Relative velocity of dark matter and baryons in clusters of galaxies and measurements of their peculiar velocities

The increasing sensitivity of current experiments, which nowadays routinely measure the thermal Sunvaev-Zel'dovich (SZ) effect within galaxy clusters, provides the hope that peculiar velocities of individual clusters of galaxies will be measured rather soon using the kinematic SZ effect. Also, the next generation of X-ray telescopes with microcalorimeters promise first detections of the motion of the intracluster medium (ICM) within clusters. We used a large set of cosmological hydrodynamical simulations, which cover a very large cosmological volume, hosting a large number of rich clusters of galaxies, as well as moderate volumes where the internal structures of individual galaxy clusters can be resolved with very high resolution. This can be used to investigate how the presence of baryons and their associated physical processes like cooling and star formation affect the systematic difference between mass-averaged velocities of dark matter and the ICM inside a cluster. We, for the first time, quantify the peculiar motion of galaxy clusters as a function of the large-scale environment. We also demonstrate that, especially in very massive systems, the relative velocity of the ICM compared to the cluster peculiar velocity adds significant scatter on to the inferred peculiar velocity, especially when measurements are limited to the central regions of the cluster. Depending on the aperture used, this scatter varies between 50 and 20 per cent, when going from the core (e.g. 10 per cent of the virial radius) to the full cluster (e.g. the virial radius).