The Swift/UVOT catalogue of NGC4321 star forming sources: A case against
  density wave theory
  We study the star forming regions in the spiral galaxy NGC4321, taking
advantage of the spatial resolution (2.5 arcsec FWHM) of the Swift/UVOT camera
and the availability of three UV passbands in the region 1600-3000 A, in
combination with optical and IR imaging from SDSS, KPNO/Ha and Spitzer/IRAC, to
obtain a catalogue of 787 star forming regions out to three disc scale lengths.
We determine the properties of the young stellar component and its relationship
with the spiral arms. The Ha 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 NUV-optical colour,
blue sources have a significant excess of flux in the IR at 8 micron, revealing
the contribution from PAHs, 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 Ha luminosity, NUV-optical
colour, or ages derived from a population synthesis model. An offset is
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.
