3 Results

Observational results are summarized in Table 2; a number of representative profiles of both [CI] and J= 2-1  ${\rm ^{13}CO}$ emission is shown in Fig. 1. For half of the galaxy sample, the distribution of [CI] was mapped beyond the central position (cf. Table 1). The [CI] emission maps of NGC 6946 and M 83 have already been published (Israel & Baas 2001); the remaining maps will be discussed in forthcoming papers. In the meantime, we have used the information contained in the maps to convolve the full-resolution central [CI] intensities (Col. 4 of Table 2) to [CI] intensities (Col. 5) appropriate to the twice larger beamsize of the J= 2-1 ${\rm ^{13}CO}$ observations. We have likewise used the map information to determine total [CI] luminosities (Col. 7) integrated over the entire central source extent.

For all but two of the sample galaxies we have also maps of the J= 4-3 ${\rm ^{12}CO}$ distribution. Again, we refer to published and forthcoming papers for a discussion of these observations. For those galaxies that were not mapped in [CI], we have used the J= 4-3 ${\rm ^{12}CO}$ maps to estimate the convolved [CI] intensity and the total luminosity by assuming identical [CI] and J= 4-3 ${\rm ^{12}CO}$ distributions. This was shown to be the case for M 83 and NGC 6946 (Petitpas & Wilson 1998; Israel & Baas 2001), but we have further verified the validity of this assumption for all galaxies that were mapped in both [CI] and J= 4-3  ${\rm ^{12}CO}$. Values obtained in this way are given in parentheses in Table 2.

In all galaxies mapped, the central neutral carbon peak is well contained within a radius $R \leq 0.6$ kpc, often as small as $R \approx$ 0.3 kpc. For only two galaxies (NGC 5713 and NGC 7331) we have no information on extent. In these two cases we have listed [CI] luminosities ranging from that observed in a single beam to that appropriate to the implied maximum source diameter of 1 kpc.

The total [CI] luminosities of the observed galaxies cover a large range. Quiescent galaxies (NGC 7331, IC 342, Maffei 2, NGC 278, NGC 5713) have modest luminosities $\approx$ $1 \leq L_{\rm [CI]} \leq 5 \,{\rm {K\,km\,s^{-1}}}$ kpc². Galaxies with a starburst nucleus (NGC 253, NGC 660, M 82, NGC 3628, NGC 6946) have luminosities $10 \leq L_{\rm [CI]} \leq 40 \,{\rm {K\,km\,s^{-1}}}$ kpc². However, M 83 has only $L_{\rm [CI]} = 3.6 \,{\rm {K\,km\,s^{-1}}}$ kpc², although it is also a starburst galaxy. The highest luminosities $L_{\rm [CI]} \geq 50 \,{\rm {K\,km\,s^{-1}}}$ kpc² are found in the active galaxies NGC 1068 and NGC 3079.

Interestingly, the ratio of the ${\rm ^{3}P_{1}}{-}{\rm ^{3}P_{0}\,[CI]}$ and J2-1  ${\rm ^{13}CO}$ line strengths exhibits a similar behaviour. The [CI] line is stronger than J= 2-1 ${\rm ^{13}CO}$ in all galaxies except Maffei 2 and NGC 7331. The highest [CI]/ ${\rm ^{13}CO}$ ratios of about five belong to the active galaxies NGC 1068 and NGC 3079. Generally, the ${\rm ^{3}P_{1}}{-}{\rm ^{3}P_{0}\,[CI]}$ line is weaker than the J= 4-3 ${\rm ^{12}CO}$ line, but not by much. In NGC 278, NGC 3079, NGC 4826 and M 51, the two lines are roughly of equal strength. Only in Maffei 2 is the [CI] line much weaker.