The role of molecular clouds in the star formation process as observed in two grand design spiral galaxies

We have compared the molecular, neutral and ionized hydrogen distributions in two nearby spiral galaxies. To estimate H$\sb2$ surface densities we acquired observations of the CO (J = 1 $\to$ 0) transition in 60 positions to a radius of 135 in the Sbc galaxy M51 (NGC 5194), and in 21 positions to a radius of 150 in the SAB galaxy M83 (NGC 5236) using the 13.7 m telescope of the Five College Radio Astronomy Observatory. The molecular component of the ISM was found to strongly dominate over the HI component in each galaxy. Extinction corrected H$\alpha$ intensities were used to compute the detailed massive star formation rates (MSFRs) in each galaxy. Estimates of the MSFR, gas density, and the ratio of these quantities, the massive star formation efficiency (MSFE), were then examined. In M51, the spiral arms exhibit an excess gas density of 1.4-1.6 times the interarm values. The MSFR contrast between the arms and interarms, measuring between 1.5 and 2.3 at the same resolution, exceeds the gas density contrast and implies a nonlinear relationship between star formation and gas surface density on the spiral arms. This follows the predictions of the cloud-cloud collision scenario fo star formation which relies on the occurrence of orbit crowding to bring clouds into close proximity. We note that the regions exhibiting the highest MSFEs are those in the spiral potential minimum inward of R = 124 , and those regions outward of R = 124 thought to be experiencing orbit crowding due to tidal distortion caused by the close passage of M51\u27s companion galaxy. The total (arm and interarm) gas content and massive star formation rates in concentric annuli in the disk of M51 were computed. The two quantities fall off together with radius, yielding a relatively constant MSFE with radius. This is consistent with the increased MSFE on the arms in that the majority of the gas shows a constant MSFE. The resulting time scale for gas depletion (total SFE$\sp{-1}$) in the disk is 2.5 $\pm$ 5 $\times$ 10$\sp9$ yr assuming a Salpeter-like initial mass function. In M83, the molecular gas component of the inner disk mimics the bar morphology. In this galaxy there is the suggestion of enhanced star formation at the ends of the central bar due to the compression of cloud orbits found there. The gas depletion time scale is 1.2 $\pm$ 0.3 $\times$ 10$\sp9$ yr