The gas distribution in the outer regions of galaxy clusters

¹ INAF – IASF – Milano, via E. Bassini 15, 20133 Milano, Italy
² ISDC Data Centre for Astrophysics, Geneva Observatory, Ch. d’Ecogia 16, 1290 Versoix, Switzerland
e-mail: Dominique.Eckert@unige.ch
³ Jacobs University Bremen, Campus Ring 1, 28759 Bremen, Germany
⁴ INAF – Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁵ INFN, Sezione di Bologna, viale Berti Pichat 6/2, 40127 Bologna, Italy
⁶ Department of Physics, Yale University, New Haven, CT 06520, USA
⁷ Shanghai Astronomical Observatory, 80 Nandan Road, 200030 Shanghai, PR China
⁸ Dipartimento di Astronomia, Università di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁹ Dipartimento di Fisica, Università degli studi di Milano, via Celoria 16, 20133 Milano, Italy
¹⁰ NASA/Goddard Space Flight Center, Code 662, Greenbelt, MD 20771, USA
¹¹ University of California at Irvine, 4129, Frederick Reines Hall, Irvine, CA 92697-4575, USA

Received: 15 October 2011
Accepted: 19 February 2012

Abstract

Aims. We present our 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.

Methods. We have exploited 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 stacked the density profiles to detect a signal beyond r₂₀₀ and measured the typical density and scatter in cluster outskirts. We also computed the azimuthal scatter of the profiles with respect to the mean value to look for deviations from spherical symmetry. Finally, we compared our average density and scatter profiles with the results of numerical simulations.

Results. As opposed to some recent Suzaku results, and confirming previous evidence from ROSAT and Chandra, we observe a steepening of the density profiles beyond  ~r₅₀₀. Comparing our density profiles with simulations, we find that bibradiative runs predict density profiles that are too steep, whereas runs including additional physics and/or treating gas clumping agree better with the observed gas distribution. We report high-confidence detection of a systematic difference between cool-core and non cool-core clusters beyond  ~0.3r₂₀₀, which we explain by a different distribution of the gas in the two classes. Beyond  ~r₅₀₀, galaxy clusters deviate significantly from spherical symmetry, with only small 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 ENZO simulations.

Conclusions. Comparing our results with numerical simulations, we find that bibradiative simulations fail to reproduce the gas distribution, even well outside cluster cores. Although their general behavior agrees more closely with the observations, simulations including cooling and star formation convert a large amount of gas into stars, which results in a low gas fraction with respect to the observations. Consequently, a detailed treatment of gas cooling, star formation, AGN feedback, and consideration of gas clumping is required to construct realistic models of the outer regions of clusters.