Atomic carbon in galaxies

We present new measurements of the ground-state fine-structure line of atomic carbon at 492 GHz in a variety of nearby external galaxies, ranging from spiral to irregular, interacting, and merging types. In comparison with CO (1-0) emission observed at the same spatial resolution, the C I (1-0) line intensity stays fairly comparable in the different environments, with an average value of the ratio of the line-integrated areas in K km s(-1) of C I (1-0)/CO (1-0) = 0.2 +/- 0.2. However, some variations can be found within galaxies or between galaxies. Relative to CO lines (J = 2-1, 3-2, 4-3), C I (1-0) is weaker in galactic nuclei but stronger in disks, particularly outside star-forming regions. Also, in NGC 891, the C I (1-0) emission follows the dust continuum emission at 1.3 mm extremely well along the full length of the major axis where molecular gas is more abundant than atomic gas. Atomic carbon therefore appears to be a good tracer of molecular gas in external galaxies, possibly more reliable than CO. Atomic carbon can contribute significantly to the thermal budget of interstellar gas. The cooling due to C and CO are of the same order of magnitude for most galaxies. However, CO is generally a more important coolant in starburst galaxies. Cooling due to C and CO amounts typically to 2 x 10(-5) of the far-IR (FIR) continuum, or 5% of the C II line. However, C and CO cooling reaches similar to 30% of the gas total in ultraluminous infrared galaxies such as Arp 220, where C II is abnormally faint. Together with C II/FIR, the emissivity ratio C I (1-0)/FIR can be used as a measure of the nonionizing UV radiation field in galaxies. The plots of C II/C I or C II/FIR versus C I/FIR show good correlations, in agreement with photodissociation region (PDR) models, except for two remarkable galaxies, Arp 220 and Mrk 231, where high opacities of the C II line and possibly the dust thermal emission may be factors reducing the C II strength below the predictions of the current PDR models.