A Massive Gas Outflow Outside the Line-of-Sight: Imaging Polarimetry of the Blue Excess Hot Dust Obscured Galaxy W0204-0506

Roberto J. Assef, Marko Stalevski, Lee Armus, Franz E. Bauer, Andrew Blain, Murray Brightman, Tanio Díaz-Santos, Peter R. M. Eisenhardt, Román Fernández-Aranda, Hyunsung D. Jun, Mai Liao, Guodong Li, Lee R. Martin, Elena Shablovinskaia, Devika Shobhana, Daniel Stern, Chao-Wei Tsai, Andrey Vayner, Dominic J. Walton, Jingwen Wu, Dejene Zewdie

First listed 2025-04-22 | Last updated 2025-04-22

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.

Short digest

VLT/FORS2 imaging polarimetry of the blue-excess Hot DOG WISE J020446.13–050640.8 (W0204–0506) finds a spatially integrated UV polarization of 24.7 ± 0.7% in R_Special, clinching a scattered-light origin for the faint blue continuum. The source is resolved, showing a polarization fraction and angle gradient aligned with the HST/WFC3 morphology, which SKIRT radiative-transfer models reproduce with a dusty conical polar outflow that starts at the sublimation radius, has half-opening angle ≤50°, and is viewed at inclination ≥45°, favoring graphite-rich dust. Inferred gas mass and outflow velocity fall within ranges seen for [O III] outflows in other Hot DOGs, though the feedback impact on the host remains uncertain. The work showcases imaging polarimetry as a geometry probe for dusty quasar outflows complementary to spectroscopy.

Key figures to inspect

• Figure 1 (SED): Check how the blue excess is modeled—heavily obscured luminous AGN plus a lightly obscured, lower-luminosity component that accounts for scattered UV—quantifying the ~2 dex mismatch between UV and mid-IR power.
• Figure 2 (Maps): Inspect the pixel-resolved polarization fraction and angle versus HST F555W/F160W structure; the alignment and cone-edge overlays constrain the outflow axis, projected ∼10 kpc extent, and confirm the scatterers lie outside our direct line of sight.
• Figure 3 (p[λ] vs dust): Compare the measured ~25% R-band point to SKIRT predictions; note that graphite-rich dust and a single approaching/receding cone can reach the observed polarization, whereas other mixtures underpredict it.
• Figure 4 (χ² grids): Read the allowed regions for inclination and half-opening angle; the hatched zones mark disallowed direct-view configurations, highlighting the preferred i ≥45° and θ ≤50° solution consistent with a torus-limited cone.