Mapping Moderate Redshift Cluster

Abstract

To test whether clusters have rising mass to light ratios at large radii and to estimate the amplitude of the density fluctuation spectrum on the scale of 10\hmpc\ the Canadian Network for Observational Cosmology (CNOC) cluster collaboration is obtaining Multi-Object Spectrograph velocities and two colour photometry for a sample of $\simeq$1000 cluster galaxies and $\simeq$2000 field galaxies in a 5\hmpc\ neighborhood of high X-ray luminosity clusters at $z\simeq 0.3$. X-ray selection of the cluster sample picks out objects on the basis of the depth of their potential well depth, and is insensitive to both projection effects and galaxy biases. The galaxy dataset, with selection controlled using automated photometry, automated mask design, spectral selection modeling, and accurate velocities, will be the best available at any redshift for our tests. Measuring mass-to-light ratios (M/L) at large radii requires a statistical removal of field galaxies projected into the redshift space of the cluster. This can be done relatively accurately for moderate redshift clusters, using the large number of foreground and background galaxies. The CNOC sample also provides a sensitive test of the density fluctuation spectrum on cluster scales. The 30 or so redshifts available in each of three clusters gives an average velocity dispersion of about 1000 \kms which indicates that the $σ_8$ normalization parameter is in the range of $0.6\lta σ_8\lta 0.9$.