The Formation Epoch of Early-Type Galaxies in the z ~ 0.9 Cl 1604 Supercluster

We analyze the cluster color-magnitude relation (CMR) for early-type galaxies in two of the richer clusters in the z ~ 0.9 supercluster system to derive average ages and formation redshifts for the early-type galaxy population. Both clusters were observed with the Advanced Camera for Surveys on board the Hubble Space Telescope through the F606W and F814W filters, which brackets the rest-frame 4000 Å break at the cluster redshifts of z ~ 0.9. We fit the zero point and slope of the red cluster sequence and model the scatter about this relation to estimate average galaxy ages and formation redshifts. We find intrinsic scatters of 0.038-0.053 mag in (V₆₀₆-I₈₁₄) for the E and E+S0 populations, corresponding to average ages of 3.5-3.7 Gyr and formation redshifts z_f = 2.4-2.6. We find at least one significant difference between the Cl 1604+4304 and Cl 1604+4321 early-type CMRs. Cl 1604+4321, the less X-ray-luminous and massive of the two, lacks bright L* elliptical galaxies. We combine the galaxy samples to fit a composite CMR down to 0.15L* and find that the slope of the combined cluster CMR is significantly steeper than for RX J0152.7-1357 but consistent with MS 1054-03, both at similar redshift. The slope of the Cl 1604 CMR at the bright end (L > 0.5L*) is flatter and consistent with the CMR slopes found for other high-redshift clusters. We find evidence for increasing scatter with increasing magnitude along the early-type CMR, consistent with a "downsizing" scenario, indicating younger mean ages with decreasing galaxy mass.