A benchmark for extreme conditions of the multiphase interstellar medium in the most luminous hot dust-obscured galaxy at z = 4.6

¹ Department of Physics, University of Crete, 70013 Heraklion, Greece
e-mail: rfernandez@physics.uoc.gr
² Institute of Astrophysics, Foundation for Research and Technology – Hellas (FORTH), Voutes, 70013 Heraklion, Greece
³ European Southern Observatory (ESO), Karl-Schwarzschild-Strasse 2, 85748 Garching b. München, Germany
⁴ School of Sciences, European University Cyprus, Diogenes Street, Engomi, 1516 Nicosia, Cyprus
⁵ Instituto de Estudios Astrofísicos, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Av. Ejército Libertador 441, Santiago, Chile
⁶ Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
⁷ Department of Physics, Winona State University, Winona, MN 55987, USA
⁸ Dipartimento di Fisica “G. Occhialini”, Università degli Studi di Milano-Bicocca, Piazza della Scienza 3, 20126 Milano, Italy
⁹ Cornell Center for Astrophysics and Planetary Science, Cornell University, Ithaca, NY 14853, USA
¹⁰ Department of Astronomy, Cornell University, Ithaca, NY 14853, USA
¹¹ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, 40129 Bologna, Italy
¹² Physics & Astronomy, University of Leicester, 1 University Road, Leicester LE1 7RH, UK
¹³ Joint ALMA Observatory, Alonso de Córdova 3101, Santiago 7630410, Chile
¹⁴ Department of Physics, Northwestern College, 101 7th St SW, Orange City, IA 51041, USA
¹⁵ National Astronomical Observatories, Chinese Academy of Sciences, 20A Datun Road, Beijing 100101, PR China
¹⁶ University of Chinese Academy of Sciences, Beijing 100049, PR China
¹⁷ Chinese Academy of Sciences South America Center for Astronomy, National Astronomical Observatories, CAS, Beijing 100101, PR China
¹⁸ Institute for Frontiers in Astronomy and Astrophysics, Beijing Normal University, Beijing 102206, PR China

Received: 4 September 2023
Accepted: 27 December 2023

Abstract

WISE J224607.6–052634.9 (W2246–0526) is a hot dust-obscured galaxy at z = 4.601, and the most luminous obscured quasar known to date. W2246–0526 harbors a heavily obscured supermassive black hole that is most likely accreting above the Eddington limit. We present observations with the Atacama Large Millimeter/submillimeter Array (ALMA) in seven bands, including band 10, of the brightest far-infrared (FIR) fine-structure emission lines of this galaxy: [OI]_63 μm, [OIII]_88 μm, [NII]_122 μm, [OI]_145 μm, [CII]_158 μm, [NII]_205 μm, [CI]_370 μm, and [CI]_609 μm. A comparison of the data to a large grid of CLOUDY radiative transfer models reveals that a high hydrogen density (n_H ∼ 3 × 10³ cm⁻³) and extinction (A_V ∼ 300 mag), together with extreme ionization (log(U) = − 0.5) and a high X-ray to UV ratio (α_ox ≥ −0.8) are required to reproduce the observed nuclear line ratios. The values of α_ox and U are among the largest found in the literature and imply the existence of an X-ray-dominated region (XDR). In fact, this component explains the a priori very surprising non-detection of the [OIII]_88 μm emission line, which is actually suppressed, instead of boosted, in XDR environments. Interestingly, the best-fitted model implies higher X-ray emission and lower CO content than what is detected observationally, suggesting the presence of a molecular gas component that should be further obscuring the X-ray emission over larger spatial scales than the central region that is being modeled. These results highlight the need for multiline infrared observations to characterize the multiphase gas in high redshift quasars and, in particular, W2246–0526 serves as an extreme benchmark for comparisons of interstellar medium conditions with other quasar populations at cosmic noon and beyond.