COLOR GRADIENTS IN ELLIPTIC GALAXIES

A recent study has shown that the B-R U-R color gradients in a homogeneous sample of 17 elliptical galaxies do not vary significantly from galaxy to galaxy. These color gradients and the well-known colorluminosity relation for ellipticals are most probably due to changes in the mean metallicity. We have investigated the dependence of color on a variety of local physical parameters. The striking result that arises is that the local color (i.e., metallicity) is a function of the local escape velocity. The relation can explain both the color gradients within the galaxies and the color-luminosity relation for ellipticals. Models with and without dark halos give comparable results. Dissipative models with inward flows of preenriched gas may have difficulty reproducing the relations directly, i.e., the escape velocity may not be the fundamental parameter. Alternative models in which star formation proceeds until the energy released by supernovae is sufficient to expel the gas may be able to produce the observed relation in a more direct way. Such models do not invoke dissipation to produce the color gradients. The relation, whatever its cause, does not necessarily exclude hierarchical models of galaxy formation. Detailed simulations are clearly needed to assess the impact of this result. Subject heading: galaxies: general © 1990. The American Astronomical Society. All rights reserved.