Clusters at Half Hubble Time: Galaxy Structure and Colors in RX J0152.7–1357 and MS 1054–03^*

We study the photometric and structural properties of spectroscopically confirmed members in the two massive X-ray-selected z ≈ 0.83 galaxy clusters MS 1054-03 and RX J0152.7-1357 using three-band mosaic imaging with the Hubble Space Telescope Advanced Camera for Surveys. The samples include 105 and 140 members of RX J0152.7-1357 and MS 1054-03, respectively, to i₇₇₅ < 24.0. A quantitative classification scheme using Sérsic indices and "bumpiness" (the ratio of the rms residuals to the model mean) was found to correlate well with visual morphological type. The color-magnitude residuals correlate well with the local density, as measured from both galaxy numbers and weak lensing. Weaker correlations are found with cluster radius (the fundamental dependence is on local density). We identify a threshold surface mass density of Σ ≈ 0.1, in units of the critical density, above which there are relatively few blue (star-forming) galaxies. In RX J0152.7-1357, there is an offset of 0.006 ± 0.002 in the mean redshifts of the early- and late-type galaxies, possibly from an infalling foreground association of late-type galaxies. A range of star-formation time-scales are needed to reproduce the galaxy loci in the color-color diagrams. We find rest-frame scatters for the ellipticals of 0.03 ± 0.01 mag in U - B and 0.07 ± 0.01 mag in U - V. The mean colors and their scatters imply ages ~3.5 Gyr, or formation at z ≈ 2.2. Thus, when the universe was half its present age, cluster ellipticals were about half the age of the universe; the same is coincidentally true of the median ages of ellipticals today. However, the most massive local ellipticals have ages ≳10 Gyr, consistent with our results for their likely progenitors at z ≳ 0.8.