A Hot DOG Forged in FIRE: Nuclear and Starburst Spectral Decomposition of a Luminous Infrared Galaxy Simulation with a Resolved Dust Torus

TL;DR

This paper uses detailed simulations to analyze the spectral features of a luminous infrared galaxy resembling Hot DOGs, revealing the roles of AGN and starburst heating, and the effects of dense ISM absorption on IR spectra.

Contribution

It presents a novel simulation that resolves the dust torus and reproduces Hot DOG spectral characteristics, linking galaxy mergers, dense ISM, and AGN activity.

Findings

Simulated galaxy matches Hot DOG IR spectrum and properties.

IR spectral shape driven by AGN-heated warm dust and starburst cold dust.

Dense ISM absorption causes near- to mid-IR cutoff.

Abstract

Ultraluminous infrared galaxies are powered by a combination of rapid star formation and active galactic nucleus (AGN) emission, but their relative importance is not always observationally clear. We study the galactic continuum spectrum of a cosmologically simulated $\sim 4 \times 10^{10} M_\odot$ stellar mass starburst galaxy at redshift $z\sim 4.4$ that refines down to resolve beyond the dust sublimation boundary of its super-Eddington-accreting $\sim 10^7 M_\odot$ supermassive black hole. We find that this system resembles the rare class of hot dust-obscured galaxy (Hot DOG), with a roughly flat (in $\nu F_\nu$) IR emission spectrum that sharply drops off at wavelengths $\lesssim 5~\mu\mathrm{m}$. Our system also matches with the observational properties of many Hot DOGs, including undergoing multiple galaxy mergers and being the most massive galaxy within a dense cosmological environment. The distinctive Hot DOG spectral shape in our system is caused by AGN-heated mid-IR warm dust, predominately starburst-heated far-IR cold dust, and a steep near- to mid-IR cutoff caused by strong absorption in the dense ISM of the galactic nucleus, rather than the dust torus itself. This system is lower luminosity ($L_\mathrm{IR} \sim 2 \times 10^{12} L_\odot$) than those detected by the WISE survey at similar redshifts, but will be a prime target for future far-IR surveys such as PRIMA. Our results show that Hot DOGs can naturally result as a transitional phase during rapid AGN accretion, but before significant AGN-driven outflows clear optically thin paths.