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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 &lt; λ &lt; 3000 Å, in combination with optical and infrared (IR) imaging from Sloan Digital Sky Survey, KPNO/Hα 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 Hα 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   μ  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 Hα luminosity, NUV–optical 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.

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