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W. Lin (UIUC), M.L. Norman (UCSD), G.L. Bryan (MIT)
Measurements of the Hubble constant to distant galaxy clusters using the Sunyaev-Zeldovich effect (SZE) are found to be systematically low relative to values obtained by other means. These measurements must assume a model for the intracluster medium (ICM), usually taken to be the spherical, isothermal beta model. Real clusters are neither spherical nor isothermal due to their formation process and possible recent mergers. We present the results of a statistical analysis of {\em temperature bias} in H0 determinations in a sample of 27 numerically simulated x-ray clusters drawn from a \LambdaCDM model at z=0.5 (the sample is online at sca.ncsa.uiuc.edu). Adaptive mesh refinement is used to provide high resolution (15 h-1 kpc) in cluster cores which dominate the x-ray and radio signals. Gas dynamics is computed using the piecewise parabolic method which provides high fidelity in the ICM temperature and density distributions. The clusters possess a variety of shapes and merger states which are computed self-consistently in a cosmological framework assuming adiabatic gas dynamics. We compute the angular diameter distance to each cluster along three orthogonal lines of sight using the same analysis procedures used by observers. Fitting synthetic x-ray and y-parameter maps to the standard isothermal beta model, we find a broad, skewed distribution in f\equiv H0(SZE)/H0(actual) with a mean, median, and standard deviation of 0.89, 0.83, and 0.32, respectively. We find that the clusters' declining temperature profiles (see poster by Loken et al.) systematically lowers estimates of H0 by 11%-17%, while cluster asphericity and clumping combine to induce scatter in H0 by ± 30%. These results refine and generalize the findings of Roettiger, Stone and Mushotzky (1997; ApJ 482, 588) and Sulkanen (1999; ApJ 522, 59) through the use of self-consistent cosmological simulations.We discuss improved analysis procedures for reducing temperature bias.
This work was supported by NASA grant NAG5-7404 and NSF grant AST-9803137. Simulations were carried out on the SGI/CRAY Origin2000 at the NCSA.