On the Nature of Spiral Arms in the Interacting Galaxy M51

We investigate the radial variation of the azimuthal propagation of star formation across the spiral arms in the nearby galaxy NGC 5194 (M51a) by analyzing the spatial distribution of young star clusters and neighboring H II regions. The radial profile in the northern arm agrees with the predictions of stationary density wave theory for trailing spirals, whereas the radial profile in the southern arm requires its pattern speed to be higher than the rotation velocity of disk matter inside the corotation circle and lower than that outside of it.