Far-infrared spectroscopy of galaxies - The 158 micron C(+) line and the energy balance of molecular clouds

Observations of the 158 microns fine-structure line of C(+) toward the nuclei of six gas-rich galaxies are presented. The observations are compared with observations of the CO J = 1-0 and H I 21 cm lines, observations of far-IR continuum emission, and observations of forbidden C II emission with the Galaxy. The forbidden C II line comes from dense, warm gas in UV-illuminated photodissociation regions at the surfaces of molecular clouds. This line is probably optically thin in all but the brightest of galactic sources. The variation of forbidden C II brightness from source to source and its ratio to the integrated infrared continuum intensity agree well with the theoretical prediction that UV absorption by dust controls the C(+) column density. The forbidden C II line is a tracer of molecular clouds, especially those near intense sources of UV radiation.