2024A&A...682A.166F

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.