7 [CI], [CII] and FIR intensities

Parameters indicating a warm and dense gas is more or less what is expected if the emission arises from photon-dominated region (PDR's - see e.g. Kaufman et al. 2000). In order to explore this possibility, we have produced Fig. 5, which presents a comparison of the ${\rm ^{3}P_{1}}{-}{\rm ^{3}P_{0}\,[CI]}$ and ${\rm ^{2}P_{3/2}}{-}{\rm ^{2}P_{1/2}\,[CII]}$ line and far-infrared continuum intensities; this Fig. 5 is directly comparable to Fig. 8 by Gerin & Phillips (2000) who performed a similar comparison. The majority of the [CII] intensities were taken from Stacey et al. (1991), as were the [CII]/FIR ratios. The Milky Way data were taken from Fixsen et al. (1999), those for the LMC from Israel et al. (1996) and Bolatto et al. (2000), and those for NGC 278 from Kaufman et al. (2000), who use this galaxy as an example for the application of their PDR model calculations. The [CII] measurements by Stacey et al. (1991) were obtained in a 55'' aperture, i.e. much larger than the [CI] beam. We have therefore used the area-integrated [CI] intensity, and assumed that all [CII] emission from the central source was contained within the 55'' aperture. As the extent of the central [CI] source is always less than this, that seems to be a reasonable assumption. If the [CII] emission is more extended than that of [CI], the relevant [CII]/[CI] ratio in Fig. 5 should be lowered correspondingly. However, we do not believe such a correction will change the picture significantly. The [CI]/FIR ratio was obtained from the [CII]/[CI] and [CII]/FIR ratio; thus [CI]/FIR may be somewhat higher than plotted.

Figure 5: Ionized carbon [CII] line to far-infrared continuum (FIR) ratio (top) and [CII]/[CI] line ratio (bottom) as a function of neutral carbon [CI] line to FIR ratio. In both diagrams, the position of the Milky Way center is marked by a cross and the positions of Magellanic Cloud objects by open hexagons.

In Fig. 5, there is no longer a clear distinction between various types of objects such as we found in Fig. 4. Rather, the [CII], [CI] and FIR intensities define a distribution in which LMC star formation regions, low-activity galaxy centers and high-activity galaxy centers are all intermingled. Nevertheless, the result shown in Fig. 5 bears a close resemblance to the the results obtained by Gerin & Phillips (2000). As the [CI]/FIR ratio increases, so does [CII]/FIR, but not the [CII]/[CI] ratio which decreases with increasing [CI]/FIR. Qualitatively, this may be explained by PDR process along the line discussed by Gerin & Phillips (2000). The horizontal location of the points in the two diagrams suggest fairly intense PDR radiation fields of about 300 to 1000 times the average UV radiation field in the Solar Neighbourhood. For the merger galaxy NGC 660 we have only upper limits (log [CII]/FIR < -3.2, log [CII]/[CI] < 2.1) which place this galaxy in the same diagram positions as the ultraluminous mergers Arp 220 and Mrk 231 observed by Gerin & Phillips, which correspond to strong radiation fields and very high gas densities.

The PDR models shown in Fig. 8 by Gerin & Phillips provide the highest [CII]/FIR ratios for model gas densities n = 10³- $10^{4}\,{\rm cm^{-3}}$. Fully half of our observed ratios are well above the corresponding curves, although they are not quite as high as the ratios observed for the three LMC starforming regions. Note that (the limits to) the quiescent cloud LMC N159-S in Fig. 5 likewise suggest high densities but only weak radiation fields, in good agreement with Israel et al. (1996) and Bolatto et al. (2000). For many of the galaxies and for the LMC starforming regions, the ratio of [CII] to [CI] intensities appears to be higher than predicted by the PDR models considered. For the LMC objects, this was already noted and discussed by Israel et al. (1996). They explain this situation by an increased mean free pathlength of energetic UV photons due the lower metallicity of the LMC. However, galaxy centers have, if anything, a higher metallicity (see Zaritsky et al. 1994). A possible explanation for the apparently similar behaviour of many galaxy centers may be a greater degree of filamentary or cirruslike structure. In spite of high metallicities, this would still allow for an effectively increased penetration depth of UV photons. If enhanced exposure results in a significantly larger fraction of carbon atoms becoming ionized, it would explain higher [CII] to [CI] emission ratios.

So far we have assumed homogeneous media, i.e. we have assumed all CO, [CI], [CII] and FIR emission to originate from the same volume. This provides in a relatively simple manner good estimates of the physical parameters characterizing the interstellar medium in the observed galaxy centers.

The LMC observations, which correspond to linear resolutions one to two orders of magnitude higher than the galaxy center observations, illustrate that homogeneity is not the case. The maps shown by Israel et al. (1996) and Bolatto et al. (2000) show that different locations in the observed regions are characterized by strongly different emission ratios indicating domination by different ISM phases (i.e. neutral atomic, ionized, molecular). A similar state of affairs applies to the Galactic Center region (Dahmen et al. 1998). Ideally, the observations should thus be modelled by physical parameters varying as a function of location in a complex geometry. Practically, we may approach reality by assuming the presence of a limited number of distinct gas components. The analysis of multitransition ${\rm ^{12}CO}$, ${\rm ^{13}CO}$ and [CI] observations of galaxy centers such as those of NGC 7331, M 83 and NGC 6946 (Israel & Baas 1999, 2001) suggests that, within the observational errors, good fits to the data can be obtained by modelling with only two components: one being dense and relatively cool, the other being relatively tenuous and warm.

The galaxy points in Fig. 5 can all be reproduced by assuming appropriate combinations of dense and cold gas (having high [CI]/FIR and [CII]/FIR ratios) with strongly irradiated gas of lower density (low [CI]/FIR and high [CII]/[CI] ratios). The distribution of points in Fig. 5 would thus not directly indicate the physical condition of the radiating gas, but rather the relative filling factors of the two components. A similar argument can be made to solve the apparent discrepancy between the relatively high kinetic temperatures suggested by Fig. 4 and the more modest dust temperatures referred to before. In the same vein, a multi-component solution requires somewhat lower beam-averaged [CI]/CO abundances than suggested by Fig. 4. The dataset presented in this paper is, however, not sufficiently detailed to warrant a more quantitative analysis such as we have presented for NGC 7331, M 83 and NGC 6946 (Israel & Baas 1999, 2001), and will present for half a dozen more in forthcoming papers.