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

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 $νF_ν$) IR emission spectrum that sharply drops off at wavelengths $\lesssim 5~μ\mathrm{m}$. 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.

Figures (5)

• Figure 1: Spatially resolved optical stellar emission (without dust attenuation) of the FORGE'd in FIRE galaxy in rest-frame optical bands. The system is clearly undergoing a merger, has a highly clumpy morphology and is undergoing a starburst causing the optical/UV stellar emission to appear blue. This system is the most massive galaxy in its dense cosmological environment at redshift $z\sim4.4$.
• Figure 2: Rest-frame sightline-mean spectrum of the galactic total emission ($r \leq 100$ kpc) when including both the fully resolved AGN-heated and starburst-heated dust (solid black line) and compare it to the rest-frame spectra of various observed ultraluminous infrared galaxies (ULIRGs). We show the most luminous hot dust obscured galaxy (Hot DOG) W2246-0526 spectral measurements from Tsai_2018 and the low-redshift Hot DOG W1904+4853 spectrum from Li_2023, with best fits in dot-dashed lines. We also show local ULIRGs Arp220 (yellow) and Mrk231 (pink) in dotted lines Polletta_2007. The dust continuum component of our spectrum is mostly flat in $\nu L_\nu$ from $\sim 5 \mu\mathrm{m}$ to $\sim 50 \mu\mathrm{m}$ and closely resembles the shape of a Hot DOG. Our simulation is lower luminosity than most observed Hot DOGs (similar to low-redshift Hot DOGs) at similar redshift ($z\sim 4$).
• Figure 3: Rest-frame sightline-mean spectrum of the galactic total emission ($r \leq 100$ kpc) as shown in Figure \ref{['fig:spectral_comparison']}, but normalized in IR luminosity for a better IR spectral shape comparison. We also include a conservative estimate for the $1\sigma$ sightline variance in our spectra (assuming most of the sightline IR variance comes from $r < 100$ pc, where most of the dust self-absorption occurs). Our spectrum is broadly consistent with W2246-0526 and the low-redshift W1904+4853 spectrum in the mid-IR, but is likely different in the near- and far-IR due to changes in the surrounding host galaxy and relative stellar heating contribution.
• Figure 4: Left panel: Intrinsic spectra vs dust reprocessed spectrum. We compare the intrinsic (non-dust attenuated) spectrum to the full galaxy spectrum (as in Figure \ref{['fig:spectral_comparison']} and \ref{['fig:spectral_comparison_renorm']}) in solid black. Most of the emergent (stellar) UV/optical emission is only slightly reddened. We note that our input AGN spectrum Bardati_2026 does not include any dust torus component since we fully resolve the dust torus in this simulation. Middle panel: Full galaxy spectrum vs galactic nucleus spectrum. We compare the emergent spectrum from the $r \lesssim 100$ kpc ($\sim 14\hbox{$.\!\!^{\prime\prime}$}7$ at $z= 4.4$) galaxy (solid black) to the spectrum emergent spectrum from the galactic nucleus at $r \lesssim 1$ kpc ($\sim 147$ mas, in dotted black). The majority of the mid-IR emission is from the galactic nucleus, whereas the UV/optical and far-IR emission is primarily produced outside the nucleus Right panel: Spectral decomposition of the stellar- vs AGN-heated dust. We compare the stellar (dust attenuated) emission and stellar-heated dust (in orange) to the AGN emission and AGN-heated dust (in green). The mid-IR is dominated by the AGN-powered nucleus dust and the far-IR by the starburst-powered cold extended galactic dust, reaching roughly equal contributions around 40 $\mu\mathrm{m}$. We also note that renormalizing the W2246-0526 spectrum to $L_\mathrm{AGN} = 5 \times 10^{45} \mathrm{~erg~s}^{-1}$ (in purple), lines up surprisingly well with our AGN-heating spectrum, consistent with the understanding of W2246-0526 being AGN-heating dominated. Overall, the system thus cleanly separates into three regions: 1. the dust "torus" Bardati_2026, 2. the dense galactic nucleus ISM (reprocessing the hot dust to the mid-IR), and 3. the extended young starburst galaxy (producing the far-IR and steep UV/optical emission).
• Figure 5: Radial profile of the escaping luminosity and dust temperatures, separated by the heating source. Top panel: We indicate the escaping luminosity of the AGN and AGN-powered dust with circles and star symbols denote the stellar and stellar-heated dust components. The emission is separated into the $8-1000~\mu\mathrm{m}$ mid- to far-IR emission, $1-8~\mu\mathrm{m}$ near- to mid-IR emission, and $912 \AA -1~\mu\mathrm{m}$ UV/optical to near-IR components. We also roughly mark the locations of the torus, the dense galactic nucleus and the more diffuse galactic ISM and CGM dust. Bottom panel: We show the mean AGN-heated (green), starburst-heated (orange), and total (black) dust temperature profiles, separated into silicates & graphites (solid lines) and stochastically-heated polycyclic aromatic hydrocarbons (PAHs, dotted lines). The cosmic microwave background (CMB) temperature at this redshift ($z=4.4$) is also indicated (purple dotted) for reference. The source of the reduced near-IR to mid-IR emission is from dust extinction from the galactic nucleus ISM. The galactic nucleus dust extinction also suppresses some of the nucleus optical emission of the starburst-heated dust, but the starburst also occurs outside of the dense nucleus ($r\sim 10$ kpc), producing significantly less reddened optical emission. Despite the luminosities being AGN-dominated at all scales, stellar heating dominates the mean dust temperatures past around $r \sim 100$ pc ($r \sim 10$ pc for PAHs) from significant nucleus dust extinction.