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We have investigated the optical and near-infrared colors of K'-selectedgalaxies in clusters at z ~ 1.2 near to the radio galaxy 3C 324 using imagesobtained with the Subaru telescope and archival HST data. The distribution ofcolors of the galaxies in the cluster region is found to be fairly broad, andit may imply significant scatter in their star-formation histories, althoughthe effect of contamination of field galaxies is uncertain. The red sequence ofgalaxies whose R-K colors are consistent with passive evolution models for oldgalaxies is found to be truncated at K' ~ 20 mag, and there are few faintergalaxies with similar red colors in the cluster region. We find that thebulge-dominated galaxies selected by quantitative morphological classificationform a broad sequence in the color-magnitude diagram, whose slope is muchsteeper than that expected from metallicity variations within a passivelyevolving coeval galaxy population. We argue that the observed color-magnitudesequence can be explained by metallicity and age variations, and the faintergalaxies with K' > 20 mag may be 1-2 Gyr younger than the brighter galaxies.Some spatial segregation of the color and K'-band luminosity is seen in the skydistribution; the redder and the brighter objects tend to be located near 3C324.Comment: 16 pages, 10 figures, accepted for publication in PAS

Color–Magnitude Sequence in the Clusters atz∼ 1.2 near the Radio Galaxy 3C 324