Testing Density Wave Theory with Resolved Stellar Populations around
  Spiral Arms in M81
  Stationary density waves rotating at a constant pattern speed $\Omega_{\rm
P}$ would produce age gradients across spiral arms. We test whether such age
gradients are present in M81 by deriving the recent star formation histories
(SFHs) of 20 regions around one of M81's grand-design spiral arms. For each
region, we use resolved stellar populations to determine the SFH by modeling
the observed color-magnitude diagram (CMD) constructed from archival Hubble
Space Telescope (HST) F435W and F606W imaging. Although we should be able to
detect systematic time delays in our spatially-resolved SFHs, we find no
evidence of star formation propagation across the spiral arm. Our data
therefore provide no convincing evidence for a stationary density wave with a
single pattern speed in M81, and instead favor the scenario of kinematic spiral
patterns that are likely driven by tidal interactions with the companion
galaxies M82 and NGC 3077.
