Clusters of GalaxiesOne of the very first discoveries of X-ray astronomy was that clusters of galaxies are strong X-ray emitters. Their X-ray spectrum is well fitted by plasma emission at a temperature of 107 - 108 K, which includes thermal bremsstrahlung and line emission, most notably the Fe K emission line at ~6.7 keV. The inferred Fe abundance is about 0.3 - 0.5 solar, with possible gradients across the cluster, but remarkable homogeneity across the cluster population. Clusters (and groups) of galaxies are therefore filled with enriched gas (likely deposited by the mass loss of the member galaxies), which appears to be trapped in the cluster potential well.The intracluster medium appears to be close to hydrostatic equilibrium Except for the core (with a scale of a few times 100 kpc), cluster gas is isothermal (or perhaps has a slowly decaying temperature) out to the distances where X-ray emission can be detected (less than the virial radius). Relaxed clusters often exhibit a "cooling flow" phenomenon, whereby the gas in the central part of the cluster is significantly cooler and the density higher probably due to a highly subsonic inflow. [Ref 4] New observations with XMM-Newton, however, show a lack of spectral evidence for large amounts of cooling and condensing gas in the centers of galaxy clusters believed to harbour strong cooling flows: the mass deposition rates in cooling cores are not as high as previously predicted. The existing cooling flow models might need to be modified introducing an additional heating process. This could possibly be achieved via a self-regulated heating model powered by the large energy output of a central AGN. [Ref 19] |
