DISTRIBUTION AND DYNAMICS OF MOLECULAR GAS IN THE GALAXY-M51 .1. DATAAND SPIRAL STRUCTURE

The 12Co(J=1-0) line emission from the region of 6' x 6' (17 kpc x 17 kpc) of M51 has been mapped with the Nobeyama 45-m telescope. The mapp ing area covers most of the optical disk. The spiral arms are clearly separated, and the CO emission from the interarm regions is well detec ted with high sensitivity. Two molecular arms are fitted by symmetrica l logarithmic spirals at 40'' < R < 120'', but are distorted at R > 12 0'' by the tidal interaction with the companion galaxy NGC 5195, where R is the galactocentric distance. The width of the molecular arms is 900-1000 pc and its displacement from Halpha arms is 440 pc. The CO(2- 1) to CO(1-0) intensity ratio is constant at 0.80 throughout the disk, except for the extreme outer region, and is almost the same as those previously measured in disks of other galaxies.