We compare the luminosity functions for red galaxies lying on the restframe ( U - V ) color-magnitude sequence in a homogeneous sample of ten X-ray luminous clusters from the MACS survey at z \sim 0.5 to a similarly selected X-ray cluster sample at z \sim 0.1 .
We exploit deep Hubble Space Telescope ACS imaging in the F555W and F814W passbands of the central 1.2-Mpc diameter regions of the distant clusters to measure precise colors for the galaxies in these regions and statistically correct for contamination by field galaxies using observations of blank fields .
We apply an identical analysis to ground-based photometry of the z \sim 0.1 sample .
This comparison demonstrates that the number of faint , M _ { V } \sim - 19 , red galaxies relative to the bright population seen in the central regions of massive clusters has roughly doubled over the 4 Gyrs between z \sim 0.5 and z \sim 0.1 .
We quantify this difference by measuring the dwarf to giant ratio on the red sequence which increases by a factor of at least 2.2 \pm 0.4 since z \sim 0.5 .
This is consistent with the idea that many faint , blue star-forming galaxies in high density environments are transforming onto the red sequence in the last half of the Hubble time .