Average radius of the atomic region (R_atomic) in all simulations (assuming they represent the same population) with respect to SFR, colour-coded by (Habing units). We binned the R_atomic values to bin sizes of ∼0.05 dex and the SFR to sizes of ∼0.2 dex. This plot supports the idea proposed by Narayanan & Krumholz (2017) that the sizes of the atomic regions (emitting [C II]) shrink with increasing SFR, which can explain the [C II] deficit. At a given SFR, the radius decreases with increasing .