Color Gradients in M99: Stellar Populations or Dust?

Gonzalez & Graham (1996a, b) discovered an azimuthal age gradient in the stellar population across one of the arms of M99, in what amounts to the first observational confirmation of a fundamental prediction of spiral density wave theory. A more exhaustive comparison of the color gradient with stellar population synthesis models seems to rule out a wave as old as 1010 yr, in favor of a duration of a few times 108 yr. By using new models that include the effects of mixed-in dust, we confirm that dust plays a minor role in the determination of the azimuthal color changes in this region of M99. We advance some possible new explanations for the overwhelming non-detection of convincing stellar population gradients across spiral arms.