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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 luminousclusters from the MACS survey at z~0.5 to a similarly selected X-ray clustersample at z~0.1. We exploit deep Hubble Space Telescope ACS imaging in theF555W and F814W passbands of the central 1.2Mpc diameter regions of the distantclusters to measure precise colors for the galaxies in these regions andstatistically correct for contamination by field galaxies using observations ofblank fields. We apply an identical analysis to ground-based photometry of thez~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 ofmassive clusters has roughly doubled over the 4 Gyrs between z~0.5 and z~0.1.We quantify this difference by measuring the dwarf to giant ratio on the redsequence which increases by a factor of at least 2.2+/- 0.4 since z~0.5. Thisis consistent with the idea that many faint, blue star-forming galaxies in highdensity environments are transforming onto the red sequence in the last half ofthe Hubble time.Comment: 8 pages, 2 figures, ApJ in pres

An Increase in the Faint Red Galaxy Population in Massive Clusters sincez∼ 0.5