4 Comparison of [CI] and $^{\mathsf{13}}$CO intensities

In various studies of Galactic objects, the similarity of ${\rm ^{3}P_{1}}{-}{\rm ^{3}P_{0}\,[CI]}$ and J= 2-1 ${\rm ^{13}CO}$ intensities and distribution is commented upon. Early such studies by the CSO group were reviewed by Keene et al. (1996). The CSO mapping of the Galactic molecular cloud complexes M 17 and Oph A show virtually identical line intensities for [CI] and ${\rm ^{13}CO}$ throughout. This is also found for most of the Orion Bar and OMC-1. The densest regions of Orion, however, show increasingly strong ${\rm ^{13}CO}$ emission whereas ${\rm ^{3}P_{1}}{-}{\rm ^{3}P_{0}\,[CI]}$ intensities level off, yielding ever lower [CI]/ ${\rm ^{13}CO}$ intensity ratios down to about 0.4. A similar range of ratios (0.3-1.1) was found by Jansen et al. (1996) towards the emission/reflection nebula IC 63. Keene et al. (1996) attributed such low values to the effects of enhanced UV radiation in photon-dominated regions (PDR's). This interpretation finds support in the results obtained by Plume et al. (1999) and Tatematsu et al. (1999) who used a reimaging device on the CSO to effectively obtain a larger beamsize suitable for large-area mapping. Their maps of clouds associated with the low-UV sources TMC-1, L 134N and IC 5146 have fairly uniform ratios I[CI]/ ${\rm ^{13}CO}$ = 1.05 $\pm$ 0.15, as do the translucent regions of the dark cloud L 183 observed by Stark et al. (1996). In contrast, maps of the molecular clouds associated with the high-UV sources W3, NGC 2024, S140 and Cep A yield I[CI]/ ${\rm ^{13}CO}$ ratios of about 0.5 for the bulk of the clouds. However, even here intensity ratios of about unity are found once again at cloud edges. The distribution of cloud-edge ratios even has a tail reaching a value of four. Only in a few globules associated with the Helix planetary nebula (Young et al. 1997) have such relatively high ratios of 3-5 also been found.

Figure 2: Left: [CI]/(J=2- $1 {\rm ^{13}CO}$) ratios versus area-integrated [CI] luminosity L[CI]. Right: [CI]/(J=4- $3 {\rm ^{12}CO}$) ratios versus L[CI]. The I([CI])/ $I({\rm ^{13}CO})$ ratio appears to be a well-defined function of log L([CI]); the I([CI])/I(4-3 ${\rm ^{12}CO}$) ratio is not. Galactic sources (not shown) would all be crowded together in the lower left corner.

Our own data on star formation regions corroborate this: towards the Galactic HII regions W 58 and ON-1 (unpublished) as well as the LMC regions N 159 and N 160 (Bolatto et al. 2000) we find intensity ratios [CI]/J= 2-1 = 0.2-0.6 for the PDR zones associated with these starforming regions. The two objects (W-58C and N 159-South) where star formation has not yet progressed to a dominating stage, in contrast, yield ratios of about unity.

As Fig. 2 shows, only a few of the observed galaxy centers obey the same linear correlations between [CI] and ${\rm ^{13}CO}$that characterize Galactic clouds. Fully two thirds of the galaxy sample has ${\rm ^{3}P_{1}}{-}{\rm ^{3}P_{0}\,[CI]}/J=2$- $1 {\rm ^{13}CO}$ ratios well in excess of unity; the galaxies thus have much stronger [CI] emission than the ${\rm ^{13}CO}$ intensity and the Galactic results would lead us to expect.

The galaxy sample, observed at 15'' resolution, discussed by Gerin & Phillips (2000) has only a little overlap with ours, but it shows the same effect: more than two thirds of the positions plotted in their Fig. 7 has a ratio [CI]/ ${\rm ^{13}CO}$ $\geq 2$. For the galaxy NGC 891, Gerin & Phillips (2000) observed various positions along the major axis, in addition to the central region. At the distance of the galaxy, their beam corresponds to a linear size of 0.5 kpc. Whereas the [CI] intensity generally drops with increasing radius, the [CI]/¹³CO intensity ratio increases, or more specifically, this ratio increases from about 2 at the central positions brightest in [CI] to about 4-6 at the disk positions weakest in [CI].

Qualitatively, low ratios are expected from regions which have low neutral carbon abundances. Low neutral carbon abundances will be found in high-UV environments where neutral carbon will become ionized, and in environments with high gas densities and column densities. Here, neutral carbon disappears because of the concomitant higher CO formation rates at high densities and the much more efficient CO (self)shielding at high column densities. Because of its lower abundance, ${\rm ^{13}CO}$ requires larger column densities for efficient shielding. Conversely, in environments characterized by low gas column densities and mild UV radiation fields, such as found in translucent clouds and at cloud edges, CO will be mostly dissociated, and most gas-phase carbon may be neutral atomic. The resultant relatively high neutral carbon abundance will then explain high [CI]/ ${\rm ^{13}CO}$ intensity ratios. In this framework, our observations and those obtained by Gerin & Phillips (2000) imply that most of the emission from galaxy centers does not come from very dense, starforming molecular cloud cores.