The Color-Magnitude Relation of Early-Type Galaxies: A Tool for Cluster Finding and Redshift Determination

The color-magnitude relation (CMR) of early-type galaxies in clusters provides a powerful tool for finding clusters and determining photometric redshifts for clusters using two-filter imaging data. We demonstrate the high accuracy of photometric redshifts attainable using the CMR by applying the technique to a sample of 45 Abell clusters with known redshifts. Furthermore, by using the red sequence of galaxies from the CMR, we have developed an extremely efficient technique for detecting clusters and groups of galaxies. We demonstrate this using both observed photometric catalogs with redshifts (from the CNOC2 Survey) and simulations.