Herschel Galactic plane survey of [NII]

The ratio of the two fine structure transitions provides a direct measurement of the electron density, yielding n(e) largely in the range 10-50 cm^-3^ with an average value of 29 cm^-3^ and N^+^ column densities 10^16^-10^17^ cm^-2^. [N II] emission is highly correlated with that of [C II], and we calculate that between 1/3 and 1/2 of the [C II] emission is associated with the ionized gas. The relatively high electron densities indicate that the source of the [N II] emission is not the warm ionized medium (WIM), which has electron densities more than 100 times smaller.

Possible origins of the observed [N II] include the ionized surfaces of dense atomic and molecular clouds, the extended low-density envelopes of H II regions, and low-filling factor high-density fluctuations of the WIM.