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

TL;DR

This paper uses cosmological hydrodynamical simulations to study the relative velocities of dark matter and baryons in galaxy clusters, assessing how baryonic physics and environment influence peculiar velocity measurements.

Contribution

It provides the first quantification of how baryonic processes and environment affect the systematic differences in velocities within galaxy clusters.

Findings

Baryonic physics introduces significant scatter in velocity measurements.

Scatter varies between 20% and 50% depending on measurement aperture.

Massive clusters show larger discrepancies in ICM and dark matter velocities.

Abstract

The increasing sensitivity of current experiments, which nowadays routinely measure the thermal SZ effect within galaxy clusters, provide the hope that peculiar velocities of individual clusters of galaxies will be measured rather soon using the kinematic SZ effect. Also next generation of X-ray telescopes with microcalorimeters, promise first detections of the motion of the intra cluster medium (ICM) within clusters. We used a large set of cosmological, hydrodynamical simulations, which cover 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 to investigate, how the presence of baryons and their associated physical processes like cooling and star-formation are affecting 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 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 add significant scatter onto 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%, when going from the core (e.g. ten percent of the virial radius) to the full cluster (e.g. the virial radius).