JWST observations and a model for the extremely luminous obscured quasar W2246-0526 at z=4.6

Charalambia Varnava, Andreas Efstathiou, Tanio Díaz-Santos, Duncan Farrah

First listed 2026-05-09 | Last updated 2026-05-09

Abstract

We present new JWST/MIRI-MRS data of the z=4.601 extremely luminous obscured quasar WISEA J224607.56-052634.9 (W2246-0526). Our fits of its spectral energy distribution (SED) with the SED fitting code SMART (Spectral energy distributions Markov chain Analysis with Radiative Transfer models) predict an active galactic nucleus (AGN) fraction in the range 72-81 per cent, an intrinsic AGN luminosity of 4.2-7.2 x 10^14 Lo, a polar dust luminosity of 1.6-1.7 x 10^14 Lo, a black hole mass of 1.3-2.3 x 10^10 Mo (assuming the quasar is accreting at the Eddington limit), a star formation rate (SFR) of 360-2900 Mo/yr and a stellar mass of 4.8-5 x 10^11 Mo. The stellar and black hole masses of W2246-0526 are typical of a giant elliptical galaxy at z=0. We find statistically significant evidence for the presence of a hot dust component, which we interpret as polar dust in the context of a torus geometry, based on recent results obtained for nearby AGN. We explore two smooth and two two-phase models for the AGN torus, to put constraints on the AGN fraction of the galaxy, the black hole mass and its SFR. We show that the presence of polar dust affects the estimate of the AGN luminosity and we recommend to take into account this component in SED fits of other high-redshift obscured AGN/quasars. Despite the large difference in luminosity, we discuss possible links between the presence of this hot dust component in W2246-0526 and in some local AGN, suggesting that they may have a different origin.

Short digest

Using new JWST/MIRI-MRS spectroscopy plus broadband SED fitting with SMART, this paper models the z=4.601 Hot DOG quasar W2246-0526 across four torus prescriptions and finds it is overwhelmingly AGN-powered, with an AGN fraction of 72-81 per cent and an intrinsic AGN luminosity of 4.2-7.2 x 10^14 Lsun. The key result is statistically significant evidence for an additional hot-dust component interpreted as polar dust, with luminosity 1.6-1.7 x 10^14 Lsun, which materially shifts the inferred AGN luminosity and therefore the derived black-hole mass. Under Eddington-limited accretion the implied black hole mass is 1.3-2.3 x 10^10 Msun, while the allowed host properties remain broad at SFR = 360-2900 Msun/yr and stellar mass about 4.8-5 x 10^11 Msun. The broader takeaway is that polar dust is not a negligible detail even in extreme high-redshift obscured quasars, and omitting it can bias physical inferences drawn from SED fitting.

Key figures to inspect

• Figure 3. This is the paper's most important evidence figure because it shows the SED fits once the polar-dust component is added, directly matching the abstract's central claim of statistically significant hot-dust emission. Use it to see how the extra component redistributes the mid-infrared luminosity budget across the different torus models and why the inferred intrinsic AGN luminosity changes when polar dust is included.
• Figure 1. This baseline comparison, without host extinction or polar dust, is useful for understanding what the standard torus-plus-starburst-plus-host framework can and cannot explain on its own. It gives the reader a clean before-state against which the later polar-dust fits can be judged, making the need for an additional hot component much more concrete.
• Figure 2. This figure isolates the effect of adding host extinction while still excluding polar dust, which is important because the paper explicitly explores modeling degeneracies before concluding that a separate hot-dust component is required. It helps the reader see that changes in line-of-sight attenuation alone do not replace the role of polar dust in the preferred interpretation.
• Figure 4. The geometry schematic matters because the paper does not only fit an empirical excess; it argues for a specific physical interpretation in which polar dust can itself suffer extinction through the torus along the observed line of sight. This figure is the clearest bridge between the radiative-transfer fitting results and the proposed obscurer geometry that explains W2246-0526's observed properties.