4.3 Towards new physics in the XMM group sample
The XMM survey will give with unprecedented quality, a sample of groups up to a redshift of 0.5. We will be able to study statistical properties of these groups such as the luminosity function, the redshift distribution, the gas mass fraction and others. Since groups are less well understood than large clusters of galaxies, the XMM sample will give us new insight into the physics at work in this important class of objects.
It is believed that cooling processes are not relevant for explaining the bulk properties of large X-ray clusters. On the contrary, due to their smaller mass and X-ray temperature, groups are likely to be dominated by cooling processes. A very powerful formalism has been introduced a few years ago (Nulsen 1987) that allows a detailed treatment of cooling in a multiphase medium. This so-called "multiphase cooling" allows one to describe the thermodynamic structure of groups or clusters, and to compute the mass deposition rate due to condensation of cooling clouds (Fig 13 & 14). The remaining hot and diffuse gas has an X-ray luminosity strongly reduced compared to the equivalent "adiabatic" cluster or group. The X-ray mass fraction is also strongly influenced by this multiphase evolution (Chièze, Alimi & Tyessier 1998). The XMM sample can give us new and powerful constraints on this theoretical description of groups and clusters.
Other physical processes are likely to be relevant for groups of galaxies. Feedback from galaxies, such as supernovae driven galactic winds, should alter the energy budget of groups and small clusters, and modify their luminosity. It is also believed that the intergalactic medium has a high initial entropy, due to different pre-heating processes prior to the group formation epoch (UV radiation field, galactic winds, etc.). This initial entropy level should be easy to detect within groups, since their rather small masses make them sensitive to their initial adiabats. Here again, the luminosity of the groups are directly affected by this initial entropy level, and the XMM group sample will give strong constraints on it.