MAPPING MODERATE REDSHIFT CLUSTERS

To test whether clusters of galaxies have rising mass-to-light ratios at large radii and to estimate the amplitude of the density fluctuatio n spectrum on the scale of 10h-1 Mpc, the Canadian Network for Observa tional Cosmology (CNOC) cluster collaboration is obtaining Multi-Objec t Spectrograph velocities and two colour photometry for a sample of co ngruent-to 1000 cluster galaxies and congruent-to 2000 field galaxies in a 5h-1 Mpc neighbourhood of high X-ray luminosity clusters at z con gruent-to 0.3. X-ray selection of the cluster sample picks out objects on the basis of the depth of their potential well, and is insensitive both to projection effects and to galaxy biases. The galaxy data set, with selection controlled using automated photometry, automated mask design, spectral selection modelling, and accurate velocities, will be the best available at any redshift for our tests. Measuring mass-to-l ight ratios (M/L) at large radii requires a statistical removal of fie ld galaxies projected into a redshift space of the cluster. This can b e done relatively accurately for moderate redshift clusters, using the large number of foreground and background galaxies. The CNOC sample a lso provides a sensitive test of the density fluctuation spectrum on c luster scales. The 30 or so redshifts available in each of three clust ers gives an average velocity dispersion of about 1000 km s-1 which in dicates that the sigma8 normalization parameter is in the range of 0.6 less than or similar sigma8 less than or similar 0.9.