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Astron. Astrophys. 341, 256-263 (1999)

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4. Discussion

In Fig. 3 we correlate the dust temperature, [FORMULA], and the infrared luminosity, [FORMULA], with the intensity ratio of the CO 3-2 and 1-0 lines. The dust temperature was determined as explained in Table 2. The IR luminosity was extrapolated from the IRAS Point Source Catalog data using a scheme described in Wouterloot & Walmsley (1986). We also included data for NGC 253 measured by Harrison et al. (1998). It is evident that only a small fraction of the observed objects have intensity ratios [FORMULA] larger than 0.7. The objects with such a high ratio also have a high [FORMULA] in excess of 35 K and tend to have a higher IR luminosity. Among the rest of the sources there appears to be no correlation between [FORMULA] or [FORMULA] and [FORMULA], the value of which scatters between 0.2 and and 0.7.

[FIGURE] Fig. 3. Correlation between the dust temperature and IR luminosity with the intensity ratio of the CO 3-2 and 1-0 lines. The only sources which appear to have [FORMULA] are NGC 2146 and NGC 3351.

There are several caveats in interpreting the diagrams in Fig. 3. First, many nearby galaxies are extended with respect to our beam and even with respect to the much larger (1´) IRAS beam. As a consequence, the CO measurements may reflect conditions in a more active nuclear region than the IRAS data. Since, however, IRAS fluxes are biased toward a warm dust component, which may also arise from a compact central region, comparing properties derived from IRAS data with those from CO measurements need not be a problem. Second, with increasing distance, the ratio [FORMULA] should decrease, as more radiation from lesser excited CO lines arising from the disk regions is picked up by the beam. This may be partly compensated if the more distant sources of our sample (which tend to be more luminous) also have a larger fraction of highly excited gas.

We have performed computations of the radiative transfer and excitation of CO for a range of kinetic temperatures and H2 densities using the large velocity gradient approximation for a spherical molecular cloud. Collision rates are taken from De Jong et al. (1975). The model used is described in Henkel et al. (1980). We took [FORMULA] [FORMULA] and 10[FORMULA]. The results of these computations can be found in Fig. 4.

[FIGURE] Fig. 4. As a result of model computations, the intensity ratios of the CO 3-2 and 1-0 lines are plotted as contours for a range of [FORMULA] and [FORMULA] for two values of the parameter [FORMULA], namely [FORMULA] and 10[FORMULA], which corresponds to velocity gradients of 1 (upper panel) and 10 [FORMULA] (lower panel) if the relative abundance of CO is 10-4.

It is evident that for the input parameters used, [FORMULA] can only become unity or higher if the gas temperatures are [FORMULA]K and average H2 densities, [FORMULA], exceed 103 cm-3. For a temperature range of 20 to 60 K, the intensity ratio is mainly related to [FORMULA]: a ratio of 0.2 would indicate densities of [FORMULA] and a ratio as high as 0.6 indicates densities in the range of 1000 cm-3. In most cases the 3-2 line is less intense than lower lying lines. Line intensity ratios close to unity are found toward the prominent starburst galaxies NGC 253, M 82 and Arp 220 indicating high densities and temperatures for a large fraction of the molecular gas in the beam. There are only few sources where the 3-2 line is stronger than the 1-0 line. Among those is NGC 2146 where [FORMULA] is as high as to 1.6. This behavior is expected for optically thin CO lines from warm gas, which seems to be exceptional.

Optical depths and thus the velocity gradient could be estimated best from observations of rare isotopic substitutions of CO. Güsten et al. (1993) pointed out that in many galaxies there exist several molecular components with different temperatures and densities and demonstrated that observations of isotopic CO lines and of higher transitions of CO will allow us to better constrain the range of possible physical parameters. The next CO line which can be observed from the ground, the CO [FORMULA] line, is 55 K above the ground level. Our observations clearly indicate that this line can be observed in many sources.

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© European Southern Observatory (ESO) 1999

Online publication: November 26, 1998