8 Conclusions

1.

We have measured the emission from the 492 GHz line corresponding to the ${\rm ^{3}P_{1}}{-}{\rm ^{3}P_{0}\,[CI]}$ transition in the centers of fifteen nearby spiral galaxies. In the same galaxies, we have also measured J = 4- $3 {\rm ^{12}CO}$ and J = 2- $1 {\rm ^{13}CO}$ intensities for comparison with the ${\rm ^{3}P_{1}}{-}{\rm ^{3}P_{0}\,[CI]}$ line within the framework of radiative transfer models.

2.

Rather unlike Galactic sources, the external galaxy centers have [CI] intensities generally exceeding J= 2-1  ${\rm ^{13}CO}$ intensities, and in a number of cases approaching J= 4-3 ${\rm ^{12}CO}$ intensities.

3.

The highest area-integrated (i.e. total central) [CI] luminosities are found in the active galaxies NGC 1068 and NGC 3079. Slightly lower luminosities occur in strong starburst nuclei, such as those of NGC 3628 and NGC 6946. Quiescent and weak-starburst nuclei have [CI] luminosities an order of magnitude lower.

4.

The observed [CI], ${\rm ^{13}CO}$ and ${\rm ^{12}CO}$ line ratios, interpreted within the context of radiative transfer models, suggest that the bulk of the observed emission arises in gas with densities $n \geq 3000 \,{\rm cm^{-3}}$ and kinetic temperatures $T_{\rm kin} \approx 30$-60 K. Depending on the actual density n, most galaxy centers should have abundances N([CI])/N(CO) = 1-3, i.e. [CI] columns just exceeding those of CO. Only relatively quiescent galaxy centers such as those of Maffei 2, IC 342 and NGC 7331 have abundances N([CI])/N(CO) $\approx 0.3$-1.0 and are dominated by CO just as the comparison starforming regions in the Milky Way and the LMC.

5.

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- $10^{4}\,{\rm cm^{-3}}$), subjected to radiation fields of various strengths ranging from a few times to several thousand times the local Galactic radiation field.

Acknowledgements

We are indebted to Ewine van Dishoeck and David Jansen for providing us with their their statistical equilibrium calculation models. We also thank Maryvonne Gerin and Tom Phillips for communicating to us their neutral carbon measurements of galaxies well before publication.