Age, Metallicity, and Star Formation History of Cluster Galaxies atz ∼ 0.3

We investigate the color-magnitude distribution in the rich cluster AC 118 atz=0.31. The sample is selected by the photometric redshift technique, allowingto study a wide range of properties of stellar populations, and is complete inthe K-band, allowing to study these properties up to a given galaxy mass. Weuse galaxy templates based on population synthesis models to translate thephysical properties of the stellar populations - formation epoch, time-scale ofstar formation, and metallicity - into observed magnitudes and colors. In thisway we show that a sharp luminosity-metallicity relation is inferred withoutany assumption on the galaxy formation scenario (either monolithic orhierarchical). Our data exclude significant differences in star formationhistories along the color-magnitude relation, and therefore confirm a puremetallicity interpretation for its origin, with an early (z~5) formation epochfor the bulk of stellar populations. The dispersion in the color-magnitudediagram implies that fainter galaxies in our sample (K~18) ceased to form starsas late as z~0.5, in agreement with the picture that these galaxies wererecently accreted into the cluster environment. The trend with redshift of thetotal stellar mass shows that half of the luminous mass in AC 118 was alreadyformed at $z~2, but also that 20% of the stars formed at z<1.Comment: 16 pages, 10 figures. ApJ in pres