The study of galaxies in the higher CO line transitions, as well as in the various isotopic species, should allow us to determine the physical conditions in the emitting regions. In the case of thermalized excitation temperatures of 55 K or 33 K are required to excite the or level, respectively. Alternatively the Large Velocity Gradient (LVG) analysis (see e. g. de Jong et al. 1975) should allow us to determine the temperature, the density and the optical depth from the observed line ratios. Following the systematic observations of many galaxies in the CO(1-0) and CO(2-1) lines, the attention has now moved to the higher line transitions. Recently 345 GHz observations of several galaxies (Mauersberger et al. 1999; Wielebinski et al. 1999) were reported, showing that this higher line transition (which in general is due to warm and dense gas) is easily detectable in many galaxies and that it can be found even far off from the centre.
There are up to now only a few published papers on CO(4-3) line observations in external galaxies. In fact only four of the most intense galaxies were observed so far. The pioneering work of Güsten et al. (1993) showed data on three points in M 82, one in IC 342 and two in NGC 253. A paper by White et al. (1994) gave CO(4-3) and CI maps of a region in extent in the centre of M 82. More extensive studies of NGC 253 in the CI and CO(3-2) line with some CO(4-3) points were reported by Israel et al. (1995). Recently Petitpas and Wilson (1998) have published a larger CO(3-2) map of M 83 and a CO(4-3) map covering of the central region of this galaxy. We note that all these four objects are considered to be active starburst galaxies.
The reason for this sparse data base is that only a few radio telescopes exist which are capable to make these sub-mm observations. We have used the 10-m Heinrich-Hertz-Telescope on Mt. Graham, Arizona, which has been designed for sub-mm observations. We have made extensive maps of several nearby galaxies in the CO(3-2) line, which we report elsewhere. In a short period of excellent weather in December 1998 we were able to make maps in the CO(4-3) transition of the central regions of six weaker, nearby galaxies. Here we report results on two of them, M 51 and NGC 6946.
Table 1. Basic parameters of M 51 and NGC 6946
© European Southern Observatory (ESO) 1999
Online publication: June 18, 1999