Clusters of Galaxies: An X-ray Perspective

There has been extensive recent progress in X-ray observations of clusters of galaxies with the analysis of the entire ASCA database and recent new results from Beppo-SAX, Chandra, and XMM-Newton. The temperature profiles of most clusters are isothermal from 0.05– 0.6 Rviral, contrary to theoretical expectations and early results from ASCA. Similarly, the abundance profiles of Fe are roughly constant outside the central regions. The luminositytemperature relation for a very large sample of clusters show that LX ∝ T 3 over the whole observable luminosity range at low redshift, but the variance increases at low luminosity, explaining the previously claimed steepening at low luminosity. Recent accurate cluster photometry in red and infrared passbands have resulted in much better correlations of optical and X-ray properties, but there is still larger scatter than one might expect between total light and X-ray temperature and luminosity. The velocity dispersion and the X-ray temperature are strongly correlated, but the slope of the relation is somewhat steeper than expected. The surface brightness profiles of clusters are very well fit by the isothermal β model out to large radii and show scaling relations, outside the central regions, consistent with a Λ-dominated Universe. At high masses the gas mass fraction of clusters is quite uniform and is consistent with the low WMAP value of Ωm. The recent analysis of cluster mass-to-light ratio and the mass-tolight ratio of stars indicates that the ratio of gas to stellar mass is ∼10:1 in massive clusters. There is an apparent decrease in gas mass fraction and increase in stellar mass fraction at lower mass scales, but the very flat surface brightness of the X-ray emission makes extension of this result to large scale lengths uncertain. The normalization of the scaling of mass with temperature, derived from measurements of density and temperature profiles and assuming hydrostatic equilibrium, is lower than predicted from simulations that do not include gas cooling or heating and has a slightly steeper slope. Detailed Chandra and XMM-Newton imaging spectroscopy of several clusters show that the form of the potential is consistent with the parameterization of Navarro, Frenk, & White (1997) over a factor of 100 in length scale and that there is no evidence for a dark matter core. Chandra X-ray images have revealed rather complex internal structures in the central regions of some clusters, which are probably due to the effects of mergers; however, their nature is still not completely clear. There are now more than 100 clusters with well-determined Fe abundance, several with accurate values at redshifts z ≈ 0.8, with little or no evidence for evolution in the Fe abun-