[13CII] and [12CII] observations: Optical depth effects and C+ column densities

The [CII] fine structure line at 158 um is, together with the [OI] 63 um fine structure line, one of the strongest cooling lines of the star forming ISM, carrying up to a few percent of the total FIR intensity. The [CII] line thus is one of the signposts for active star formation also in distant galaxies, where ALMA can observed the line, which is shifted to the submm-atmospheric windows for high redshift sources.

Since its first detection, the question of how optically thick this line may be has been an open issue. This has been resolved recently, after a few pioneering observations with the KAO and Herschel/HIFI, with the opportunity to now routinely observe the [13CII] hyperfine lines with the upGREAT instrument onboard SOFIA. In this talk, I will summarize the [13CII] results obtained with SOFIA, which show that in many bright star forming regions in the Milky Way, the line shows strong optical depth and self-absorption effects. The column densities are significantly larger than expected from standard Photodissociation Region models for star forming clouds. The large number of PDR surfaces along the line of sight corresponding to these column densities requires a clumpy or fractal, surface dominated structure of the clouds. The nature of the absorbing cold foreground gas is puzzling as it is not explained by any standard model of the ISM phases.