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

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 (<ns0:sc>TUNER</ns0:sc>) 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 Tk/Td ∼ 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.