A Far-Infrared Spectral Sequence of Galaxies: Trends and Models
  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 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 H$_{2}$O column densities
with trends found in the well-studied sample of ten nearby galaxies from the
IRAS Bright Galaxy Sample with infrared luminosities greater than 10$^{10}$
L$_{\odot}$ and IRAS 60 micron fluxes equal to or greater than that of the
nearby ULIRG Arp 220. We find that the spectral sequence extending from normal
starburst-type emission line 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 10$^{21}$
cm$^{-2}$ to as high as 10$^{24.8}$ cm$^{-2}$ 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 H$_{2}$O column densities in Arp 220. Our results
suggest that the molecular ISM in ULIRG-like, 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.
