The color-magnitude relation has been determined for the RDCS J0910+5422 cluster of galaxies at redshift z = 1.106 .
Cluster members were selected from HST Advanced Camera for Surveys ( ACS ) images , combined with ground–based near–IR imaging and optical spectroscopy .
Postman et al .
( 2005 ) morphological classifications were used to identify the early-type galaxies .

The observed early–type color–magnitude relation ( CMR ) in ( i _ { 775 } - z _ { 850 } ) versus z _ { 850 } shows an intrinsic scatter in color of 0.060 \pm 0.009 mag , within 1 \arcmin from the cluster X–ray emission center .
Both the ellipticals and the S0s show small scatter about the CMR of 0.042 \pm 0.010 mag and 0.044 \pm 0.020 mag , respectively .
From the scatter about the CMR , a mean luminosity–weighted age \overline { t } > 3.3 Gyr ( z _ { f } > 3 ) is derived for the elliptical galaxies , assuming a simple stellar population modeling ( single burst solar metallicity ) .
This is consistent with a previous study of the cluster RDCS 1252.9-292 at z { = } 1.24 ( Blakeslee et al .
) .

Strikingly , the S0 galaxies in RDCS J0910+5422 are systematically bluer in ( i _ { 775 } { - } z _ { 850 } ) by 0.07 \pm 0.02 mag , with respect to the ellipticals .
The blue S0s are predominantly elongated in shape ; the distribution of their ellipticities is inconsistent with a population of axisymmetric disk galaxies viewed at random orientations , suggesting either that they are intrinsically prolate or there is some orientation bias in the S0 classification .
The ellipticity distribution as a function of color indicates that the face-on S0s in this particular cluster have likely been classified as elliptical .
Thus , if anything , the offset in color between the elliptical and S0 populations may be even more significant .

The color offset between S0 and E corresponds to an age difference of \approx 1 Gyr , for a single-burst solar metallicity model .
Alternatively , it could be the result of a different star formation history ; a solar metallicity model with an exponential decay in star formation will reproduce the offset for an age of 3.5 Gyr , i.e .
the S0s have evolved gradually from star forming progenitors .
The color offset could also be reproduced by a factor of \sim 2 difference in metallicity , but the two populations would each need to have very small scatter in metallicity to reproduce the small scatter in color .

The early–type population in this cluster appears to be still forming .
The blue early-type disk galaxies in RDCS J0910+5422 likely represent the direct progenitors of the more evolved S0s that follow the same red sequence as ellipticals in other clusters .

Thirteen red galaxy pairs are observed and the galaxies associated in pairs constitute \sim 40 % of the CMR galaxies in this cluster .
This finding is consistent with the conclusions of van Dokkum and Tran et al .
that most of the early–type galaxies grew from passive red mergers .