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

Abstract

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 TelescopeAdvanced Camera for Surveys. The samples include 105 and 140 members of RX J0152.7‐1357 and MS 1054‐03, respectively, with ACS F775W magnitude i775 < 24.0. We fit the 2-D galaxy light profiles to determine effective radii and Se rsic indices; deviations from the smooth profiles are quantified by the ratio of the rms residuals to the mean of t he galaxy model. Galaxies are then classified according to a combination of this rms/mean ratio and the Sersic index; the resulting classes correlate well with visually classified morphological types, but are less affec ted by orientation. We find the size‐surface brightness relations in the two clusters to be very similar, supporting recent results on the evolution of this relationship with redshift. We examine in detail the color‐magnitude relations in these clusters and systematic effects on the residuals with respect to these relations. The color-ma gnitude residuals correlate with the local density, as measured from both galaxy numbers and weak lensing. These correlations are strongest for the full galaxy samples (commensurate with the morphology‐density relation), but are also present at lower significance levels for the early- and late-type samples individually. Weaker correlations are found with cluster radius, resulting from the more fundamental dependence on local density. We identify a threshold surface mass density of � ≈ 0.1, in units of the critical density, above which there are rel atively 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, which produces a trend in the color residuals with velocity and may result from an infalling foreground association of late-type galaxies. Comparison of the color‐color diagrams for these clusters to stellar population models implies that a range of star formation time-scales are needed to reproduce the locus of galaxy colors. We also identify two galaxies, one in each cluster, whose colors can only be explained by large amounts, AV ∼ 1 mag, of internal dust extinction. Converting to rest-frame bandpasses, we find elliptical galaxy color scatters of 0.03 ± 0.01 mag in (U-B) and 0.07 ± 0.01 mag in (U-V), indicating mean ages of ∼ 3.5 Gyr, similar to the estimates from the mean colors and corresponding to formation at z ≈ 2.2. Thus, when the universe was half its present age, cluster ellipticals were half the age of the universe at that epoch; the same is coincidentally true of the median ages of ellipticals today. However, the most massive local cluster ellipticals have ages & 10 Gyr, consistent with our results for their likely progenitors at z & 0.8.