Spectroscopy of Moderately High Redshift RCS-1 Clusters

We present spectroscopic observations of 11 moderately high-redshift (z~0.7-1.0) clusters from the first Red-Sequence Cluster Survey (RCS-1). We confirm that at least 10 of the 11 systems represent genuine overdensities in redshift space and show that for the remaining system, the spectroscopy was not deep enough to confirm a cluster. This is in good agreement with the estimated false positive rate of <5% at these redshifts from simulations. We find excellent agreement between the red-sequence-estimated redshift and the spectroscopic redshift, with a scatter of 10% at z>0.7. At the high-redshift end (z>~0.9) of the sample, we find that two of the systems selected are projections of pairs of comparably rich systems, with red sequences too close to discriminate in (R-z^') color. In one of these systems, the two components are close enough to be physically associated. For a subsample of clusters with sufficient spectroscopic members, we examine the correlation between B_gcR (optical richness) and the dynamical mass inferred from the velocity dispersion. We find these measurements to be compatible, within the relatively large uncertainties, with the correlation established at lower redshift for the X-ray-selected Canadian Network for Observational Cosmology clusters and also for a lower redshift sample of RCS-1 clusters. Confirmation of this and calibration of the scatter in the relation will require larger samples of clusters at these and higher redshifts.

This paper includes data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile.

This work is based on observations obtained at the Gemini Observatory, which is operated by the Association of Universities for Research in Astronomy (AURA), Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (United States), the Particle Physics and Astronomy Research Council (United Kingdom), the National Research Council (Canada), CONICYT (Chile), the Australian Research Council (Australia), CNPq (Brazil), and CONICET (Argentina).