2002A&A...383...82I

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=10³-10⁴cm^–3), subjected to radiation fields of various strengths.