SIMBAD references

We investigate the color-magnitude distribution in the rich cluster AC 118 at z=0.31. The sample is selected by the photometric redshift technique, allowing us to study a wide range of properties of stellar populations, and is complete in the K band, allowing us to study these properties up to a given galaxy mass. We use galaxy templates based on population synthesis models to translate the physical properties of the stellar populations–formation epoch, timescale of star formation, and metallicity–into observed magnitudes and colors. The distributions of galaxies in color-magnitude space thus map into distributions in physical parameter space. This is achieved by means of a statistical procedure that constrains the photometric properties of AC 118 galaxies to reproduce those of a nearby rich cluster once evolved at z∼0. In this way we show that a sharp luminosity-metallicity relation is inferred without any assumption on the galaxy formation scenario (either monolithic or hierarchical). Our data exclude significant differences in formation redshift along the color-magnitude relation and therefore confirm a pure metallicity interpretation for its origin, with an early (z∼5) formation epoch for the bulk of stellar populations. The dispersion in the color-magnitude diagram implies that fainter galaxies in our sample (K∼18) ceased to form stars as late as z∼0.5, in agreement with the picture that these galaxies were recently accreted into the cluster environment. The trend with redshift of the total stellar mass shows that half of the luminous mass in AC 118 was already formed at z∼2 and also that 20% of the stars formed at z<1.