2004ApJS..154..184S

We report the detection of strong absorption and weak emission features in the 4-27 µm Spitzer Infrared Spectrograph (IRS) spectrum of the distant ultraluminous infrared galaxy IRAS F00183-7111 (z=0.327). The absorption features of CO₂and CO gas, water ice, hydrocarbons, and silicates are indicative of a strongly obscured (A_9.6≥5.4; A_V≥90) and complex line of sight through both the hot diffuse interstellar medium and shielded cold molecular clouds toward the nuclear power source. From the profile of the 4.67 µm CO fundamental vibration mode, we deduce that the absorbing gas is dense (n∼10⁶/cm3) and warm (720 K) and has a CO column density of ∼10^19.5/cm2, equivalent to N_H∼10^23.5/cm2. The high temperature and density, as well as the small inferred size (<0.03 pc), locates this absorbing gas close to the power source of this region. Weak emission features of molecular hydrogen, polycyclic aromatic hydrocarbons (PAHs), and Ne⁺, likely associated with star formation, are detected against the 9.7 µm silicate feature, indicating an origin away from the absorbing region. Based on the 11.2 µm PAH flux, we estimate the star formation component to be responsible for up to 30% of the IR luminosity of the system. While our mid-infrared spectrum shows no telltale signs of active galactic nucleus (AGN) activity, the similarities to the mid-infrared spectra of deeply obscured sources (e.g., NGC 4418) and AGN hot dust (e.g., NGC 1068), as well as evidence from other wavelength regions, suggest that the power source hiding behind the optically thick dust screen may well be a buried AGN.