Efficient Star Formation in the Spiral Arms of M51

We have compared the molecular, neutral, and ionized hydrogen distributions in the Sbc galaxy M51 (NGC 5194). To estimate H_2_ surface densities we made observations of the CO (J = 1 -> 0) transition in 60 positions out to a radius of 155" using the 13.7 m telescope of the Five College Radio Astronomy Observatory. Extinction-corrected Hα intensities were used to compute the detailed massive star formation rates (MSFRs) in the disk. Estimates of the gas surface density, the MSFR, and the ratio of these quantities, MSFR/σ_p_, were then examined. The spiral arms were found to exhibit an excess gas density, measuring between 1.4 and 1.6 times the interarm values at 45" resolution. The MSFR contrast between the arms and interarms, measuring between 1.5 and 2.3 at the same resolution, exceeds the gas density arm-interarm contrast. The enhanced Hα/CO ratios in the spiral arms imply a higher massive star formation rate per unit gas surface density in these regions. This follows the predictions of the cloud-cloud collision scenario of star formation which relies on the occurrence of orbit crowding to bring clouds into close proximity. We note that the regions exhibiting the highest massive star formation rates per unit gas surface density 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's 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 MSFR/σ_p_ with radius. This behavior is not explained by current models of star formation in galactic disks. The resulting time scale for gas depletion, total (SFR/σ_p_)^-1^, in the disk is 2.5 +/- 0.5 x 10^9^ yr assuming a Salpeter-like initial mass function.