What is X-COP?

Because of their deep gravitational well, galaxy clusters are expected to retain all the material accreted since the formation epoch. The intracluster medium (ICM) contains the majority (~ 90%) of their baryonic mass, the rest being in the form of stars. The ICM is visible in the X-ray range through thermal bremsstrahlung and line emission, and at millimeter wavelengths through the distortion of the cosmic microwave background (CMB) induced by inverse Compton scattering with the hot ICM electrons (the Sunyaev-Zeldovich effect, SZ). Since they trace the main baryonic component, X-ray and SZ observations are crucial to advance our understanding of clusters and filaments.

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In the local Universe, structure formation is expected to be still ongoing through the accretion of galaxies and groups of galaxies onto massive clusters. These processes should be observable in the outer regions of local clusters, allowing us to study the growth of structures in the Universe. Additionally, cluster outskirts host the transition region between the virialized, self-gravitating ICM and the material infalling from the surroundings. As such, they are key to understanding the processes responsible for heating the gas to its present temperature.

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X-COP is a Very Large Program (VLP) on ESA’s cornerstone X-ray observatory, XMM-Newton (PI: Eckert). The aim of the project is to exploit jointly X-ray and millimeter observations to track the state of the hot baryons in the close environment of galaxy clusters, in combination with SZ data from ESA’s Planck cosmology mission. The project will address the following scientific questions:

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• Measure the universal radial profiles of the thermodynamic properties of the ICM out to the virial radius;

• Estimate the level of non-thermal pressure support and quantify deviations from hydrostatic equilibrium;

• Assess the level of inhomogeneities in the gas distribution (clumping) as a function of radius;

• Detect infalling galaxies and galaxy groups to study the stripping and virialization of the infalling gas;

• Assess the occurrence and mass distribution of infalling structures to constrain the growth rate of galaxy clusters;

• Search for asymmetries in the gas distribution to detect the gas associated with filaments connected to massive clusters.

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The X-COP collaboration involves about 20 scientists working on X-ray, SZ and optical observations as well as cosmological simulations

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