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Internal Dynamics of Galaxy Clusters in the Red-Sequence Cluster Survey

Kris Blindert

Doctor of Philosophy 2006
Graduate Department of Astronomy and Astrophysics, University of Toronto

We present a spectroscopic survey of clusters of galaxies, designed to explore the distribution of matter, and its relation to light, in these massive systems. Our sample is drawn from the Red-sequence Cluster Survey (RCS), and is composed of 33 clusters with a wide range of optical richness and a moderate range of redshift. We obtained spectra of 56 to 529 objects per cluster, which yielded over 3500 redshifts of which 1014 are cluster members. To account for survey incompleteness due to sparse sampling, we constructed several empirical corrections so that our redshift catalogue is suitable for use in dynamical analysis.

We compute the global dynamical properties of the clusters in our survey, such as velocity dispersion and mass, and use them to explore the properties of RCS clusters as functions of total cluster mass. In particular, we first examine the optical richness as an inexpensive proxy for mass. The calibration of this relation is an important ingredient in the construction of the cluster mass function, which is used to constrain cosmological parameters. Our relation and scatter are consistent with self-calibrated cosmological parameter results. We also study the cluster mass-to-light ratios, and how they scale with mass. We compare our results to other observations and to structure formation models. Although the scatter in this relation is large, we find agreement among remarkably different types of galaxy clusters and groups; however, current structure formation models do not accurately reproduce the variation of M/L with mass.

To boost statistics and allow us to perform a more detailed dynamical analysis of clusters, we stack our data into ``ensemble'' clusters. Cosmological simulations predict a roughly universal density profile for dark matter haloes over many scales, and a relation between halo mass and the concentration of the profile. We analyse the ensembles using the Jeans equations, and recover density profiles for several ensemble clusters of different masses. Although the sampling of our data at small radii is insufficient to constrain some quantities of interest such as the inner core slope of the density profile, we can constrain the concentrations and orbital anisotropies for the ensemble clusters. We find they agree with predictions and previous observations: namely, that galaxies in clusters follow roughly isotropic orbits, and the concentrations of cluster density profiles are consistent with the prediction one would obtain from their masses. We also estimate the variation of concentration with redshift, finding a slightly stronger evolution than expected.

Reproduced with permission. library@astro.utoronto.ca
June 28 2006