A massive gas outflow outside the line of sight: Imaging polarimetry of the blue excess Hot Dust-Obscured Galaxy W0204–0506

Aims. Hot Dust-Obscured Galaxies (Hot DOGs) are a population of hyper-luminous, heavily obscured quasars. Although nearly Compton-thick nuclear obscurations are typical for Hot DOGs, 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 used imaging polarization observations in the RSpecial band obtained with the FORS2 instrument at VLT. We compared 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 ± 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 active galactic nucleus sublimation radius with a half-opening angle of ≲50 deg viewed at an inclination ≳45 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 [O <ns0:sc>III</ns0:sc>] 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 of polarization imaging for studying dusty quasar outflows and providing complementary constraints to those obtained through traditional spectroscopic studies.