[CII]-to-[NII] ratio as a function of IR luminosity for the sources in our sample as well as for galaxies taken from the literature that are detected in both [CII] 158 μm and [NII] 205 μm, assuming the commonly adopted [CII]${}_{158\mathrm{\mu }\mathrm{m}}^{\mathrm{ion}}$/[NII]_205 μm value of approximately three, following the study from Oberst et al. (2006). The color scheme adopted is the same as in Fig. 2. Most of the sources in our sample show a value greater than ten, indicating that the bulk of the [CII] emission of our sources arises from the neutral medium.