Neutral Atomic Carbon in Centers of Galaxies
We present measurements of the emission from the centers of fifteen spiral galaxies in the 3P1-3P0 [CI] fine-structure transition at 492 GHz. Observed galaxy centers range from quiescent to starburst to active. The intensities of neutral carbon, the J=2-1 transition of 13CO and the J=4-3 transition of 12CO are compared in matched beams. Most galaxy centers emit more strongly in [CI] than in 13CO, completely unlike the situation pertaining to Galactic molecular cloud regions. [CI] intensities are lower than, but nevertheless comparable to J=4-3 12CO intensities, again rather different from Galactic sources. The ratio of [CI] to 13CO increases with the central [CI] luminosity of a galaxy; it is lowest for quiescent and mild starburst centers, and highest for strong starburst centers and active nuclei. Comparison with radiative transfer model calculations shows that most observed galaxy centers have neutral carbon abundances close to, or exceeding, carbon monoxide abundances, rather independent from the assumed model gas parameters. The same models suggest that the emission from neutral carbon and carbon monoxide, if assumed to originate in the same volumes, arises from a warm and dense gas rather than a hot and tenuous, or a cold and very dense gas. The observed [CI] intensities together with literature [CII] line and far-infrared continuum data likewise suggest that a significant fraction of the emission originates in medium-density gas (n = 10**3 - 10**4 cm**-3, subjected to radiation fields of various strengths.