A COMPLETELY SAMPLED APERTURE SYNTHESIS MAP OF THE CO EMISSION IN M51

Interferometric observations from the Berkeley-Illinois-Maryland Association millimeter interferometer are combined with single-dish observations from the Onsala Space Observatory to create high-resolution fully sampled maps of CO (J = 1-0) emission from two 2' fields in the spiral galaxy M51. In previous interferometer CO maps of M51, greater-than-or-equal-to 65% of the CO flux density in each field is missing but assumed to be uniformly distributed through the field of view. In contrast, our maps recover all of the flux density and show directly that 75% of the CO flux density from the inner few kiloparsecs of M51 come from the spiral arms. The CO arms are wider (less-than-or-similar-to 1.3 kpc) and more extended than the CO arms presented in previous interferometer maps. The CO arm is clumpy along the arm with a linear scale approximately 1 kpc. The average arm-to-interarm intensity contrast over the two fields is greater-than-or-equal-to 4.6: 1; peak arm-to-interarm CO integrated intensity contrasts of greater-than-or-equal-to 20:1 are seen in the inner arms. Some large "clumps" of interarm CO emission are seen. Streaming motions of up to 70 km s-1 (in the plane of the galaxy) are seen in the arms; these motions are consistent with those expected from the spiral density wave theory. Both the intensity contrast and streaming motions are consistent with numerical simulations which show that orbit crowding is the dominant physical mechanism for gathering the molecular clouds into the arms. Dissipative cloud-cloud collisions and gaseous self-gravity must be included in the numerical models to account for the observed clumpiness along the spiral arms. Adopting the Galactic conversion ratio of H-2 to CO integrated intensity, we find that the inferred gas mass of the molecular cloud complexes in the spiral arms of M51 is systematically larger than the virial mass. An H-2-to-CO conversion ratio in the inner spiral arms of M51 2.5 times smaller than the Galactic value will correct the disrepancy.