Intergalactic matter

The review begins with a brief discussion of the state of our own interstellar medium and follows with a detailed study of the diffuse medium in clusters of galaxies. The distribution of the intracluster medium and the role of cooling in the cores of many clusters are given special attention. The similar properties of the hot gas in massive early-type galaxies are also discussed. Clusters and massive early-type galaxies contain the best-studied bodies of diffuse gas known. Observations made over a wide range of wavelengths, from the radio to X-ray bands, show that the intracluster medium is complex and multiphase, consisting mainly of hot diffuse gas, but with cold embedded clouds in the cluster core. The possible properties of a diffuse intergalactic medium are then reviewed, including the sources and the allowed degree of clumping. The most crucial observations are the absence of any measurable distortion to the microwave background spectrum and the lack of an absorption trough shortward of the wavelength of Lyman alpha in the spectra of quasars. There is as yet no clear evidence for any diffuse intergalactic medium, although it is widely predicted to exist and could be the dominant baryonic component in the Universe. Finally, the intergalactic absorption-line clouds, both those forming the 'Lyman alpha forest' in the spectra of distant quasars and the metal-rich systems, are discussed in detail.