3.1. Results on M 51
The galaxy M 51 is a grand design spiral galaxy at a distance of . The CO(4-3) spectra shown in Fig. 1 have been smoothed to a velocity resolution of . Even far from the centre of the galaxy (at or galactocentric distance), on a dust lane of a spiral arm between two regions, CO(4-3) is detected. The spectra of the CO(3-2) transition obtained by Wielebinski et al. (1999) are also plotted in Fig. 1 as dotted lines. The shapes of the two transitions and their widths do not show significant variations, which confirms the high pointing accuracy.
We made a comparison of the integrated line emission in the two transitions CO(4-3) and CO(3-2) correcting for the different beams. For the central region we find a line ratio [(4-3)/(3-2)] of . Though there are not enough points for good statistics, a slight decrease in the line ratio can be seen from the very centre to larger radii where we find . This is the first indication that the density and excitation temperature of the molecular gas fall off from the centre of M 51. In the outer spiral arm the line ratio is which is an upper limit due to the different beams.
The critical conditions to excite the level are concentrated to special regions of the galaxy, in contrast to which can be found almost everywhere where CO(1-0) and CO(2-1) are detected (see Wielebinski et al. 1999). Using the CO(1-0) map of Nakai et al. (1994) we see a constant line ratio of for the whole region covered by the CO(4-3) map, which also indicates a shrinking of the radial extension for the state compared to lower levels.
3.2. Results on NGC 6946
The spiral galaxy NGC 6946 at distance has been studied at various wavelengths. Up to now little is known about the molecular distribution in this galaxy because of the lack of extensive observations. The CO(1-0) map of Sofue et al. (1988) covered the central region only. The work of Casoli et al. (1990) added information about two regions in spiral arms from the centre in the CO(1-0) and CO(2-1) transitions. Fig. 2 shows the spectra of the CO(4-3) emission smoothed to a resolution of . In addition observations of the CO(3-2) transition by Dumke et al. (in prep.) at the same telescope are plotted as dashed lines. 19 positions at the centre of the galaxy and three positions in the outer spiral arms in regions of strong HI emission were observed. The observations at / and / offsets are situated on the inner edge of an optical spiral arm in the east of the galaxy, which also shows emission within the beams of our CO observations. The third position was placed in the western part in a star-forming region. In the first of these spectra there is a 5 detection at a distance of away from the nucleus, corresponding to a deprojected galactocentric distance of .
Up to now it was not expected to find molecular gas of high temperature and density so far from the centre of an external galaxy. At the moment it is too early to draw any conclusion on the physical conditions from the detection or non-detection in the different regions in the spiral arms. Presumably the emission arises from a small part of our beam.
We have made a comparison between the integrated line intensity of the CO(4-3) and CO(3-2) transition after a correction for the different beams. Again we find a slight decrease in the line ratio from in the centre to at radius. Therefore we have studied the extension of the emission in NGC 6946 at the two transitions. The CO(4-3) intensity falls off much faster. Fitting a two-dimensional gaussian through the integrated spectra, we find extensions of 26" by 23" along the major and minor axis, respectively. The position angle was calculated to be . For the lower transition these values are 34" and 32" (, the uncertainty of the gaussians fitting is ). Now we can deconvolve the apparent widths with the beams of and and get the sizes of the sources (see Table 2). Using the spectra published by Sofue et al. (1988) we have also calculated these numbers for the CO(1-0) transition. Here the deconvolved source size is by with a position angle of . Though this angle differs from the one for the higher transitions, we can state that in the direction of the radial extension of the integrated CO emission is nearly the same for and . For the transition however, we find the emission significantly more concentrated to the centre of the galaxy, which was also suggested by the decrease in the line ratio. Thus we conclude that the physical conditions of density and temperature of the molecular gas are above the critical values to excite the level, but CO(4-3) needs special conditions that are restricted to regions like the centre of the galaxy or local warm cores.
Table 2. FWHP size of the central peak in NGC 6946.
© European Southern Observatory (ESO) 1999
Online publication: June 18, 1999