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 MS1054’s redshift and velocity dispersion to be z = 0.8307± 0.0004 and σz = 1156 ± 82 km s . The absorption-line, post-starburst (“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 ∼ 3.5 magnitudes in i775: their intrinsic scatter in (V606− i775) 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 (σV i = 0.055± 0.008), but measurably smaller than that of the combined E+S0 sample (σV i = 0.072± 0.010). The color scatter of MS1054’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 ∼ 2, and all of them by z ∼ 1.2. For red members, we find a trend (> 95% confidence) of weakening Hδ absorption with redder colors that we conclude is due to age: in MS1054, the color scatter on the red sequence is driven by differences in mean stellar age of up to ∼ 1.5 Gyr. We also generate composite spectra and estimate that the average S0 in MS1054 is ∼ 0.5− 1 Gyr younger than the average elliptical; this difference in mean stellar age is mainly due to a number of S0s that are blue (18%) and/or are post-starburst systems (21%). Subject headings: galaxies: clusters: individual (MS 1054-03) – galaxies: elliptical and lenticular, cD – galaxies: fundamental parameters – galaxies: evolution