A WIDE-FIELD STUDY OF THE z ∼ 0.8 CLUSTER RX J0152.7−1357: THE ROLE OF ENVIRONMENT IN THE FORMATION OF THE RED SEQUENCE

We present the first results from the largest spectroscopic survey to date of an intermediate redshift galaxy cluster, the z = 0.834 cluster RX J0152.7−1357. We use the colors of galaxies, assembled from a D ∼ 12 Mpc region centered on the cluster, to investigate the properties of the red sequence as a function of density and clustercentric radius. Our wide-field multislit survey with a low-dispersion prism in the Inamori Magellan Areal Camera and Spectrograph at the 6.5 m Baade telescope allowed us to identify 475 new members of the cluster and its surrounding large-scale structure with a redshift accuracy of σz/(1 + z) ≈ 1% and a contamination rate of ∼2% for galaxies with i′ <23.75 mag. We combine these new members with the 279 previously known spectroscopic members to give a total of 754 galaxies from which we obtain a mass-limited sample of 300 galaxies with stellar masses M>4 × 1010 M☉ (log M/M☉>10.6). We find that the red galaxy fraction is 93 ± 3% in the two merging cores of the cluster and declines to a level of 64 ± 3% at projected clustercentric radii R ≳ 3 Mpc. At these large projected distances, the correlation between clustercentric radius and local density is nonexistent. This allows an assessment of the influence of the local environment on galaxy evolution, as opposed to mechanisms that operate on cluster scales (e.g., harassment, ram-pressure stripping). Even beyond R>3 Mpc we find an increasing fraction of red galaxies with increasing local density. The red galaxy fraction at the highest local densities in two large groups at R>3 Mpc matches the red galaxy fraction found in the two cores. Strikingly, galaxies at intermediate densities at R>3 Mpc, that are not obvious members of groups, also show signs of an enhanced red galaxy fraction. Our results point to such intermediate-density regions and the groups in the outskirts of the cluster, as sites where the local environment influences the transition of galaxies onto the red sequence.