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

Understand

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.