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 R _ { F 606 W } and I _ { F 814 W } , 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 \pm 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 \pm 0.02 magnitudes , and the scatter in their colors is 0.07 \pm 0.04 magnitudes .
Spiral galaxies in MS 1054 - 03 exhibit a large range in their colors .
The bluest spiral galaxies are \sim 0.7 magnitudes bluer than the early-type galaxies , but the majority is within \pm 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 : f _ { B } = 0.22 \pm 0.05 , similar to intermediate redshift clusters and much lower than previously reported values for clusters at z \sim 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 .