Hubble Space TelescopePhotometry and Keck Spectroscopy of the Rich Cluster MS 1054−03: Morphologies, Butcher‐Oemler Effect, and the Color‐Magnitude Relation atz= 0.83

Abstract

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 rest frame. 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 mag in rest frame 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 mag, and the scatter in their colors is 0.07 ± 0.04 mag. Spiral galaxies in MS 1054-03 exhibit a large range in their colors. The bluest spiral galaxies are ~0.7 mag bluer than the early-type galaxies, but the majority is within ±0.2 mag 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.