The Swift/UVOT catalogue of NGC 4321 star-forming sources: a case against density wave theory

We study the star-forming regions in the spiral galaxy NGC?4321 (M100). We take advantage of the spatial resolution (2.5?arcsec full width at half-maximum) of the Swift/Ultraviolet/Optical Telescope camera and the availability of three ultraviolet (UV) passbands in the region 1600 < lambda< 3000 angstrom, in combination with optical and infrared (IR) imaging from Sloan Digital Sky Survey, KPNO/H alpha and Spitzer/IRAC, to obtain a catalogue of 787 star-forming regions out to three disc scalelengths. We use a large volume of star formation histories, combined with stellar population synthesis, to determine the properties of the young stellar component and its relationship with the spiral arms. The H alpha luminosities of the sources have a strong decreasing radial trend, suggesting more massive star-forming regions in the central part of the galaxy. When segregated with respect to near-UV (NUV)optical colour, blue sources have a significant excess of flux in the IR at 8 mu m, revealing the contribution from polycyclic aromatic hydrocarbons, although the overall reddening of these sources stays below E(B - V) = 0.2?mag. The distribution of distances to the spiral arms is compared for subsamples selected according to H alpha luminosity, NUVoptical colour or ages derived from a population synthesis model. An offset would be expected between these subsamples as a function of radius if the pattern speed of the spiral arm were constant as predicted by classic density wave theory. No significant offsets are found, favouring instead a mechanism where the pattern speed has a radial dependence.