X-ray emission from clusters of galaxies and cosmological parameters

Clusters of galaxies are excellent probes of cosmic structure and evolution . X-ray studies of clusters provide some of their key parameters, viz., tem perature of the hot intra-cluster gas, its metallicity, X-ray luminosity an d surface brightness giving mass distribution and mass-flow rate in the cas e of cooling flows. X-ray measurements for a large sample of clusters have lead to estimates of the total gravitating mass in them, which can be compa red to the virial masses derived from dynamical considerations and gravitat ional lensing in some of them. X-ray derived total masses are consistent wi th masses obtained from the other methods after the effects due to the pres ence of cooling flows are taken into account in the analyses. Estimated vir ial masses, lack of evolution in X-ray properties, and detection of several very hot clusters at high redshifts indicate a Universe with a low value ( less than or equal to 0.3) for the Omega parameter.