X-rays from Clusters of Galaxies

The X-ray emission from clusters of galaxies enables them to be used as good cosmological probes and as an example for massive galaxy formation. The gas mass fraction in clusters should be a universal standard which by means of Chandra observations enables Omega_m to be determined to better than 15 per cent accuracy. Future observations of its apparent variation with redshift will enable Omega_Lambda to be measured. The interplay of radiative cooling and heating in cluster cores may reveal the dominant processes acting during the formation of the baryonic part of massive galaxies.