Spectral evolution of galaxies. I. An observational approach

In the present paper, two major new surveys are discussed, taking into account a photometric and a spectrophotometric approach. It is demonstrated that the optical color selection of galaxies is a superior technique for selecting a more homogeneous sample of distant galaxies than other methods such as radio flux density, which is rather arbitrary, or selecting rich clusters of galaxies. The comparison of phenomenological models of spectral evolution with the observed data indicates that the present epoch age for giant elliptical galaxies is much older than 8 Gyr. These models provide an adequate description of the evolution of giant elliptical galaxies with a present epoch age of about 16 Gyr and an exponentially decaying star-formation rate with an e-folding time of about 0.9 Gyr. 76 references