Radio galaxies and quasars in clusters of galaxies.

The interactions of extended radio sources with the diffuse-gas intracluster medium (ICM) of clusters of galaxies (COG) and the relationship of QSOs to COGs are examined in a review of recent observational and theoretical studies. The evidence for the existence of an ICM is summarized, and the use of radio-source observations to estimate the temperature and pressure of the ICM is discussed. A dense ICM can best explain why sources with steep low-frequency spectra are often members of rich COGs. Possible effects of cluster membership on the radio luminosity function of a galaxy are analyzed. QSOs with redshifts of 0.5 or less are found to occur in groups and to be associated with galaxies but to 'avoid' the centers of rich COGs, probably because ICM effects prevent QSO formation or activity. Techniques to search for higher-redshift QSOs in COGs are suggested.