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
(
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.
| Clusters | Groups | |
| Temperature | 3-10 keV | 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.