Probes of the warm ionized medium in the disk of the Galaxy

Although emission in the [S ii] λ6716 forbidden line is commonly assumed to trace ionized gas, we propose state‐selective photoionization of sulfur within neutral regions as a mechanism to explain the 1.5 power scaling between optical forbidden lines of N ii and S ii in M 82 and identify it with the 1.5 power scaling found between the far infrared emission lines of N ii and C ii associated with large scale structure of the Galaxy. While N ii emission can be attributed uniquely to ionized regions, C ii emission arises in neutral photodissociation regions as well. Boundaries between ionized and neutral regions are found to contribute significantly to the Galactic luminosity of both species. Forbidden line emission of ionized sulfur, it is suggested, similarly probes neutral photodissociation regions.