In this paper we study the colour-magnitude relation ( CMR ) for a sample of 172 morphologically-classified elliptical and S0 cluster galaxies from the ESO Distant Cluster Survey ( EDisCS ) at 0.4 \lesssim z \lesssim 0.8 .
The intrinsic colour scatter about the CMR is very small ( \langle \sigma _ { int } \rangle = 0.076 ) in rest-frame U - V .
However , there is a small minority of faint early-type galaxies ( 7 % ) that are significantly bluer than the CMR .
We observe no significant dependence of \sigma _ { int } with redshift or cluster velocity dispersion .
Because our sample is strictly morphologically-selected , this implies that by the time cluster elliptical and S0 galaxies achieve their morphology , the vast majority have already joined the red sequence .
The only exception seems to be the very small fraction of faint blue early-types .
Assuming that the intrinsic colour scatter is due to differences in stellar population ages , we estimate the galaxy formation redshift z _ { F } of each cluster and find that z _ { F } does not depend on the cluster velocity dispersion .
However , z _ { F } increases weakly with cluster redshift within the EDisCS sample .
This trend becomes very clear when higher redshift clusters from the literature are included .
This suggests that , at any given redshift , in order to have a population of fully-formed ellipticals and S0s they needed to have formed most of their stars \simeq 2 – 4 Gyr prior to observation .
That does not mean that all early-type galaxies in all clusters formed at these high redshifts .
It means that the ones we see already having early-type morphologies also have reasonably-old stellar populations .
This is partly a manifestation of the “ progenitor bias ” , but also a consequence of the fact that the vast majority of the early-type galaxies in clusters ( in particular the massive galaxies ) were already red ( i.e. , already had old stellar populations ) by the time they achieved their morphology .
Elliptical and S0 galaxies exhibit very similar colour scatter , implying similar stellar population ages .
The scarcity of blue S0s indicates that , if they are the descendants of spirals whose star-formation has ceased , the parent galaxies were already red when they became S0s .
This suggests the red spirals found preferentially in dense environments could be the progenitors of these S0s .
We also find that fainter early-type galaxies finished forming their stars later ( i.e. , have smaller z _ { F } ) , consistent with the cluster red sequence being built over time and the brightest galaxies reaching the red sequence earlier than fainter ones .
Combining the CMR scatter analysis with the observed evolution in the CMR zero point we find that the early-type cluster galaxy population must have had their star formation truncated/stopped over an extended period \Delta t \gtrsim 1 Gyr .