ROSAT observations of clusters of galaxies.

Observations of four rich clusters of galaxies made using the Position Sensitive Proportional Counter (PSPC) instrument onboard the German Rontgensatellit (ROSAT) observatory were analysed. These data were complemented by ROSAT Wide Field Camera (WFC), Ginga and optical Automated Plate Measuring Machine (APM) information. Cluster properties, including the temperature profile, gas mass and total gravitational mass were investigated. Estimates for the mass deposition rates of central cooling flows, where appropriate, were made. Substructure was investigated using isophotal analysis and the employment of hardness ratios. The presence of central, intrinsic absorption, discovered previously in some similar clusters was confirmed. This is probably due to cold gas clouds. In the case of A2199, the WFC/PSPC data provided useful constraints to the partial covering fraction of these putative gas clouds. Abell 2142, which is classified in the optical as a binary cluster was shown to have a complex substructure. It is highly elliptical out to ~ 2 Mpc which can be explained in terms of a merger between two subclusters of differing sizes. The temperature and luminosities measured for A2218 and A1061, both of which have high velocity dispersions, indicate that these dispersion measurements are overestimated. Substructure is the most plausible explanation; spatial analysis supports this view. The galaxy density profiles obtained from the APM data were found to be consistent with the King approximation, and their luminosity functions were in reasonable agreement with the 'universal' Schechter model with a characteristic absolute magnitude of Bj = -21.6. Finally, the x-ray data showed that the gas mass fraction continues to rise with radius at the limit of detectability of ~ 2 Mpc. Including the optical data, the baryonic fraction at this radius is = 0.3. This result is difficult to reconcile with the universal fraction determined from standard cosmological nucleosynthesis models.