Distribution and Dynamics of Molecular Gas in the Galaxy M51. I. Data and Spiral Structure

The (12) CO(J=1--0) line emission from the region of 6' times 6' (17 kpc times 17 kpc) of M51 has been mapped with the Nobeyama 45-m telescope. The mapping area covers most of the optical disk. The spiral arms are clearly separated, and the CO emission from the interarm regions is well detected with high sensitivity. Two molecular arms are fitted by symmetrical logarithmic spirals at 40'' < R < 120'', but are distorted at R > 120'' 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 Hα 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.