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Session 37 - Clusters of Galaxies - X-Rays.
Display session, Tuesday, January 14
We have used ROSAT HRI observations to study the structure and properties of clusters of galaxies at redshifts of z = 0.2 - 0.33. In our sample of thirteen clusters, seven show obvious substructure in the form of double peaked emission, elliptical iso-intensity contours, and offset centers. For a subset of nine clusters in our sample, the standard isothermal-hydrostatic model has been used to determine physical properties of the hot gas distribution of the clusters. From the measured high, central gas densities and short cooling times, we conclude that at least six of these nine clusters possess cooling flows in their cores. The gas density distributions were used to determine the gas masses of the clusters. With temperatures determined from ASCA observations (Rines, 1997), the total gravitational masses for Abell 963, CL 1358+6245, Abell 2219, Abell 115, and Abell 2390 have been computed within 1 Mpc (H_0 = 50 km s^-1 Mpc^-1. The gas mass fractions within 1 Mpc measured for these clusters average \sim25%, and range from 0.16% to 0.32%. As galaxies only contribute a small fraction of the total mass of the clusters, we can essentially take this to be the baryon fraction, f_b. Assuming light-element nucleosynthesis, the f_b for these clusters, and others (e.g. David et al. 1995), requires Ømega to be much smaller than unity, and thus does not support the standard inflationary model for a closed universe. In a comparison with a sample of nearby clusters we find that the weighted average core radius for the clusters in our sample falls below the average nearby core radius. But, scatter in our sample and a strong possibility of sample bias towards clusters with small core radii (cooling flow clusters) places doubt on an actual increase in core radius size from z = 0.2 to the present. The cosmological implications of these results will be discussed.
Program listing for Tuesday