Azimuthal propagation of star formation in nearby spiral galaxies: NGC 628, NGC 3726, and NGC 6946

Star formation induced by a spiral shock wave, which in turn is generated by a spiral density wave, produces an azimuthal age gradient across the spiral arm, which has opposite signs on either side of the corotational resonance. An analysis of the spatial separation between young star clusters and nearby H ii regions has made it possible to determine the position of the corotation radius in the galaxies studied. Fourier analysis of the gas velocity field in the same galaxies independently confirmed the corotation radius estimates obtained by the morphological method presented here.