Clusters of galaxies are the largest bound structures of the universe. They range from rich groups (with masses above some 1013 solar masses) to large superclusters (with masses of more than 1015 solar masses).
Most of the baryonic matter in galaxy clusters is in the form of a hot ionized gas, of typical temperature few keV. Photons of the Cosmic Microwave Background interact by inverse Compton on free electrons of the intracluster gas. In this interaction, they gain energy from the electron, which induces a distortion of the blackbody spectrum of the primordial CMB. This effect, the thermal Sunyaev-Zel'dovich (or thermal SZ) effect, can be observed in the millimeter wavelength range. The Planck mission has already detected hundreds of galaxy clusters in this way, and is expected to detect one or two thousand more.
Most of the galaxy clusters have angular sizes of order 1 arcminute. The Planck instruments can resolve only a limited number of clusters. Detailed observations from the ground using large antennas such as the 30-meter IRAM telescope, however, make it possible to investigate the structure of the hot intracluster gaz. Such observations are complementary to the all-sky survey possible with the Planck space mission.
By stacking WMAP observations in the direction of known ROSAT clusters, we have been able to derive scaling laws that connect the SZ flux Y to the X-ray luminosity and mass of clusters in different redshift bins. This work is described in this paper.
Cluster number counts as a function of redshift provide strong constraints of cosmological parameters. A sensitive space mission such as Planck, with its detection of bright and high redshift over a very large fraction of sky, is expected to enable us to improve dramatically the constraints on cosmological parameters obtained with galaxy clusters. This is the subject of the thesis work of Matthieu Roman. The following figure (prepared by Matthieu) shows the improvement on cosmological parameter constraints expected with the Planck cluster survey (this work), as compared to previous constraints from present CMB observations and SPT clusters.