Homogeneity of early–type galaxies across clusters

We studied the homogeneity, across clusters, of the color of the red sequence (the intercept of the color–magnitude relation) of 158 clusters and groups detected in the Early Data Release of the Sloan Digital Sky Survey (EDR–SDSS) in the redshift range 0.06 < z < 0.34. We found a high degree of homogeneity: the color of the red sequence shows an intrinsic scatter of 0.02 mag across clusters, suggesting that either galaxies on the red sequence formed a long time ago (z > 2) or else their star formation is universally delayed with preservation of a small spread in age formation. The latter possibility is ruled out by the mere existence of galaxies at high redshift. While the old age of ellipticals was already been claimed for a small heterogeneous collection of clusters, most of which are rich ones, we found that it holds for ten to one hundred large sample, representative of all clusters and groups detected on the EDR–SDSS. Hence we suggest the possible universality of the color of the galaxies on the red sequence. Furthermore, the sample includes a large number of very poor clusters (also called groups), not studied in previous works, for which the hierarchical and monolithic scenarios of elliptical formation predict different colors for the brightest ellipticals. The observed red sequence color does not depend on cluster/group richness at a level of 0.02 mag, while a ∼ 0.23 mag effect is expected according to the hierarchical prediction. Therefore, the stellar population of red sequence galaxies is similar in clusters and groups, in spite of different halo histories. Finally, since the observed rest–frame color of the red sequence does not depend on environment and redshift, it can be used as a distance indicator, with an error σz = 0.018, a few time better than the precision achieved by other photometric redshift estimates and twice better than the precision of the Fundamental Plane for a single galaxy at the median redshift of the EDR–SDSS.