The [CII] 158 um line is a powerful tool to understand the evolution of the interstellar medium and star formation in galaxies. The [CII] line traces different phases of the interstellar medium (ISM), including the diffuse ionized medium, warm and cold atomic clouds, clouds in transition from atomic to molecular, and dense and warm photon dominated regions (PDRs). In particular, the [CII] line is a tracer of the CO-dark H2 gas, in which hydrogen is molecular but carbon is ionized and thus is not traced by CO. This CO-dark H2 gas is the likely precursor of dense molecular clouds that will eventually form stars. The [CII] 158 um line is also the brightest far–infrared cooling line in galaxies and reprocesses UV starlight from newly formed stars. Thus it is also a potentially powerful tracer of star formation activity. In this talk I will review recent work on using velocity resolved observations of the [CII] line taken with Herschel/HIFI and SOFIA/GREAT to characterize the evolution of the interstellar medium and star formation in the Milky Way and nearby galaxies. I will also show preliminary results on a survey devoted to image the grand design spiral galaxy M51 in [CII] with the upGREAT and FIFI-LS instruments on SOFIA.
Characterizing the Life Cycle of the Interstellar Medium and Star Formation in Galaxies with the [CII] 158 um Line
N232, Room 227