Color–magnitude relations have been derived in two high redshift clusters at z \sim 1.26 .
These clusters , RX J0849+4452 and RX J0848+4453 ( with redshifts of z=1.26 and 1.27 , respectively ) lie in the highest redshift cluster superstructure known today , the Lynx supercluster .
The color-magnitude relation was determined from Advanced Camera for Surveys ( ACS ) imaging in the WFC ( Wide Field Camera ) F775W ( i _ { 775 } ) and F850LP ( z _ { 850 } ) 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 color–magnitude relation very similar in color , although the two clusters present different X–ray luminosities and shapes , with RX J0849+4452 being three times more X–ray luminous and more compact , and having a temperature two times higher .
The elliptical galaxy color–magnitude relations ( CMR ) in RX J0849+4452 and RX J0848+4453 show an intrinsic ( i _ { 775 } - z _ { 850 } ) color scatter of 0.026 \pm 0.012 mag and 0.024 \pm 0.015 mag , respectively , within 2 \arcmin ( \sim 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 \overline { t } > 2.5 Gyr ( z _ { f } > 2.75 ) and \overline { 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 \arcmin from the cluster X–ray emission centers , elliptical CMR scatters imply mean luminosity–weighted age \overline { 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 lookback 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 \sim 3 .