During the summer of 1996 I worked at the Harvard-Smithsonian Center for Astrophysics, in Cambridge, Massachusetts as part of the SAO summer intern research experience for undergraduates (REU) program. My advisors were Christine Jones and Bill Forman and my mentor was Hank Donnelly. I also worked with another one of the interns, Ken Rines, on this project. The following is a brief abstract of the project and its results.
We have used ROSAT HRI observations to study the structure 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 those without structure, 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 the nine clusters in our sample which we were able to model, possess cooling flows in their cores. The gas density distributions were used to determine the gas masses of the clusters within 1 and 3 Mpc. With temperatures determined from $ASCA$ observations (Rines, 1996), the total gravitational masses for Abell 963, CL 1358+6245, Abell 2219, Abell 115, and Abell 2390 have been computed. The gas mass fractions measured for these clusters average sim 25%. 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 Omega to be much smaller than unity, and thus, does not support the standard inflationary model for a closed universe. On the other hand, the high fraction of structure-rich clusters would support a high-density universe. Comparison with nearby clusters suggests evolution in cluster core radius, thus also supporting a high-density universe.