A Keck spectroscopic survey of MS 1054-03 (z=0.83):: Forming the red sequence

Using a magnitude-limited, spectroscopic survey of the X-ray-luminous galaxy cluster MS 1054 - 03, we isolate 153 cluster galaxies and measure MS 1054's redshift and velocity dispersion to be z = 0: 8307 +/- 0: 0004 and sigma(z) = 1156 +/- 82 km s(-1). The absorption-line, poststarburst ('' E+ A ''), and emission-line galaxies, respectively, make up 63% +/- 7%, 15% +/- 4%, and 23% +/- 4% of the cluster population. With photometry from HST ACS, we find that the absorption-line members define an exceptionally tight red sequence over a span of similar to 3.5 mag in i(775): their intrinsic scatter in (V-606 - i(775)) color is only 0: 048 +/- 0: 008, corresponding to a (U - B)(z) scatter of 0.041. Their color scatter is comparable to that of the ellipticals (sigma(Vi) = 0: 055 +/- 0: 008), but measurably smaller than that of the combined E+ S0 sample (sigma(Vi) = 0: 072 +/- 0: 010). The color scatter of MS 1054's absorption-line population is approximately twice that of the ellipticals in Coma; this difference is consistent with passive evolution where most of the absorption-line members (> 75%) formed by z similar to 2, and all of them by z similar to 1: 2. For red members, we find a trend (> 95% confidence) of weakening H delta absorption with redder colors that we conclude is due to age: in MS 1054, the color scatter on the red sequence is driven by differences in mean stellar age of up to similar to 1.5 Gyr. We also generate composite spectra and estimate that the average S0 in MS 1054 is similar to 0.5-1 Gyr younger than the average elliptical; this difference in mean stellar age is mainly due to a number of S0's that are blue (18%) and/or are poststarburst systems (21%).