Galaxy clusters are the largest gravitational bound systems. They contain dark matter (~80% of total mass) and baryonic matter in the form of a hot ionised gas (ICM) and galaxies. Their study if of primary importance to understand the formation of cosmic structure and to probe the cosmological model. The dominant baryonic component (~15% of total mass), the ICM, can be observed in the X-rays or through the inverse Compton effect of CMB photons off free electrons, i.e. Sunyaev-Zeldovich (SZ) effect. Since the first blind detection of clusters through the SZ effect, this domain has witnessed a revolution.

With our project, ISSI will provide the unique possibility to gather for the first time an international team of scientists, in four countries in Europe and US, deeply involved in two major SZ surveys namely the Atacama Cosmology Telescope (ACT) and the Planck satellite. We will work together on a detailed investigation of the SZ signal in the direction of 30 massive clusters from redshift 0.1 to 0.87 common to both surveys. We will combine for the first time high resolution SZ data from ACT to lower resolution data from Planck in order to derive the first SZ-based reference model of the pressure distribution in clusters. We will investigate the effects of the pressure profile, on cosmological analyses based either on SZ cluster counts and/or on SZ power spectrum.

Our work will allow us to achieve a harmonized view and a unified understanding of the SZ signal. It will improve the quality of SZ detection methods, the robustness of the SZ photometry and its inter-calibration across experiments, and most importantly, it will enable us to understand the intrinsic limitations of the datasets and optimise their combination for an optimal use of SZ clusters in cosmology and cluster physic studies.

The SZ community has matured and high quality data are available. SZ-selected clusters are successfully used to probe either the gravitational collapse or the cosmological models, limited only by astrophysical systematic effects. Our research programme is timely. Through the ISSI project, our scientific results and product deliveries to the community will gain a high-level visibility. They will (i) lead to a greatly improved understanding of the SZ view of galaxy clusters, (ii) allow the community to improve cosmological constraints by reducing the amount of modeling uncertainties and (iii) help in defining the prospects for future CMB space or ground-based missions in terms of SZ science.