Distribution and Dynamics of Molecular Gas in the Galaxy M51. II. Formation of Giant Molecular Associations and Massive Stars

We have investigated the distributions of molecular gas, HI-to-H_2 conversion and the formation mechanisms of Giant Molecular Associations (GMAs) and of massive stars in M51 using mapping data of CO obtained with the NRO 45-m telescope. The variations of the molecular-to-total gas mass fraction in the disk and its dependence on the total gas density and the radiation field are shown. The possibilities of a gravitational instability and cloud-cloud collisions as the formation mechanisms of GMAs are examined. A gravitational instability is supported from a comparison between the surface density of the total gas and the critical density of the gravitational instability, as well as from the spatial separations of the GMAs. On the other hand, since the time scale of a cloud-cloud collision is much shorter than the arm-crossing time, cloud coagulation may also work for GMA formation together with a gravitational instability. Point-by-point comparisons between the star-formation rates (SFRs) and the gas surface densities show that the SFR is correlated with the surface density of the total gas better than with that of the molecular gas alone. Observations of (13) CO and HCN indicate denser gas in the spiral arms than in the interarms.