Hot Dust Obscured Galaxies with Excess Blue Light: Dual AGN or Single
  AGN Under Extreme Conditions?
  Hot Dust-Obscured Galaxies (Hot DOGs) are a population of hyper-luminous
infrared galaxies identified by the WISE mission from their very red mid-IR
colors, and characterized by hot dust temperatures ($T>60~\rm K$). Several
studies have shown clear evidence that the IR emission in these objects is
powered by a highly dust-obscured AGN that shows close to Compton-thick
absorption at X-ray wavelengths. Thanks to the high AGN obscuration, the host
galaxy is easily observable, and has UV/optical colors usually consistent with
those of a normal galaxy. Here we discuss a sub-population of 8 Hot DOGs that
show enhanced rest-frame UV/optical emission. We discuss three scenarios that
might explain the excess UV emission: (i) unobscured light leaked from the AGN
by reflection over the dust or by partial coverage of the accretion disk; (ii)
a second unobscured AGN in the system; or (iii) a luminous young starburst.
X-ray observations can help discriminate between these scenarios. We study in
detail the blue excess Hot DOG WISE J020446.13-050640.8, which was
serendipitously observed by Chandra/ACIS-I for 174.5 ks. The X-ray spectrum is
consistent with a single, hyper-luminous, highly absorbed AGN, and is strongly
inconsistent with the presence of a secondary unobscured AGN. Based on this, we
argue that the excess blue emission in this object is most likely either due to
reflection or a co-eval starburst. We favor the reflection scenario as the
unobscured star-formation rate needed to power the UV/optical emission would be
$\gtrsim 1000~\rm M_{\odot}~\rm yr^{-1}$. Deep polarimetry observations could
confirm the reflection hypothesis.
