2. IMPORTANCE OF STUDIES OF THE HOT INTRAGLUSTER MEDIUM

In rich clusters the hot intracluster medium (ICM) is the dominant, luminous component of the mass, i.e. the gas mass exceeds that in the stellar component of the galaxies in the cluster. Therefore, to understand clusters (and the galaxies in them), we must understand the origin and evolution of the gaseous component.

More specifically, studies of the ICM yield information on a rich variety of phenomena including:

• Mapping the structure and mass distribution of clusters - Since the X-ray emission emphasizes the potential wells, the X-ray images are relatively insensitive to superpositions of low mass systems. X-ray cluster surveys show that substructure in optically selected clusters is very common and, therefore, that many clusters are dynamically young.

• Role of the central galaxy - The X-ray observations of clusters have emphasized the special nature of bright centrally located galaxies (like M87 in Virgo, N4696 in Centaurus) and have suggested the presence of "cooling flows" around central galaxies.

• Interactions of gas with radio emitting plasmas - The detection of the ICM and detailed maps have permitted studies related to confinement and morphology of radio emitting plasmas (e.g., WAT and head-tail sources).

• Measurement of H₀ through the Sunyaev-Zel'dovich Effect - Combination of imaging and spectroscopy of the X-ray emitting plasma with radio measurements of the microwave decrement have provided estimates of H₀ and suggest that the next generation of X-ray observatories (e.g., ASTRO-D, BBXRT, and AXAF) will have adequate sensitivity to yield precise (10-20%) estimates of the value of the Hubble constant for clusters with redshifts up to z ~ 0.5.

• Trace large scale structure - Clusters are luminous X-ray sources and are readily identifiable to large redshift (with moderate angular resolution) because of their large physical sizes. Therefore, they can be used to trace the large scale distribution of matter.

• Study evolution of clusters with redshift - One of the dominant-factors controlling the X-ray properties of clusters (e.g., substructure and X-ray luminosity) is the gravitational collapse of the cluster system. Therefore, X-ray cluster studies at different epochs can provide information on the initial perturbations from which large scale structures form.

• Determine the properties of the X-ray emitting gas at various epochs. The X-ray emitting gas in clusters has been found to contain heavy elements which can only be produced through nucleosynthesis in stellar systems. Thus, studies of the ICM can tell us about galaxy evolution, the IMF of galaxies, the formation efficiency of galaxies in clusters (and groups), and the mass loss history of galaxies.

The above list of topics shows the rich variety of problems which can be addressed through studies of the ICM. The X-ray study of clusters has become sufficiently vast that it is no longer possible to review all of the material in a single presentation and, hence, this contribution emphasizes four topics - the dynamical evolution of clusters (and the role of the central galaxy), substructure in clusters, the origin of the ICM, and concludes with a brief review of what we can expect to learn from future X-ray astronomy missions.