2016MNRAS.463.2085M

Current predictions for the line ratios from photodissociative 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 [C II] fine-structure line in luminous and ultraluminous infrared galaxies. While there are a number of potential explanations for the [C II] deficit observed in the highest IR surface-brightness systems, such as 'dust-bounded' H II regions and grain charging, we propose a simpler solution where the [C II] deficit arises from saturating the upper fine-structure transition state at gas temperatures above 91 K. To reproduce the measured amplitude of the [C II]/FIR ratio in deficit galaxies, we require that [C II] trace approximately 10-17 per cent of all gas in these systems, roughly independent of IR surface brightness and consistent with observed [C II] 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 [O I]63 µm 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.