2014ApJ...795..117F

We present a framework for the interpretation of the far-infrared spectra of galaxies in which we have expanded the model parameters compared with previous work by varying the ionization parameter U, column density N(H), and gas density at the cloud face n(H⁺) for a central starburst or active galactic nucleus (AGN). We compare these models carried out with the Cloudy spectral synthesis code to trends in line-to-total far-infrared luminosity ratios, far-infrared fine-structure line ratios, IRAS colors, and OH and H₂ O column densities found in the well-studied sample of 10 nearby galaxies from the IRAS Bright Galaxy Sample with infrared luminosities greater than 10¹⁰ L_☉and IRAS 60 µm fluxes equal to or greater than that of the nearby ultraluminous infrared galaxy (ULIRG) Arp 220. We find that the spectral sequence extending from normal starburst-type emission-line-dominated spectra to ULIRG-type absorption-dominated spectra with significant absorption from excited levels can be best explained by simultaneously increasing the hydrogen column density, from as low as 10²¹/cm2 to as high as 10^24.8/cm2 or greater, and the ionization parameter, from as low as 10^–4 to as high as 1. The starburst models best reproduce most of the sequence, while AGN models are somewhat better able to produce the high OH and H₂ O column densities in Arp 220. Our results suggest that the molecular interstellar medium in ULIRG-like, molecular-absorption-dominated systems is located close to and at least partially obscures the source of power throughout much of the far-infrared, which must be taken into account in order to properly interpret diagnostics of both their sources of power and of feedback.