The [CII] Deficit in LIRGs and ULIRGs is Due to High-Temperature Saturation

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 [OI]63 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.