DETECTION OF DYNAMICAL STRUCTURES USING COLOR GRADIENTS IN GALAXIES

We describe a technique that uses radial color gradients in disk galax ies to detect the presence of bulk noncircular motion or elliptical or bits. In a disk galaxy with both a radial color gradient and noncircul ar motion, isochromes, or isocolor contours, should follow the shape o f closed stellar orbits, and the ellipticity of the isophotes should v ary as a function of wavelength. A difference in the ellipticity of is ochromes and the isophotes can be used to detect the presence of nonci rcular motion. A model galaxy is constructed which demonstrates this p henomenon. The difference between isochrome and isophote ellipticity i s directly related to the ellipticity of the potential. This provides a new way to measure the ellipticity of the dark matter in the outer p arts of galaxies. As an example, we apply this technique to two dwarf galaxies, NGC 1800 and NGC 7764. We detect a bar in NGC 1800 which has only previously been suggested from the H I velocity held. In NGC 776 4 there is no color gradient along its bar, so we cannot detect noncir cular motion in this region; however, ellipticities observed in a star -forming ring at the end of the bar are consistent with this ring bein g located near the corotation resonance.