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Clusters and groups of galaxies do not only contain galaxies but all the space between the galaxies is filled with hot gas. This gas is also often referred to as intra-cluster medium (ICM) or intra-group medium (IGM). The high temperature of the gas of 1 -10 keV is in good correspondence with the depth of the potential wells of groups and clusters (see Table 1). According to the shallower potential of groups also the temperature is lower in groups (approx 1 keV) compared to clusters (3-10 keV). The gas is almost fully ionised and has very low densities of 10-2 - 10-4 cm-3 decreasing outwards. It is optically thin. In the spectra of both, groups and clusters, lines of heavy elements have been detected corresponding to metallicities of about 0.2 to 0.4 in solar units. These metals indicate that the gas cannot be of purely primordial origin, but part of it must have been previously the inter-stellar medium (ISM) in galaxies and then it must have been transported from the galaxies into the ICM and IGM, respectively, by certain processes. The processes will be discussed in the next section.

Table 1. Properties of the gas in cluster and groups of galaxies.

  Clusters Groups

Temperature 3-10 keV approx 1 keV
Extent few Mpc 0.1-1 Mpc
Metallicity 0.2-0.4 solar 0.2 solar
X-ray luminosity 1043-45 erg/s 1042-43 erg/s

Gas with such properties emits thermal bremsstrahlung in the X-ray range (see Table 1). Therefore groups and clusters are currently studied extensively by the X-ray observatories XMM and CHANDRA. While in almost all of the clusters X-ray emission has been found, the X-ray emission from groups is somewhat harder to detect due to their lower luminosity. So far in about 50% of the nearby groups X-ray emission has been found.

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