The gas distribution in the outer regions of galaxy clusters

TL;DR

This study measures the gas density in the outskirts of galaxy clusters using ROSAT data, compares it with simulations, and finds differences in profiles and symmetry, highlighting the impact of physical processes and cluster types.

Contribution

First detection of systematic differences in gas distribution between cool-core and non-cool core clusters beyond 0.3r200, and comparison of observed profiles with various simulation models.

Findings

Density profiles steepen beyond r500, consistent with previous ROSAT and Chandra results.

Non-radiative simulations predict too steep profiles, while physics-inclusive runs match observations better.

Clusters deviate from spherical symmetry beyond r500, with minimal differences between relaxed and disturbed systems.

Abstract

We present the analysis of a local (z = 0.04 - 0.2) sample of 31 galaxy clusters with the aim of measuring the density of the X-ray emitting gas in cluster outskirts. We compare our results with numerical simulations to set constraints on the azimuthal symmetry and gas clumping in the outer regions of galaxy clusters. We exploit the large field-of-view and low instrumental background of ROSAT/PSPC to trace the density of the intracluster gas out to the virial radius. We perform a stacking of the density profiles to detect a signal beyond r200 and measure the typical density and scatter in cluster outskirts. We also compute the azimuthal scatter of the profiles with respect to the mean value to look for deviations from spherical symmetry. Finally, we compare our average density and scatter profiles with the results of numerical simulations. As opposed to some recent Suzaku results, and confirming previous evidence from ROSAT and Chandra, we observe a steepening of the density profiles beyond \sim r500. Comparing our density profiles with simulations, we find that non-radiative runs predict too steep density profiles, whereas runs including additional physics and/or treating gas clumping are in better agreement with the observed gas distribution. We report for the first time the high-confidence detection of a systematic difference between cool-core and non-cool core clusters beyond \sim 0.3r200, which we explain by a different distribution of the gas in the two classes. Beyond \sim r500, galaxy clusters deviate significantly from spherical symmetry, with only little differences between relaxed and disturbed systems. We find good agreement between the observed and predicted scatter profiles, but only when the 1% densest clumps are filtered out in the simulations. [Abridged]