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 gaseou s disk: a self-induced wave and a density wave. We show that the surfa ce brightness profiles produced by both scenarios of ring formation ar e similar and are strongly sensitive to the velocity of the wave. The results of our computations are compared with the observational quanti ties sensitive to the young and old stellar populations in the ring ga laxies A0035-335 (the Cartwheel Galaxy) and VII Zw 466. The best fit t o the observed radial Ha surface brightness distribution in the Cartwh eel Galaxy is obtained for a wave velocity of about 90 km s(-1). The r ed continuum brightness of the ring can be fully explained by the evol ving stars present in the trailing part of the wave. However, the red continuum brightness in regions internal to the ring indicates that th e wave of star formation propagates in a preexisting stellar disk in t he Cartwheel. The Ha and K-band surface brightness profiles in VII Zw 466 match very well the values expected from stellar populations produ ced by a wave of star formation propagating in a purely gaseous disk. We conclude that VII Zw 466 is probably experiencing the first event o f star formation in the disk.