Search For Star Cluster Age Gradients Across Spiral Arms of Three LEGUS
  Disk Galaxies
  One of the main theories for explaining the formation of spiral arms in
galaxies is the stationary density wave theory. This theory predicts the
existence of an age gradient across the arms. We use the stellar cluster
catalogues of the galaxies NGC 1566, M51a, and NGC 628 from the Legacy
Extragalactic UV Survey (LEGUS) program. In order to test for the possible
existence of an age sequence across the spiral arms, we quantified the
azimuthal offset between star clusters of different ages in our target
galaxies. We found that NGC 1566, a grand-design spiral galaxy with bisymmetric
arms and a strong bar, shows a significant age gradient across the spiral arms
that appears to be consistent with the prediction of the stationary density
wave theory. In contrast, M51a with its two well-defined spiral arms and a
weaker bar does not show an age gradient across the arms. In addition, a
comparison with non LEGUS star cluster catalogues for M51a yields similar
results. We believe that the spiral structure of M51a is not the result of a
stationary density wave with a fixed pattern speed. Instead, tidal interactions
could be the dominant mechanism for the formation of spiral arms. We also found
no offset in the azimuthal distribution of star clusters with different ages
across the weak spiral arms of NGC 628.
