Stellar populations and morphology on the red sequence at z≈1

We present results from a detailed study of cluster red sequence at z≈1 from the ACS Intermediate Redshift Cluster Survey (Mei et al. 2009). Our analysis shows that the red sequence is well defined at z≈1 and elliptical and lenticular galaxies lie on similar color-magnitude relations. We analyze the parameters of the early-type color-magnitude relations -scatter, slope and zero-point - as a function of redshift, galaxy properties and cluster mass. Our results suggest that bright elliptical galaxies in cluster cores are on average older than S0 galaxies and peripheral elliptical galaxies (by about 0.5 Gyr, using a simple, single burst solar metallicity Bruzual & Charlot (2003) stellar population model). The red sequence does not show significant evolution out to redshift z≈1.3 nor significant dependence on cluster mass. The fraction of morphological early-type galaxies on the red sequence is 80% to 90% of the total red sequence population for most of our clusters. In the highest redshift, low mass cluster of our sample, early-type/late-type fractions on the red sequence are similar (≈50%), with most of the late-type population composed of galaxies classified as S0/a. This trend is not correlated with the cluster's X-ray luminosity, nor with its velocity dispersion, and could be a real evolution with redshift.