Effects of Non-Circular Motions on Azimuthal Color Gradients
  Assuming that density waves trigger star formation, and that young stars
preserve the velocity components of the molecular gas where they are born, we
analyze the effects that non-circular gas orbits have on color gradients across
spiral arms. We try two approaches, one involving semi-analytical solutions for
spiral shocks, and another with magnetohydrodynamic (MHD) numerical simulation
data. We find that, if non-circular motions are ignored, the comparison between
observed color gradients and stellar population synthesis models would in
principle yield pattern speed values that are systematically too high for
regions inside corotation, with the difference between the real and the
measured pattern speeds increasing with decreasing radius. On the other hand,
image processing and pixel averaging result in systematically lower measured
spiral pattern speed values, regardless of the kinematics of stellar orbits.
The net effect is that roughly the correct pattern speeds are recovered,
although the trend of higher measured $\Omega_p$ at lower radii (as expected
when non-circular motions exist but are neglected) should still be observed. We
examine the Martinez-Garcia et al. (2009) photometric data and confirm that
this is indeed the case. The comparison of the size of the systematic pattern
speed offset in the data with the predictions of the semi-analytical and MHD
models corroborates that spirals are more likely to end at Outer Lindblad
Resonance, as these authors had already found.
