Abstract
(Aims) Hot Dust Obscured Galaxies (Hot DOGs) are a population of
hyper-luminous, heavily obscured quasars. Although nuclear obscurations close
to Compton-thick are typical, a fraction show blue UV spectral energy
distributions consistent with unobscured quasar activity, albeit two orders of
magnitude fainter than expected from their mid-IR luminosity. The origin of the
UV emission in these Blue excess Hot DOGs (BHDs) has been linked to scattered
light from the central engine. Here we study the properties of the UV emission
in the BHD WISE J020446.13-050640.8 (W0204-0506). (Methods) We use imaging
polarization observations in the $R_{\rm Special}$ band obtained with the FORS2
instrument at VLT. We compare these data with radiative transfer simulations to
constrain the characteristics of the scattering material. (Results) We find a
spatially integrated polarization fraction of $24.7\pm 0.7$%, confirming the
scattered-light nature of the UV emission of W0204-0506. The source is
spatially resolved in the observations and we find a gradient in polarization
fraction and angle that is aligned with the extended morphology of the source
found in HST/WFC3 imaging. A dusty, conical polar outflow starting at the AGN
sublimation radius with a half-opening angle $\lesssim 50~\rm deg$ viewed at an
inclination $\gtrsim 45~\rm deg$ can reproduce the observed polarization
fraction if the dust is graphite-rich. We find that the gas mass and outflow
velocity are consistent with the range of values found for [OIII] outflows
through spectroscopy in other Hot DOGs, though it is unclear whether the
outflow is energetic enough to affect the long-term evolution of the host
galaxy. Our study highlights the unique potential for polarization imaging to
study dusty quasar outflows, providing complementary constraints to those
obtained through traditional spectroscopic studies.