Infall, the Butcher‐Oemler Effect, and the Descendants of Blue Cluster Galaxies atz ∼ 0.6

Abstract

Using wide-field Hubble Space Telescope WFPC2 imaging and extensive Keck LRIS spectroscopy, we present a detailed study of the galaxy populations in MS 2053-04, a massive, X-ray-luminous cluster at z = 0.5866 ± 0.0011. Analysis of 149 confirmed cluster members shows that MS 2053 is composed of two structures that are gravitationally bound to each other; their respective velocity dispersions are 865 ± 71 km s-1 (113 members) and 282 ± 51 km s-1 (36 members). MS 2053's total dynamical mass is 1.2 × 1015 M☉. MS 2053 is a classic Butcher-Oemler cluster with a high fraction of blue members (24% ± 5%) and an even higher fraction of star-forming members (44% ± 7%), as determined from their [O II] λ3727 emission. The number fraction of blue/star-forming galaxies is much higher in the infalling structure than in the main cluster. This result is the most direct evidence to date that the Butcher-Oemler effect is linked to galaxy infall. In terms of their colors, luminosities, estimated internal velocity dispersions, and [O II] λ3727 equivalent widths, the infalling galaxies are indistinguishable from the field population. MS 2053's deficit of S0 galaxies combined with its overabundance of blue spirals implies that many of these late-type galaxies will evolve into S0 members. The properties of the blue cluster members in both the main cluster and infalling structure indicate that they will evolve into low-mass, L 3.