The build-up of the red sequence in the galaxy cluster MS1054−0321 at z= 0.831




Using one of the deepest data sets available, we determine that the red sequence of the massive cluster MS1054−0321 at z= 0.831 is well populated at all studied magnitudes, showing no deficit of faint (down to M*+ 3.5) red galaxies: the faint end of the colour–magnitude relation is neither empty nor underpopulated. The effect is quantified by the computation of the luminosity function (LF) of red galaxies. We found a flat slope, showing that the abundance of red galaxies is similar at faint and at intermediate magnitudes. Comparison with present-day and z∼ 0.4 LFs suggests that the slope of the LF is not changed, within the errors, between z= 0.831 and 0. Therefore, the analysis of the LF shows no evidence for a decreasing (with magnitude or redshift) number of faint red galaxies. The presence of faint red galaxies in high-redshift clusters disfavours scenarios where the evolution of red galaxies is mass-dependent, because the mass dependency should differentially depopulate the red sequence, while the MS1054−0321 colour–magnitude relation is populated as in nearby clusters and as in z∼ 0.4 clusters. The presence of abundant faint red galaxies in the high-redshift cluster MS1054−0321 restricts the room for allocating descendants of Butcher–Oemler galaxies, because they should change the faint end slope of the LF of red galaxies, while instead the same faint end slopes are observed in MS1054−0321, at z∼ 0 and at z∼ 0.4. In the rich MS1054−0321 cluster, the colour–magnitude relation seems to be fully in place at z= 0.831 and therefore red galaxies of all magnitudes were wholly assembled at higher redshift.