An Increase in the Faint Red Galaxy Population in Massive Clusters since z ~ 0.5

We compare the luminosity functions for red galaxies lying on the rest-frame (U - V) color-magnitude sequence in a homogeneous sample of 10 X-ray-luminous clusters from the MACS survey at z ~ 0.5 to a similarly selected X-ray cluster sample at z ~ 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 ~ 0.1 sample. This comparison demonstrates that the number of faint, MV ~ -19, red galaxies relative to the bright population seen in the central regions of massive clusters has roughly doubled over the 4 Gyr between z ~ 0.5 and z ~ 0.1. We quantify this difference by measuring the dwarf-giant ratio on the red sequence, which increases by a factor of at least 2.2 ± 0.4 since z ~ 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.