Detection of Dynamical Structures Using Color Gradients in Galaxies

We describe a technique that uses radial color gradients in disk galaxies todetect the presence of bulk non-circular motion or elliptical orbits. In a diskgalaxy with both a radial color gradient and non-circular motion, isochromes oriso-color contours should follow the shape of closed stellar orbits, and theellipticity of the isophotes should vary as a function of wavelength. Adifference in the ellipticity of isochromes and the isophotes can be used todetect the presence of non-circular motion. A model galaxy is constructed whichdemonstrates this phenomenon. The difference between isochrome and isophoteellipticity is directly related to the ellipticity of the potential. Thisprovides a new way to measure the ellipticity of the dark matter in the outerparts of galaxies. As an example, we apply this technique to two dwarf galaxies NGC 1800 and NGC7764. We detect a bar in NGC 1800 which has only previously been suggested fromthe HI velocity field. In NGC 7764 there is no color gradient along its bar sowe cannot detect non-circular motion in this region; however ellipticitiesobserved in a star forming ring at the end of the bar are consistent with thisring being located near the corotation resonance.Comment: AAS Latex, submitted to ApJ