Average G₀ (left panel) and n (right panel) plotted as a function of galaxy redshift. We compare the values derived from our unadjusted observations (blue solid circles) and observations including the adjustments applied to the [Cii] and CO(1–0) emission (see ), assuming either 70% (red semi-open circles) or 50% (red open circles) of the total [Cii] emission arises from PDR regions. Downward triangles represent upper limits. Comparative galaxy samples of the literature are shown with the symbols specified in the legends, together with the ranges seen in spatially resolved studies of M 51 (blue line; Parkin et al. 2013), Cen A (red line; Parkin et al. 2014) and NGC 891 (green line; Hughes et al. 2015). To facilitate a comparison, we apply the same standard adjustments (i.e. adopting and 2CO) to the high redshift (z> 1) systems as those made for galaxies at lower redshift.