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

TL;DR

This paper investigates the origin and geometry of blue UV emission in the Blue excess Hot DOG W0204-0506. Using spatially resolved imaging polarimetry with VLT/FORS2 and SKIRT radiative-transfer modeling, the authors show that the UV light is dominated by scattered quasar light from a dusty polar outflow. The measured integrated polarization p=$24.7\pm0.7$% and the spatial gradients in p and $\chi$ require graphite-rich dust in a polar cone with $\psi_{\rm Cone}\lesssim 50^{\circ}$ and $\eta\gtrsim 45^{\circ}$, implying a gas mass of $(5\times10^{8}$–$4\times10^{9})\ M_{\odot}$, and mass-outflow rates of $\dot M_{\rm out}\gtrsim 100\ M_{\odot}\ \rm yr^{-1}$ (with a fiducial case near $\dot M_{\rm out}\approx 430\ M_{\odot}\ \rm yr^{-1}$). The inferred kinetic power and momentum flux are $\dot E_{\rm out}\gtrsim 10^{43.6}$–$10^{44.2}$ erg s$^{-1}$ and $\dot P_{\rm out}\gtrsim 10^{35.9}$–$10^{36.5}$ dyn, respectively, and the outflow may only partially couple to the AGN luminosity ($\sim$0.1% in the fiducial model). The study demonstrates that polarization imaging provides unique, complementary constraints on dusty quasar outflows and AGN feedback in extreme systems.

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.

Figures (4)

• Figure 1: SED of W0204--0506. The green circles show the measured flux densities and their 1$\sigma$ uncertainties in the bands described in the text. The solid black line shows the best-fit SED model composed of a heavily obscured luminous AGN (dashed magenta line), a lightly obscured lower luminosity AGN that accounts for the scattered light component (solid blue line) and a host galaxy from the Sbc template (dashed green line).
• Figure 2: (Top) The two leftmost panels show the resolved maps of the polarization angle and fraction of W0204-0506 measured with FORS2. Polarization values are shown for every pixel where the combined flux from all PSF-matched o- and e-beam images is detected above 5$\sigma$. The two rightmost panels show the HST/WFC3 images in the F555W and F160W bands. All cutouts have a size of 3$^{\prime\prime}$$\times$3$^{\prime\prime}$ and have been astrometrically aligned using stars within the field of view. The cross shows the position of the brightest F160W pixel in all panels. (Bottom) Representative best-fit model intensity and polarization maps, convolved with the FORS2, F555W or F160W PSF as appropriate. The cross shows the position of the quasar. The cyan-black dashed lines in the top panels show where the edges of a cone with $\psi_{\rm Cone}=40~\rm deg$ and an axis length of 10kpc projected by the 60 degree inclination of the model (see §\ref{['sec:discussion']} for details).
• Figure 3: Polarization fraction as a function of wavelength for the different dust models tested with SKIRT assuming the scattering medium consists of a polar outflow. The left panel shows the expected polarization fraction for bipolar cones for a number of different dust mixtures, while the right panel show the expected values for single approaching or receding cones for the preferred dust mixture. The blue circle shows the measurement for W0204--0506 in the $R_{\rm Special}$ band. The horizontal error-bar shows the width of the photometric band.
• Figure 4: Maps of the minimum $\chi^2$ found as a function of the three parameters fit: $\psi_{\rm Torus}$, $\psi_{\rm Cone}$ and $\eta$. The hatched regions show non-allowed combinations of parameters, either because we would have a direct line of sight to the accretion disk or because the cone would be larger than the torus opening. Approaching (receding) cones are presented as having positive (negative) inclination angles $\eta$.