Molecular Clouds in M51 and in the Galaxy

Studies of molecular clouds in nearby galaxies require high angular resolution. Ten arcseconds corresponds to 0.5 kpc at the distance of M51. Typical gigant molecular clouds (GMC:s) have a size of 5-30 pc (Solomon et al. 1985). However complexes of GMC: s (Superclouds) can be several hundred parsecs (Elmegreen 1985; Rivolo et al. 1985). The higest angular resolution achived in CO(J=1-0) line observations of external galaxies is 7” (Lo et al 1984,1985). The resolution problem can be eased by observing M31 with a distance of only ⋍ 690 kpc (10” corresponds to 34 pc), which has been done by Combes et al. 1977a,b; Boulanger et al. 1984; Ryden and Stark 1985; Stark 1985; Blitz 1985; Ichikawa et al. 1985. In M31 the CO emission is strongly concetrated to the spiral arms with a arm interarm ratio of ≥ 25 (Ryden and Stark 1985; Stark 1985). The emission is caused by many small clouds unresolved with present resolution together with some larger clouds. Streaming is observed to occur across the arms. Extragalatic studies have the advantage of being more easy to interpret in terms of arm interarm contrast, noncircular motion, and galatic structure. They also make possible studies of the mass fraction of gas as a function of radius in different morphological types of galxies. Answers to questions like “Do any relation exist between galaxy type and molecular abundance?” are very important for our understanding of galatic evolution.