GLOBAL MORPHOLOGY AND PHYSICAL RELATIONS BETWEEN THE STARS, GAS AND DUST IN THE DISC AND ARMS OF M100

We study star formation processes in the disc of the weakly barred gra nd design spiral galaxy M100 (NGC 4321) from a variety of images traci ng recent massive star formation, old and young stars, dust and neutra l hydrogen. Differences between arm and interarm regions are specifica lly studied by decomposing the images into arm and non-arm zones. We f ind from a comparison of the morphology in H alpha, H I and dust that while the first two are coincident over most of the disc, they are off set from the dust lanes especially along the inner parts of the spiral arms: a picture which is indicative of a density wave shock moving th rough the arms. H I is formed near the young massive stars as a result of photodissociation. From radial profiles we find that in the region of the star-forming spiral arms the exponential scalelengths for H al pha, blue and near-infrared light, and 21-cm radio continuum are equal within the fitting errors. The scalelengths for the interarm region a re also equal for all these tracers, but the arm scalelengths are sign ificantly longer. This points to a common origin of the profiles in st ar formation, with little or no influence from radial population gradi ents or dust in the disc of this galaxy. The longer arm scalelengths a re equivalent to an outwardly increasing arm-interarm contrast. We arg ue that the radial profiles of radio continuum and H I, as well as CO, are also directly regulated by star formation, and discuss the possib le implications of this result for the interpretation of observed CO i ntensities inside and outside spiral arms. We discuss the radial atomi c hydrogen profile in some detail. Its almost perfectly flat shape in the region of the star-forming spiral arms may be explained by photodi ssociation and recombination processes in the presence of a limited qu antity of interstellar dust, controlling the equilibrium between the m olecular and atomic form of hydrogen. Over most of the inner part of t he disc, H I seems to be a product of the star formation processes rat her than the cause of enhanced star formation.