Within the large scale distribution of galaxies, groups and clusters provide highly relevant landmarks. Indeed, the deep potential wells of the universe are found to be at the intersection of sheets or filaments and, thus, can be viewed as the nodes of the large scale structures. Clusters originated from the initial density fluctuations of large amplitude and the evolution of their physical properties as well as of the topology of their overall distribution strongly depend on , the type of dark matter and the primordial spectrum. On the one hand, clusters may be systematically identified in optical galaxy surveys, but corresponding galaxy overdensity with respect to the mean galaxy background is hardly detectable beyond z ~0.8. On the other hand, high galactic latitude fields observed in the X-ray band contain basically only QSOs (pointlike) and clusters (extended) and are thus ideal for detecting high redshift clusters, all the more so since projection effects are negligible.
The X-ray satellite XMM - one of ESA's cornerstones for the 2000's - will be launched at the end of 1999 and is to provide a major contribution to cosmology. It covers the 0.1-10 keV energy range with an effective area of ~6,000 cm2 at 1keV, i.e. some 7.5 times more sensitive than AXAF. The on-axis PSF FWHM is ~12'' (AXAF ~1"), and consequently can flag clusters as extended sources up to z = 1 - 2 . The energy resolution of the imaging detectors is ~0.1 keV at 1 keV. XMM is ideally suited to the dectection and study of high redshift clusters which are basically low surface brightness extended X-ray sources having temperatures between 1-10 keV.
In the context of XMM guaranteed time, we are planning to perform a medium survey covering some 20 sq. deg. The foreseen X-ray depth should enable the detection of low luminosity groups up to z = 1 and bright clusters will be visible up to z ~ 2 (if any !). Assuming no evolution in the luminosity function and physical properties of clusters, some 200-250 clusters between 0 z 1 are expected for 20 sq. deg. In addition, QSOs, which are more easily detectable than clusters and may be very X-ray luminous, should be the dominant source population (may reach a density of 200-400 per sq. deg.) and be seen much further away than clusters.
Thanks to its high quality, depth and multi-colour coverage, the MEGACAM survey will ensure the necessary complementary optical data base for the XMM medium survey. The spectroscopic follow-up is foreseen at the VLT with VIRMOS. The whole will provide the cosmology community with a unique opportunity to study:
-1- The distribution and properties of the potential wells at early epochs, when the universe was only 1/3 of its present age. X-ray evolution compared to optical evolution.
-2- The evolution of the large scale distribution of AGNs, as a function of optical/X-ray spectral signatures. This is a still open question (e.g. as to the presence of BL Lacs in galaxy clusters).
-3- The presence of "dark clusters" (dark matter + hot gas without optical counterpart).
-4- The existence and evolution of a hot diffuse medium between clusters. The temperature of such a medium is still much debated and the high XMM sensitivity is required to detect high z faint diffuse gas.
-5- Large scale weak lensing effects. The XMM survey will provide the necessary X-ray conterpart for interpreting the optical Megacam investigations of weak shear distortions on scales 100 Mpc, another fundamental and very powerful way of probing large scale mass distributions.
-6- Such a project will also yield strong constraints on the evolution of the X-ray luminosity function of various classes of objects and clues on the temperature function of clusters, which is directly connected to the value.
-7- Finally, combined with the future Planck survey in the millimeter wave band in which clusters will leave typical signatures through the Sunyaev-Zel'dovich effect, the project will yield unprecedented information on the physical properties of galaxy clusters on all scales.