2009ApJ...694..512M

Azimuthal color (age) gradients across spiral arms are one of the main predictions of density wave theory; gradients are the result of star formation triggered by the spiral waves. In a sample of 13 spiral galaxies of types A and AB, we find that ten of them present regions that match the theoretical predictions. By comparing the observed gradients with stellar population synthesis models, the pattern speed and the location of major resonances have been determined. The resonance positions inferred from this analysis indicate that nine of the objects have spiral arms that extend to the outer Lindblad resonance; for one of the galaxies, the spiral arms reach the corotation radius. The effects of dust, and of stellar densities, velocities, and metallicities on the color gradients are also discussed.