The 158 Micron [C ii] Line: A Measure of Global Star Formation Activity in Galaxies

We report 55" resolution KAO observations of the 158 micron [C II] fine-structure line emission from a sample of 14 gas-rich galaxies. Of this sample, five were studied with a new spatial array which gives a simultaneous three-point measurement. Fully sampled velocity channel maps at 125 km s^-1^ resolution were obtained for the central 2 kpc of the archetypical starburst galaxy, M82. The galaxies sampled include a wide variety of spectral types from the early RSab galaxy NGC 4736 to the late Scd spiral galaxy NGC 6946 and span a range of far-infrared luminosity classes from the lower luminosity Sc NGC 5907 to the ultraluminous IRAS galaxies NGC 3690 and NGC 6240. (L_FIR_ ~ 5 x 10^11^ L_sun_). Our results are combined and analyzed together with a previous sample of six infrared bright galaxies, observations of the LMC 30 Doradus region, and a sample of Galactic sources. The [C II] line is an important cooling line in galaxies, accounting for 0.1%-1% of the far-infrared luminosity of the nuclear regions. The brightness of the [C II] line indicates that it is produced in the warm (T-gas_ > 200 K), dense (n_H_ > 10^3^ cm^-3^) photodissociated gas at the interface regions between giant molecular clouds and fully ionized media. The photodissociated gas may represent a substantial fraction (up to ~ 40%) of the total gas mass in starburst nuclei. Much of the observed 21 cm line emission from starburst nuclei may originate in these dense photodissociation regions, lending support to the contention that much of the atomic gas in galaxies may result from the photodissociation of molecular clouds by nearby OB stars. The [C II] line is correlated with the FIR continuum intensity and is therefore a measure of the excitation of the neutral interstellar medium by the average external UV radiation field. The [C II] to ^12^CO (1 -> 0) line intensity ratio is a diagnostic of global star formation activity in galaxies. The ratio obtained for starburst nuclei is constant and equals the ratio measured for Galactic star-forming regions. This implies that, as in Galactic star- forming regions, the excitation temperature of the ^12^CO (1 -> 0) transition in the molecular clouds in starburst nuclei is substantially higher than in Galactic disk molecular clouds. The ^12^CO (1 -> 0) line may, therefore, not be a reliable tracer of molecular mass in starburst nuclei. The observed intensities of the [C II] line in starburst nuclei indicate that the bulk of the molecular media in these regions is exposed to UV fields which are more than 1000 times the intensity of the average UV field in the solar neighborhood. The [C II]/^12^CO (1 -> 0) line intensity ratio in nonstarburst nuclei is a factor of 3 smaller than the value observed for starburst nuclei similar to the ratio found for Galactic giant molecular clouds. This suggests that in nonstarburst galaxies the ^12^CO (1 -> 0) emission is not severely affected by external UV radiation fields. The ultraluminous IRAS galaxy NGC 6240 has a small [C II]/^12^CO (1 -> 0) line intensity ratio which implies that this galaxy is not currently undergoing a global burst of star formation activity.