Hst Photometry and Keck Spectroscopy of the Rich Cluster Ms 1054 – 03 : Morphologies

We present a study of 81 I-band selected, spectroscopically-confirmed members of the X-ray cluster MS 1054−03 at z = 0.83. Redshifts and spectral types were determined from Keck spectroscopy. Morphologies and accurate colors were determined from a large mosaic of HST WFPC2 images in RF606W and IF814W , corresponding to U and B in the restframe. Early-type galaxies constitute only 44 % of this galaxy population. This fraction is much lower than in comparable rich clusters at low redshift. Thirty-nine percent are spiral galaxies, and 17 % are mergers. The early-type galaxies follow a tight and well-defined color-magnitude relation, with the exception of a few outliers. The observed scatter is 0.029± 0.005 magnitudes in restframe U − B. Most of the mergers lie close to the CM relation defined by the early-type galaxies. They are bluer by only 0.07± 0.02 magnitudes, and the scatter in their colors is 0.07± 0.04 magnitudes. Spiral galaxies in MS 1054−03 exhibit a large range in their colors. The bluest spiral galaxies are ∼ 0.7 magnitudes bluer than the early-type galaxies, but the majority is within ±0.2 magnitudes of the early-type galaxy sequence. The red colors of the mergers and the majority of the spiral galaxies are reflected in the fairly low Butcher-Oemler blue fraction of MS 1054−03: fB = 0.22±0.05, similar to intermediate redshift clusters and much lower than previously reported values for clusters at z ∼ 0.8. The slope and scatter of the CM relation of early-type galaxies are roughly constant with redshift, confirming previous studies that were based on ground-based color measurements and very limited membership information. However, the scatter in the combined sample of early-type galaxies and mergers (i.e., the sample of future early-type galaxies) is twice as high as the scatter of the early-type galaxies alone. This is a direct demonstration of the “progenitor bias”: high redshift early-type galaxies seem to form a homogeneous, old population because the progenitors of the youngest present-day early-type galaxies are not included in the sample. Subject headings: galaxies: evolution, galaxies: elliptical and lenticular, cD, galaxies: structure of, galaxies: clusters: individual (MS 1054−03)