Astron. Astrophys. 347, L5-L8 (1999)

4. Discussion

The present observations open new possibilities of molecular studies of galaxies. The ability to map many CO line transitions (and their isotopic species) will allow us to narrow down the parameter space of temperature, density and optical depth in the context of e. g. Large Velocity Gradient analysis (see e. g. de Jong et al. 1975).

Here we report that the size of the central emission region of NGC 6946 decreases towards higher line transitions. A similar effect was found by Mauersberger et al. (1996) for the nucleus of NGC 4945. The slight decrease of the line ratio in the centre of the two galaxies shows that the excitation temperature is going down. This effect may be due to a variation of the kinetic temperature of the molecular gas as well as due to a change of the optical depth. We can state that the central molecular clouds have different physical conditions than the outer ones.

Furthermore, detection of the CO(4-3) line emission away from the centre, in the spiral arms of galaxies, has not been reported before. In view of the fact that mm and sub-mm bolometric observations of galaxies suggest the presence of cold dust () in the spiral arms we must investigate why the kinetic temperatures of the molecular gas are around (LVG). Possibly even warmer cores can be found by high angular resolution interferometric observations. Data on more objects of different morphological type are also required to advance this subject. In any case, mapping of larger parts of the galaxies and also observing different isotopes are necessary to improve our studies and to use the LVG model to calculate the physical conditions of the molecular gas in galaxies.

© European Southern Observatory (ESO) 1999

Online publication: June 18, 1999
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