrared Survey Explorer (WISE) mid-infrared (mid-IR) color-color
selection. Hubble Space Telescope images are also presented for ten of the
galaxies. Based on Sloan Digital Sky Survey spectroscopy, six galaxies in our
sample have optical evidence for hosting AGNs and five are classified as
star-forming. We detect X-ray point sources with luminosities above that
expected from X-ray binaries in the nuclei of five of the six galaxies with
optical evidence of AGNs. However, the X-ray emission from these AGNs is
generally much lower than expected based on AGN scaling relations with infrared
and optical tracers. We do not find compelling evidence for AGNs in the five
optically-selected star-forming galaxies despite having red mid-IR colors. Only
two are detected in X-rays and their properties are consistent with
stellar-mass X-ray binaries. Based on this multiwavelength study, we conclude
that two-color mid-IR AGN diagnostics at the resolution of WISE cannot be used
to reliably select AGNs in optically-star-forming dwarf galaxies. Future
observations in the infrared with the James Webb Space Telescope offer a
promising path forward.
