Hot Diggity Dog: Simultaneous CO and dust modelling of the most luminous WISE hot dust-obscured galaxy unveils extreme molecular gas excitation

¹ Joint ALMA Observatory, Alonso de Córdova 3107, Vitacura, Santiago RM, Chile
² National Astronomical Observatory of Japan, Los Abedules 3085 Oficina 701, Vitacura, 763 0414 Santiago, Chile
³ European Southern Observatory, Alonso de Córdova 3107, Vitacura, Casilla, 19001 Santiago RM, Chile
⁴ Instituto de Estudios Astrofísicos, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Av. Ejército Libertador 441, Santiago, Chile
⁵ Department of Physics, University of Crete, 70013 Heraklion, Greece
⁶ Institute of Astrophysics, Foundation for Research and Technology–Hellas (FORTH), Heraklion 70013, Greece
⁷ Leiden Observatory, Leiden University, PO Box 9513 NL-2300 RA, Leiden, The Netherlands
⁸ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁹ School of Sciences, European University Cyprus, Diogenes Street, Engomi, 1516 Nicosia, Cyprus
¹⁰ Millenium Nucleus for Galaxies (MINGAL), Av. Ejército Libertador 441, Santiago, Chile
¹¹ National Astronomical Observatories, Chinese Academy of Sciences, 20A Datun Road, Beijing 100101, China
¹² Institute for Frontiers in Astronomy and Astrophysics, Beijing Normal University, Beijing 102206, China
¹³ School of Astronomy and Space Science, University of Chinese Academy of Sciences, Beijing 100049, China
¹⁴ Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, 91109 CA, USA

^⋆ Corresponding author: kevin.harrington@alma.cl

Received: 22 April 2025
Accepted: 25 September 2025

Abstract

Hot dust-obscured galaxies (Hot DOGs), the most infrared (IR) luminous objects selected by the WISE all-sky mid-IR survey, have yielded a sample of intrinsically luminous quasars (QSOs) with obscured nuclear activity and hot dust temperatures. The molecular gas excitation properties have yet to be examined in detail under such extreme conditions. Here we study the most far-IR luminous WISE Hot DOG W2246–0526, focusing on ALMA observations of the central host galaxy. Multi-J CO transition measurements at J = 2 − 1, 5–4, 7–6, 12–11, and 17–16 provide the first self-consistent modelling constraints on the molecular gas and dust properties of any WISE Hot DOG to date, providing a benchmark for future studies of dust-obscured QSOs. We implement a state-of-the-art TUrbulent Non-Equilibrium Radiative transfer model (TUNER) that simultaneously models both the line and dust continuum measurements. Due to a combination of high molecular gas densities and high gas kinetic temperatures, this extreme CO spectral line energy distribution seems to turnover around the CO(12–11) transition, likely making this among the most highly excited galaxies ever reported. The model infers a molecular gas mass ∼8 × 10¹⁰ M_⊙, and we conclude that J ≥ 3 CO line luminosities trace the bulk of the molecular gas mass for this extreme system unlike low-excitation conditions calibrated locally. W2246–0526 is a rapidly evolving system, with a high mean value of the molecular gas kinetic temperature versus dust temperature T_k/T_d ∼ 4.3. This may be due to the shocks and outflows injecting kinetic energy within the central kiloparsec of this host that have previously been reported. These signs of highly excited molecular gas in W2246–0526 motivates obtaining well-sampled CO ladders in larger samples to better understand the conditions within these short-lived episodes associated with the most obscured supermassive black hole activity.