Abstract
Current predictions for the line ratios from photo-dissociative regions
(PDRs) in galaxies adopt theoretical models that consider only individual
parcels of PDR gas each characterized by the local density and far-UV radiation
field. However, these quantities are not measured directly from unresolved
galaxies, making the connection between theory and observation ambiguous. We
develop a model that uses galaxy-averaged, observable inputs to explain and
predict measurements of the CII fine structure line in luminous and
ultra-luminous infrared galaxies. We find that the CII deficit observed in
the highest IR surface-brightness systems is a natural consequence of
saturating the upper fine-structure transition state at gas temperatures above
91 K. To reproduce the measured amplitude of the CII/FIR ratio in deficit
galaxies, we require that CII trace approximately 10-17% of all gas in these
systems, roughly independent of IR surface brightness and consistent with
observed CII to CO(1--0) line ratios. Calculating the value of this fraction
is a challenge for theoretical models. The difficulty may reside in properly
treating the topology of molecular and dissociated gas, different descriptions
for which may be observationally distinguished by the OI63 micron line in
yet-to-be-probed regions of parameter space, allowing PDR emission lines from
to probe not only the effects of star formation but also the state and
configuration of interstellar gas.
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