Surface Brightness Gradients Produced by the Ring Waves of Star Formation

We compute surface brightness profiles of galactic disks for outwardly propagating waves of star formation with a view to investigating the stellar populations in ring galaxies. We consider two mechanisms that can create outwardly propagating star-forming rings in a purely gaseous disk: a self-induced wave and a density wave. We show that the surface brightness profiles produced by both scenarios of ring formation are similar and are strongly sensitive to the velocity of the wave. The results of our computations are compared with the observational quantities sensitive to the young and old stellar populations in the ring galaxies A0035-335 (the Cartwheel Galaxy) and VII Zw 466. The best fit to the observed radial Hα surface brightness distribution in the Cartwheel Galaxy is obtained for a wave velocity of about 90 km s-1. The red continuum brightness of the ring can be fully explained by the evolving stars present in the trailing part of the wave. However, the red continuum brightness in regions internal to the ring indicates that the wave of star formation propagates in a preexisting stellar disk in the Cartwheel. The Hα and K-band surface brightness profiles in VII Zw 466 match very well the values expected from stellar populations produced by a wave of star formation propagating in a purely gaseous disk. We conclude that VII Zw 466 is probably experiencing the first event of star formation in the disk.