**AAS 197, January 2001**

*Session 107. Galaxy Clusters and Large Scale Structure II*

Display, Thursday, January 11, 2001, 9:30-4:00pm, Exhibit Hall
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## [107.09] Temperature Bias in Measurements of the Hubble Constant Using the Sunyaev-Zeldovich Effect

*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 H_{0}
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 H_{0}(SZE)/H_{0}(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
H_{0} by 11%-17%, while cluster asphericity and clumping
combine to induce scatter in H_{0} 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.

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