2006ApJ...644..759M

Color-magnitude relations (CMRs) have been derived in two high-redshift clusters, RX J0849+4452 and RX J0848+4453 (with redshifts of z=1.26 and 1.27, respectively), that lie in the highest redshift cluster superstructure known today, the Lynx Supercluster. The CMR was determined from ACS imaging in the WFC F775W (i₇₇₅) and F850LP (z₈₅₀) filters combined with ground-based spectroscopy. Early-type cluster candidates have been identified according to the Postman et al. morphological classification. In both clusters the bright red early-type population defines a tight CMR very similar in color, although the two clusters present different X-ray luminosities and shapes. The elliptical galaxy CMRs in RX J0849+4452 and RX J0848+4453 show an intrinsic (i₇₇₅-z₈₅₀) color scatter of 0.026±0.012 and 0.024±0.015 mag, respectively, within 2' (∼1 Mpc at z=1.26) from the cluster X-ray emission centers. Simple modeling of the scatters using stellar population models from Bruzual and Charlot gives a mean luminosity-weighted age t>2.5 Gyr (z_f>2.75) and t>2.6 Gyr (z_f>2.8) for ellipticals in RX J0849+4452 and RX J0848+4453, respectively. S0 galaxies follow the elliptical CMR; they show larger scatters about the CMR. The intrinsic scatter decreases and the CMR slopes are steeper at smaller radii, within both clusters. Within 1' from the cluster X-ray emission centers, elliptical CMR scatters imply a mean luminosity-weighted age t>3.2 Gyr (z_f>3.7). We conclude that old stellar populations in cluster elliptical galaxies are already in place at z=1.26, both in the more evolved cluster RX J0849+4452 and in its less evolved companion RX J0848+4453. Even at a look-back time of 9 Gyr, in the early merging and buildup of massive clusters, the bulk of the stellar content of the bright elliptical galaxy population was in place–apparently formed some 2.5 Gyr earlier at z∼3.