CaltechAUTHORS
Published October 2025 | Version Published
Journal Article Open

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

Creators

  • 1. ROR icon Diego Portales University
  • 2. ROR icon Astronomical Observatory
  • 3. ROR icon Ghent University
  • 4. ROR icon Infrared Processing and Analysis Center
  • 5. ROR icon University of Tarapacá
  • 6. ROR icon University of Leicester
  • 7. ROR icon California Institute of Technology
  • 8. ROR icon European University Cyprus
  • 9. ROR icon Jet Propulsion Lab
  • 10. ROR icon University of Crete
  • 11. ROR icon Northwestern College - Iowa
  • 12. ROR icon National Astronomical Observatories
  • 13. ROR icon University of Chinese Academy of Sciences
  • 14. ROR icon Beijing Normal University
  • 15. ROR icon University of Hertfordshire
  • 16. ROR icon North-West University

Abstract

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 III] 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.

Copyright and License

© The Authors 2025. Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Acknowledgement

RJA was supported by FONDECYT grant number 1231718 and by the ANID BASAL project FB210003. MS acknowledges support by the Ministry of Science, Technological Development and Innovation of the Republic of Serbia (MSTDIRS) through contract no. 451-03-136/2025-03/200002 with the Astronomical Observatory (Belgrade). DJW acknowledges support from the Science and Technology Facilities Council (STFC; grant code ST/Y001060/1). Part of this research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration (80NM0018D0004). ML was supported by the grants from the Rubin-Chile Fund (DIA3324), the National Natural Science Foundation of China (Nos.11988101, 11973051, 12041302), the China Postdoctoral Science Foundation (No. 2024M753247) and the International Partnership Program of Chinese Academy of Sciences, Program No.114A11KYSB20210010. TDS acknowledges the research project was supported by the Hellenic Foundation for Research and Innovation (HFRI) under the “2nd Call for HFRI Research Projects to support Faculty Members & Researchers” (Project Number: 3382). This research made use of Photutils, an Astropy package for detection and photometry of astronomical sources Bradley et al. 2024.

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Additional details

Related works

Is new version of
Discussion Paper: arXiv:2504.15913 (arXiv)

Funding

Agencia Nacional de Investigación y Desarrollo
1231718
Agencia Nacional de Investigación y Desarrollo
FB210003
Ministry of Education, Science and Technological Development
451-03-136/2025-03/200002
Science and Technology Facilities Council
ST/Y001060/1
National Aeronautics and Space Administration
80NM0018D0004
Rubin Chile Fund
DIA3324
National Natural Science Foundation of China
11988101
National Natural Science Foundation of China
11973051
National Natural Science Foundation of China
12041302
Chinal Postdoctoral Science Foundation
2024M753247
Chinese Academy of Sciences
114A11KYSB20210010
Hellenic Foundation for Research and Innovation
3382

Dates

Accepted
2025-07-11
Available
2025-10-16
Published online

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Caltech groups
Infrared Processing and Analysis Center (IPAC), Division of Physics, Mathematics and Astronomy (PMA)
Publication Status
Published