Annu. Rev. Astron. Astrophys. 1994. 32: 277-318 Copyright © 1994 by Annual Reviews. All rights reserved

1.1 The Hot Intracluster Medium

Enormous quantities of diffuse hot gas are observed in clusters and groups of galaxies. The gas can only support itself close to hydrostatic equilibrium in the gravitational field of a virialized cluster if its sound speed is similar to the typical velocity of a cluster galaxy (the velocity dispersion of the cluster). This is generally in the range 300 to 1200 km s^-1, implying that the gas temperature is 10⁷-10⁸ K. The main energy loss of gas at such high temperatures is through bremsstrahlung radiation, which produces the diffuse X-radiation from clusters of galaxies and is our principal source of information on their intracluster medium (ICM). Further indirect evidence for the gas is found in "head-tail" radio sources and from theories of the propagation of double-lobed radio sources. The general properties of the ICM have been reviewed by Sarazin (1986, 1988, 1992) and Fabian (1988b).

Most of the observed intracluster gas has an electron density, n_e, in the range of 10^-4-10^-2 cm^-3 and a temperature T ~ 2 x 10⁷-10⁸ K, and is contained within a radius of 1 to 2 Mpc. ⁽¹⁾ The total mass of gas in rich clusters ranges from 5 x 10¹³-5 x 10¹⁴ M with an X-ray luminosity of ~ 10⁴³-3 x 10⁴⁵ erg s^-1. Emission lines due to highly ionized iron are observed in all clusters that are bright enough to permit detection (e.g. see Rothenflug & Arnaud 1985, Edge 1989) showing that the gas has ~ 0.3 times solar abundance in iron (the abundance appears to be higher in low temperature clusters). The work of Canizares et al (1979, 1982, 1988), Mushotzky et al (1981), and Mushotzky (1992) on cooling regions in clusters shows that Si and S are also present at abundances close to solar, and that O may exceed solar abundance.

The origin of the gas is uncertain. Being metal-enriched, it cannot all be primordial. Some of the gas must have been processed through an early population of stars before being released back into intracluster space in supernova explosions (Larson & Dinerstein 1975, Mathews 1989, M. Arnaud et al 1992). It could also have been stripped from young galaxies during the formation of the cluster. In all cases, the gas receives a similar kinetic energy per unit mass as the galaxies, which makes the sound speed similar to the galaxy velocities. This energy is ultimately gravitational in origin, and there is no need for additional heating of the gas to account for its temperature and distribution. The presence of so much gas, comparable to, or exceeding, the total mass in observable stars and 10-30% of the virial mass of the cluster, suggests that galaxy formation is no more than 50% efficient (e.g. see David et al 1990).

¹ A Hubble constant of H₀ = 50 km s^-1 Mpc^-1 is assumed here and throughout this review. Back.