We analyse the cluster color-magnitude relation ( CMR ) for early-type galaxies in two of the richer clusters in the z \sim 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 aboard the Hubble Space Telescope through the F606W and F814W filters , which brackets the rest-frame 4000 Å break at the cluster redshifts of z \sim 0.9 .
We fit the zeropoint 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 ( V _ { 606 } - I _ { 814 } ) 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 Cl1604+4304 and Cl1604+4321 early-type CMRs .
Cl1604+4321 , the less X-ray luminous and massive of the two , lacks bright L ^ { * } ellipticals .
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 Cl1604 CMR at the bright end ( L > 0.5 L ^ { * } ) 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 .