Clusters of galaxies are the largest gravitationally bound systems in
the Universe. They appear at optical wavelengths as over-densities of
galaxies with respect to the field average density. In addition to
the galaxies, they contain an intracluster medium (ICM) of hot
(T 
108 K),
low-density (ne
10-3
cm-3)
gas, detected through its luminous X-ray emission (LX
1043 -
1045 erg s-1), produced by thermal bremsstrahlung
radiation.
The visible galaxies and the ICM are important components of clusters, however most of the cluster mass is in dark matter. Although dark matter has not been directly observed at any wavelength and its nature remains unknown, X-ray and visible light observations provide clues to its amount and distribution in clusters.
X-ray images, starting with the Einstein satellite and
continuing with ROSAT and ASCA, and now with Chandra
and XMM-Newton, provide a powerful technique to trace the global
cluster gravitational potential and to probe the dynamics, morphology
and history of clusters. In the hierarchical scenario of the
structures formation, clusters of galaxies are formed by the
gravitational merger of smaller units e.g. groups and
sub-clusters. Such mergers are spectacular events involving kinetic
energies as large as
1064
ergs. In these mergers a large
portion of energy is dissipated in the ICM, generating shock,
turbulence and bulk motions, and heating it.
Substructure in the X-ray images as well as complex gas temperature
gradients are all signatures of cluster mergers.
A significant fraction of clusters of galaxies shows the X-ray surface
brightness strongly peaked at the center. This implies a high density,
and cooling times of the hot ICM within the inner
100 kpc of
much less than the Hubble time. To maintain hydrostatic equilibrium,
an inward flow may be required. X-ray observations with
XMM-Newton indicate no spectral evidence for large amounts of
cooling
and condensing gas in the centers of galaxy clusters believed to
harbour strong cooling flows. The cooling flow seems to be hindered
by some mechanism, whose nature is still debated. Thus, there is no
consensus on the actual existence of material "cooling" and
"flowing". What is generally agreed upon is that cooling core
clusters are more dynamically relaxed than non cooling core clusters,
which often show evidence of cluster merger.
One of the most important results obtained with the Chandra satellite on clusters of galaxies was the discovery of sharp surface brightness discontinuities in the images of merging clusters, called "cold fronts". Initially, one might have suspected these features to be merger shocks but spectral measurements showed that these are a new kind of structure. These cold fronts are apparently contact discontinuities between the gas which was in the cool core of one of the merging sub-clusters and the surrounding intracluster gas. Cold fronts and merger shocks offer unique insights into the cluster physics, including the determination of the gas bulk velocity, its acceleration, the growth of plasma instabilities, the strength and structure of magnetic fields and the thermal conductivity.
A precise physical description of the ICM necessitates also adequate
knowledge of the role of non-thermal components. The most detailed
evidence for these phenomena comes from the radio observations. A
number of clusters of galaxies is known to contain wide diffuse
synchrotron sources (radio halos, relics and mini-halos) which have no
obvious connection with the cluster galaxies, but are rather
associated with the ICM. The synchrotron emission of such sources
requires a population of
GeV relativistic
electrons and cluster magnetic fields on µG levels. An
indirect evidence of the
existence of cluster magnetic fields is also derived from studies of
the Rotation Measure of radio galaxies located within or behind
clusters of galaxies.
A probe of the existence of a population of relativistic electrons in the ICM is also obtained from the detection of non-thermal emission of inverse Compton origin in the hard X-ray and possibly in the extreme ultraviolet wavelengths. The combination of the observed diffuse radio and hard X-ray emissions from clusters of galaxies is used to estimate the intracluster magnetic field strengths.