Submillimeter CI and CO Lines in Galaxies

Together with C⁺ and CO, C is an important reservoir of carbon in the interstellar medium. We present recent results on the ground state transition of atomic carbon at 492 GHz, and on submillimeter CO lines in nearby galaxies. We show that atomic carbon is a good tracer of molecular gas in spiral galaxy disks. It has also a contribution to the molecular gas cooling: the cooling due to C and CO are of the same order of magnitude, and amounts typically to 4× 10^-5 of the FIR continuum. C and CO cooling becomes significant in ULIRG galaxies like Arp220.It is possible to use CI measurements to diagnose the physical conditions in galaxies. Together with CII/CI, the emissivity ratio CI/FIR can be used as a measure of the non-ionizing UV radiation field in galaxies.