Large Infrared and Optical Color Gradients in the Cartwheel Ring Galaxy: Evidence for the First Epoch of Star Formation in the Wake of an Expanding Ring

Substantial near-infrared and optical color gradients have been observed within the disk of the classical ring galaxy A0035-324 (the "Cartwheel"). The slope of the radial B - V, V - K color gradient coincides with starburst color evolution predictions, indicating that the age of the disk population is a function of radius. Observations indicate that the youngest stars are on the leading edge of the outer ring, and the stellar population grows increasingly older toward the nucleus. The radial color gradient, taken together with other evidence, strongly support the collisional picture for the formation of ring galaxies. Strong IR emission observed in the outer ring appears to be directly associated with the young star-forming regions rather than an evolved stellar population. We conclude that star formation is occurring in a mainly gaseous expanding density wave and that the Cartwheel is undergoing its first epoch of star formation since its formation.