ON THE ORIGIN OF THE DIFFUSE C+ 158 MICRON LINE EMISSION

We consider the relative importance of three types of region as global sources of the diffuse C+ 158 mu m emission in the Galactic interior. We find that they rank as follows: the extended low-density warm ionized medium (ELDWIM) ranks first in importance, the cold neutral medium (CNM) ranks second, and photodissociation regions (PDRs) rank third. Above a threshold intensity, we interpret the data as showing that the C+ line is well correlated with other tracers of the ELDWIM. The C+ intensity below the threshold is easily accounted for by the CNM. PDRs must also contribute. These conclusions should also apply to the inner portions of many external nonstarburst galaxies. The opacity of the C+ line over long lines of sight in the Galactic plane is not negligible.