Optical Color Gradients in Star‐forming Ring Galaxies

Abstract

We compute radial color gradients produced by an outwardly propagating circular wave of star formation and compare our results with color gradients observed in the classical ring galaxy, the "" Cartwheel.ÏÏ We invoke two independent models of star formation in the ring galaxies. The Ðrst one is the conventional density wave scenario, in which an intruder galaxy creates a radially propagating density wave accompanied by an enhanced star formation following the SchmidtÏs law. The second scenario is a pure self-propagating star formation model, in which the intruder sets o † only the Ðrst burst of stars at the point of impact. Both models give essentially the same results. Systematic reddening of B[V , V [K colors toward the center, such as that observed in the Cartwheel, can be obtained only if the abundance of heavy elements in the star-forming gas is a few times below solar. The B[V and V [K color gradients observed in the Cartwheel can be explained as a result of mixing of stellar populations born in a star-forming wave propagating through a low-metallicity gaseous disk, and a preexist-ing stellar disk of the size of the gaseous disk with color properties typical to those observed in nearby disk galaxies.