MOLECULAR GAS OBSERVATIONS AND ENHANCED MASSIVE STAR-FORMATION EFFICIENCIES IN M-100

We present new J = 1 --> 0(12)CO observations along the northern spira l arm of the grand-design spiral galaxy M 100 (NGC 4321), and study th e distribution of molecular hydrogen as derived from these observation s, comparing the new data with a set of data points on the southern ar m published previously. We compare these measurements on both spiral a rms and on the interarm regions with observations of the atomic and io nized hydrogen components. We determine massive star formation efficie ncy parameters, defined as the ratio of H alpha luminosity to total ga s mass, along the arms and compare the values to those in the interarm regions adjacent to the arms. We find that these parameters in the sp iral arms are on average a factor of 3 higher than outside the arms, a clear indication of triggering of the star formation in the spiral ar ms. We discuss possible mechanisms for this triggering, and conclude t hat a density wave system is probably responsible for it. We discuss s everal possible systematical effects in some detail, and infer that th e conclusions on triggering are sound. We specifically discuss the pos sible effects of extinction in H alpha, or a non-standard CO to H-2 co nversion factor (X), and find that our conclusions on the enhancement of the efficiencies in the arms are reinforced rather than weakened by these considerations. A simple star forming scheme involving threshol d densities for gravitational collapse is discussed for NGC 4321, and comparison is made with M 51. We find that the gas between the arms is generally stable against gravitational collapse whereas the gas in th e arms is not, possibly leading to the observed arminterarm difference s in efficiency, but also note that these results, unlike the others o btained in this paper, do depend critically on the assumed value for t he conversion factor.