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L. Grego (SAO), J. E. Carlstrom, E. D. Reese, G. P. Holder (U. Chicago), M. K. Joy (NASA MSFC), S. Patel (U. Alabama Huntsville), W. L. Holzapfel (UC Berkeley)
Galaxy clusters are the most massive known virialized objects in the universe, as evidenced by the large velocity dispersions (\sigmav > 1000 km/s) of their constituent galaxies and their high measured intracluster medium (ICM) temperatures (kTe > 7 keV). The intracluster medium is the dominant visible baryonic mass component; a cluster's ICM generally contributes several times more mass to the cluster than do its galaxies. It is difficult to reconcile proposed mechanisms for segregating baryonic matter from dark matter on cluster mass scales with observations and with standard cosmological and large scale structure models. Hence, the mass composition of clusters within the virialized region is expected to closely reflect the universal mass composition. The ICM mass fraction of clusters therefore provides a useful cosmological probe and also allows insight into cluster formation processes.
Here we present measurements of the cluster gas mass fraction in a sample of 18 galaxy clusters, which range in redshift from z=0.14 to z=0.83. Each cluster's gas mass fraction is derived from its measured Sunyaev-Zel'dovich (SZ) effect and its ICM temperature as derived from x-ray spectral observations. The SZ effect measurements were obtained as part of a survey of x-ray-luminous galaxy clusters for which we use interferometric techniques at centimeter wavelengths. We present the gas fraction calculation method and results, examine possible sources of systematic uncertainty, and discuss the implications of these measurements for cosmology and for the physics of galaxy clusters.
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