Homogeneity of Early-Type Galaxies Across Clusters

Abstract

We studied the scatter across clusters of the color of the red sequence in a representative and large sample of clusters (more than 200) detected on the Early Data Release of the Sloan DigitalSky Survey (EDR-SDSS) in the redshift range 0.06 < z < 0.34. We found an extreme degree of homogeneity in the color of the red sequence (the intrinsic scatter is about 0.02 to 0.03 mag) 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 a ten to one hundred larger sample, representative of all clusters and groups detected on the EDR-SDSS. Hence, we claim 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 σ^z = 0.018, a few times 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.