CHEMICALLY CONSISTENT EVOLUTION OF GALAXIES - BROAD-BAND COLORS AND MG-2

The evolution of optical colours UBVRI in galaxies with various star f ormation histories from galaxy formation to the present time is invest igated using a chemically consistent photometric method for the calcul ation of evolving stellar systems. Most recent complete and consistent stellar evolutionary tracks and empirical colour and Mg-2 calibration s are used. The increase with time of the initial stellar metallicitie s is taken into account in our simulation of composite stellar systems . We show that the present day colours of elliptical galaxies reflect a true metallicity distribution of stars, while the metallicity of sta rs in spiral galaxies corresponds more closely to the actual gas metal licity. We demonstrate the large difference between chemically consist ent colour evolution and colour evolution at constant (solar) metallic ity. Using recently published Mg-2 calibrations for stars we also inve stigate the evolution of the stellar Mg-2 with time and its relation t o the metallicity of composite stellar systems and tentatively point t o cosmological implications.