GALAXY CLUSTERS IN FORMATION: DETERMINING THE AGE OF THE RED-SEQUENCE IN OPTICAL AND X-RAY CLUSTERS AT z ∼ 1 WITH HST 1

Using deep two-band imaging from the Hubble Space Telescope, we measure the color-magnitude relations (CMR) of E/S0 galaxies in a set of 9 optically-selected clusters principally from the RedSequence Cluster Survey (RCS) at 0.9 < z < 1.23. We find that the mean scatter in the CMR in the observed frame of this set of clusters is 0.049±0.008, as compared to 0.031±0.007in a similarly imaged and identically analyzed X-ray sample at similar redshifts. Single-burst stellar population models of the CMR scatter suggest that the E/S0 population in these RCS clusters truncated their star-formation at zl ≃ 1.6, some 0.9 Gyrs later than their X-ray E/S0 counterparts which were truncated at zl ≃ 2.1. The notion that this is a manifestation of the differing evolutionary states of the two populations of cluster galaxies is supported by comparison of the fraction of bulge-dominated galaxies found in the two samples which shows that optically-selected clusters contain a smaller fraction of E/S0 galaxies at the their cores. Subject headings: