XMM-Newton XMM-LSS  

Index - ... - Simulations (2/3) - Simulations (3/3) - The Optical Follow-up - ...

4. Simulations (3/3)

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.

cascade3d.ps.gz (gzipped Postscript, 29196 bytes)

Figure 13: Numerical simulation of dense, cooling gas in the core of an X-ray cluster. The cooling instability is responsible for this fragmentation of the gas in a multiphase medium.

cascade.ps.gz (gzipped Postscript, 5850 bytes)

Figure 14: Semi-analytical model describing the temporal evolution of the temperature distribution in an isobaric, multiphase, cluster core. The initial temperature distribution is in green. As cooling proceeds, a net mass flow from the hot phase to the cool phase develops, although some hot, diffuse gas always remains in the cluster. This remaining X-ray emitting gas allows us to avoid a cooling catastrophe in the cluster core. After a few cooling times, a self-similar evolution builds in (pink curves), allowing us to describe the multiphase cooling flow using very simple analytical models (Teyssier 1996).

Index - ... - Simulations (2/3) - Simulations (3/3) - The Optical Follow-up - ...
Web Pages : Alain Detal, Oct 2001.