A Keck Spectroscopic Survey of MS 1054-03 (z = 0.83): Forming the Red Sequence

Abstract

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 σ_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 ~3.5 mag 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 (σ_Vi=0.055+/-0.008), but measurably smaller than that of the combined E+S0 sample (σ_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~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 MS 1054, 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 MS 1054 is ~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%).

Based on observations obtained at the W. M. Keck Observatory, which is operated jointly by Caltech and the University of California. Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.