3.1. Results for M51
The galaxy M51 is classified as SA(s)bc pec (in RC2) and has an optical diameter . The CO(3-2) spectra for M51 shown in Fig. 1 have been smoothed to 20 km s-1 and can best be compared with the spectra of CO(2-1) given in Garcia-Burillo et al. (1993). The comparison of the shapes of the spectra and their widths shows that they do not vary significantly, neither in the nucleus, nor in the spiral arms. The spectra (Fig. 1) were transformed into an integrated [K km s-1] contour plot shown in Fig. 4. This plot shows a striking similarity to the colour plot of the CO(2-1) distribution published in Garcia-Burillo et al. (1993). There is also great similarity with the CO(1-0) map in Nakai et al. (1994). Since the CO(3-2) transition is expected to originate in warmer gas, the immediate conclusion is that warm CO gas is extended in M51, a result which was not expected on the basis of the few published results so far.
In the report of Bash et al. (1990) a claim was made that at two positions near the nucleus of M51 the CO(3-2) intensity was in fact higher than the CO(1-0) one. Our calibration accuracy of better than 20% allows us to make significant conclusions about the line ratios. We have made comparisons of the integrated line intensity ratios by smoothing the CO(1-0) data of Nakai et al. (1994). For the central region we find the line ratio of . In the inner spiral arms of M51 the line ratios are higher, . There seems to be no variation in these ratios along the inner spiral arms. In the outermost spiral arms, however, the line ratio increases to . Determinations in ten positions made by Garcia-Burillo et al. (1993) for the CO(3-2)/(2-1) line ratios were in the centre of M51 and in the spiral arms. We consider these ratios to be consistent with our determinations since 345 GHz Pico Veleta line intensity values could be underestimated because of the error beam which is present in the 30-m telescope observations.
3.2. Results for NGC 278
The galaxy NGC 278 is type SAB(rs)b with an optical diameter . The CO(3-2) spectra smoothed to 20 km s-1 shown in Fig. 2 are present across a large part of NGC 278. This observational fact immediately leads to the suggestion that the warm gas is widely distributed in this galaxy. The observations of Braine et al. (1993) suggested a line ratio . This high line ratio continues to our CO(3-2) observations. In fact relating the CO temperatures to the same beam area we get . Braine et al. (1993) claimed double structure of the central spectrum of NGC 278 which is suggested in our observations also, but slightly off the nucleus. A map of the CO(3-2) line emission suggests the existence of a ring-like structure, a fact that was discussed by Braine et al. (1993). Two spectra in the CO(1-0) line are published by Young et al. (1995) which also confirm the high line ratio after appropriate convolution. We note that the line width in NGC 278 is very narrow, possibly as narrow as 10 km s-1, but similar at 115, 230, and 345 GHz. The study of NGC 278 by Schmidt et al. (1990) showed that the nucleus of this galaxy is dominated by young components. A recent starburst was suggested by the emission line spectrum presented in that paper. This would make NGC 278 more similar to M82 and not to M51. A more recent study of NGC 278 (Rhoads 1998) uses CO absorption at m to study the stellar content. This NIR study shows that the ring-like structure is seen in the optical range also.
3.3. Results for NGC 4631
The galaxy NGC 4631 is type SB(s)d sp seen edge-on with . The CO(3-2) spectra, again smoothed to 20 km s-1, are shown in Fig. 3. The CO(1-0) data set for NGC 4631 can be found in Sofue et al. (1989) and in Golla & Wielebinski (1994). The CO(1-0) data in the Golla & Wielebinski paper have been collected simultaneously with the CO(2-1) data and hence on a grid of 6". The comparison of the data sets gives for the nuclear area and for the regions of the CO maxima some from the nucleus along the major axis. The published `global' line ratio in Golla & Wielebinski (1994) is . We suggest that given the quoted calibration errors the line ratios are compatible. Certainly the line ratios are high for the "mild starburst" galaxy. We also see similar line widths at all positions in our respective maps. Some spectra were observed by Golla & Wielebinski (1994) along the major axis further out from the nucleus than in our present observations. We have used short integrations in our observations only and we have concentrated on the central area of NGC 4631.
© European Southern Observatory (ESO) 1999
Online publication: June 30, 1999