Global morphology and physical relations between the stars , gas and dust in the disc and arms of M 100

We study star formation processes in the disc of the weakly barred grand design spiral galaxy M100 (NGC 4321) from a variety of images tracing recent massive star formation, old and young stars, dust, and neutral hydrogen. Differences between arm and interarm regions are specifically studied by decomposing the images into arm and non-arm zones. We find from a comparison of the morphology in Hα, H i and dust that while the first two are coincident over most of the disc, they are offset from the dust lanes especially along the inner parts of the spiral arms: a picture which is indicative of a density wave shock moving through the arms. H i is formed near the young massive stars as a result of photo-dissociation. From radial profiles we find that in the region of the star-forming spiral arms the exponential scale lengths for Hα, blue and near-infrared light, and 21 cm radio continuum are equal within the fitting errors. The scale lengths for the interarm region are also equal for all these tracers, but the arm scale lengths are significantly longer. This points to a common origin of the profiles in star formation, with little or no influence from radial population gradients or dust in the disc of this galaxy. The longer arm scale lengths are equivalent to an outwardly increasing arm-interarm contrast. We argue that the radial profiles of radio continuum and H i, as well as CO, are also directly regulated by star formation, and discuss the possible implications of this result for the interpretation of observed CO intensities in and outside spiral arms. We discuss the radial atomic hydrogen profile in some detail. Its almost perfectly flat shape in the region of the star-forming spiral arms may be explained by pho-todissociation and recombination processes in the presence of a limited quantity of interstellar dust, controlling the equilibrium between the molecular and atomic form of hydrogen. Over most of the inner part of the disc, Hi seems to be a product of the star formation processes, rather than the cause of enhanced star formation.