M51 - MOLECULAR SPIRAL ARMS, GIANT MOLECULAR ASSOCIATIONS, AND SUPERCLOUDS

We present an aperture synthesis image of the prototypical grand-design spiral galaxy M51 in the CO 1 → 0 line made with the Owens Valley Millimeter Interferometer. The image is a mosaic of 30 1′ fields. Narrow (10″-20″) spiral arms are seen which are coincident with the optical dust lanes and nonthermal radio emission, but are offset from the ridges of Hα and thermal radio emission. There are many dense concentrations of emission, termed giant molecular associations (GMAs), both along and between the arms. The GMAs have masses of 107 to 6 × 107 M⊙, and most of the on-arm ones appear to be bound. They consist of several spectral components, termed molecular superclouds, which also appear to be bound. The interarm GMAs are unbound and are probably due to a secondary compression of the density wave. Streaming motions in the GMAs are consistent with density wave theory.