3.6. The hot IC gas
It was Limber  in 1959 the first to suggest that diffuse gas must be present among galaxies, and clusters be filled with a hot IC diffuse gas component. He argued that galaxy formation from gas cannot be 100 % efficient, and some gas must be lost from galaxies through collisions. The first detection of an X-ray source associated with a cluster of galaxies came from Byram et al. , in 1966. They detected M 87, the central giant galaxy of the Virgo cluster. In the same year, Boldt et al.  claimed detection of the Coma cluster in X-ray. It took just one year to Friedman & Byram  to show that Boldt et al.'s detection was spurious. However, Boldt et al.'s spurious result inspired Felten et al. 's correct theoretical estimate. Felten et al. estimated that a thermalized diffuse gas in the Coma cluster should have a temperature 7 × 107 K, and would therefore emit in the X-ray via thermal bremsstrahlung.
In 1971, Cavaliere et al.  suggested that many extragalactic X-ray sources are probably associated with clusters of galaxies. The same year, the extended X-ray emission from the Coma IC gas was detected, by Meekins et al. , with observations from an Aerobee 150 rocket, and, independently, by Gursky et al. , with the Uhuru satellite. Thanks to Uhuru many more clusters were X-ray detected, and as early as in 1972, Gursky et al.  suggested that
``most, if not all, rich clusters include an X-ray emission region of large size and of net luminosity 1043-1044 erg s-1''
A first indication about the nature of the diffuse cluster X-ray emission came from Solinger & Tucker  in 1972, with an early indication of a correlation between the X-ray luminosities of clusters and the velocity dispersions of their member galaxies. Such a correlation is naturally expected if the gas is thermalized, in equilibrium with the cluster gravitational potential, and the emission mechanism is thermal bremsstrahlung. This correlation was later improved by Cooke & Maccagni .
Always in 1972, Syunyaev & Zel'dovich  proposed The observation of relic radiation as a test of the nature of X-ray radiation from the clusters of galaxies. Immediately after, an over-enthusiast Parijsky  gave a start to a series of spurious detections of the Syunyaev-Zel'dovich effect. Other early controversial detections were claimed by Gull & Northover , Lake & Partridge [265, 266], Birkinshaw et al. [65, 64], all regarded with much scepticism by theorists (Gould & Rephaeli , Tarter ). White & Silk  noted that the combined X-ray and microwave observations of Abell 576 would have implied an improbable value for the Hubble constant of 1.5 km s-1 Mpc-1!
There has been an impressive observational progress in this field over the last decade. Nowadays, the rate of reliable Syunyaev-Zel'dovich detections of clusters is very high, and techniques allow Syunyaev-Zel'dovich imaging of galaxy clusters (see CARLSTROM, these proceedings).
In 1973, Lea et al.  analysed the distribution of the IC gas and showed the gas to be less centrally concentrated than galaxies. Their model of the IC gas distribution was the first of a long series [272, 197, 48], among which the -model of Cavaliere & Fusco-Femiano [98, 99] proved the most successful. Lea et al. 's result was confirmed by Bahcall , and by Gorenstein et al. , who estimated the slope of the galaxy number density profile in Coma to be twice the slope of the gas density profile. Bahcall  also showed that the peak of the diffuse X-ray emission coincides with the centre of the galaxy distribution, or with the position of the cD galaxy.
Bahcall  started a systematic comparison of optical and X-ray cluster properties. She found richer clusters to be more likely associated with X-ray sources, and cD-type clusters to have higher X-ray luminosities. On the other hand, she confirmed Kellogg et al. 's result that clusters of a given richness class span a wide range of X-ray luminosities. Later, she found a relation between the fraction of spirals in clusters and the X-ray luminosity .
Figure 16. The Coma cluster X-ray brightness distribution, according to two different reconstruction algorithms (contours and boxes). The straight line is the major axis of the galaxy luminosity distribution. From Johnson et al. (1979).
Wolff et al.  were possibly the first to record a deviation of the X-ray surface brightness distribution from spherical symmetry. They showed the X-ray emission of Perseus to be elongated along the E-W direction, like the galaxy distribution. Some years later, in 1979, Gorenstein et al. , and Johnson et al.  found a good correspondence between the shape of the X-ray emission and the galaxy distribution in Coma - see Fig. 16. The Einstein IPC observations of Jones et al.  finally revealed all the complex cluster X-ray morphologies. The close correspondence between the X-ray emission and the galaxy distribution was interpreted by Gioia et al.  as evidence for equilibrium of both the IC gas and the cluster galaxies in the cluster gravitational potential.
Figure 17. The deviation of the flux as a function of energy from the flux predicted by the best fitting single temperature continuum in the Perseus cluster. The Iron line feature is evident at around 7 keV. From Mitchell et al. (1976).
The thermal bremsstrahlung interpretation received further support by the lack of detection of hard (>20 keV) X-ray emission from Coma and Perseus by Scheepmaker et al. 's balloon-borne X-ray experiment. The thermal origin of the X-ray emission was finally demonstrated in 1976 and 1977, with the Ariel V detection of the 7 keV Iron line in Perseus and Centaurus by Mitchell et al.  and Mitchell & Culhane  (see Fig. 17), and with the analogous OSO 8 detections in Virgo, Perseus and Coma, by Serlemitsos et al. . In 1977, 30 clusters had been identified as X-ray sources, 10 of them with extended emission . Mitchell et al.  and Mushotzky et al.  produced the first relations between the X-ray temperatures and velocity dispersions of eight, and, respectively, 13 clusters. With much scatter, these relations looked however consistent with TX v2 (where TX is the X-ray temperature and v the galaxy velocity dispersion), as expected if the X-ray emission was produced by an IC gas in equilibrium with the gravitational potential traced by cluster galaxies.
In 1980, Schwartz et al. [404, 405] detected X-ray emission from poor clusters and compact groups, at temperatures consistent with the low velocity dispersions of their member galaxies. The nature of the X-ray emission from poor galaxy systems is still debated. Both the contribution of individual galaxies to the total emission and Supernova heating must be considered (see PONMAN, these proceedings).