Spectroscopic characterization of galaxy clusters in RCS-1: Spectroscopic confirmation, redshift accuracy, and dynamical mass-richness relation

We present follow-up spectroscopic observations of galaxy clusters from the first Red-sequence Cluster Survey (RCS-1). This work focuses on two samples, a lower redshift sample of ~30 clusters ranging in redshift from z~0.2-0.6 observedwith multiobject spectroscopy (MOS) on 4-6.5-m class telescopes and a z ~ 1 sample of ~10 clusters 8-m class telescope observations. We examine the detection efficiency and redshift accuracy of the now widely used redsequence technique for selecting clusters via overdensities of red-sequence galaxies. Using both these data and extended samples including previously published RCS-1 spectroscopy and spectroscopic redshifts from SDSS, we find that the red-sequence redshift using simple twofilter cluster photometric redshifts is accurate to σ_z ≈ 0.035(1 + z) in RCS-1. This accuracy can potentially be improved with better survey photometric calibration. For the lower redshift sample, ~5 per cent of clusters show some (minor) contamination from secondary systems with the same red-sequence intruding into the measurement aperture of the original cluster. At z ~ 1, the rate rises to ~20 per cent. Approximately ten per cent of projections are expected to be serious, where the two components contribute significant numbers of their red-sequence galaxies to another cluster. Finally, we present a preliminary study of the mass-richness calibration using velocity dispersions to probe the dynamical masses of the clusters. We find a relation broadly consistent with that seen in the local universe from the WINGS sample at z ~ 0.05.