Theoretical [N II]/[C II] ratios as a function of density. The ratio values include ionic abundance ionization corrections (see text). The red curve indicates ratio values with [C II] originating fully in the ionized gas. The solid black lines show the observed upper limits for all pointings observed with Herschel/PACS (except #5 for [N II] 205μm/[C II] shown as a dashed line).