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 t̄ > 2.5 Gyr (zf > 2.75)and t̄ > 2.6Gyr(zf > 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 t̄ > 3.2 Gyr (zf > 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 10849+4452 and in its less evolved companion RX 10848+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.
All Science Journal Classification (ASJC) codes
- Astronomy and Astrophysics
- Space and Planetary Science