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gas clouds out of which galaxies, groups, and clusters form were
heated by the energy released during their initial gravitational
collapse. Some of the gas then cooled to form the objects
readily observed today. In the case of ordinary galaxies much of
the gas cooled rapidly, but in massive galaxies and clusters the
gas cooled more slowly and a quasi-hydrostatic atmosphere
formed. The mass of uncooled hot gas exceeds that in visible
stars in groups and clusters of galaxies. The temperature of the
atmosphere is close to the virial temperature (typically several
million K and greater) and is directly observable only in the
X-ray waveband. The hot atmosphere continues to lose energy by
the emission of radiation (principally X rays); in the central
region, where the atmosphere is naturally densest, a cooling
flow forms. This region, which grows with time, is where the
cooling rate of the gas is sufficiently high that the gas
particles lose their ener