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We present a framework for the interpretation of the far-infrared spectra ofgalaxies in which we have expanded the model parameters compared with previouswork by varying the ionization parameter $U$, column density $N$(H), and gasdensity at the cloud face $n$(H$^{+}$) for a central starburst or AGN. Wecompare these models carried out with the $Cloudy$ spectral synthesis code totrends in line-to-total far-infrared luminosity ratios, far-infraredfine-structure line ratios, IRAS colors, and OH and H$_{2}$O column densitieswith trends found in the well-studied sample of ten nearby galaxies from theIRAS Bright Galaxy Sample with infrared luminosities greater than 10$^{10}$L$_{\odot}$ and IRAS 60 micron fluxes equal to or greater than that of thenearby ULIRG Arp 220. We find that the spectral sequence extending from normalstarburst-type emission line spectra to ULIRG-type absorption-dominated spectrawith significant absorption from excited levels, can be best explained bysimultaneously increasing the hydrogen column density, from as low as 10$^{21}$cm$^{-2}$ to as high as 10$^{24.8}$ cm$^{-2}$ or greater, and the ionizationparameter, from as low as 10$^{-4}$ to as high as 1. The starburst models bestreproduce most of the sequence, while AGN models are somewhat better able toproduce the high OH and H$_{2}$O column densities in Arp 220. Our resultssuggest that the molecular ISM in ULIRG-like, absorption-dominated systems islocated close to and at least partially obscures the source of power throughoutmuch of the far-infrared, which must be taken into account in order to properlyinterpret diagnostics of both their sources of power and of feedback.

A Far-Infrared Spectral Sequence of Galaxies: Trends and Models