The outer regions of galaxy clusters: Chandra constraints on the X-ray surface brightness

¹ INAF, Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy e-mail: stefano.ettori@oabo.inaf.it
² INFN, Sezione di Bologna, viale Berti Pichat 6/2, 40127 Bologna, Italy
³ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstr. 1, 85748 Garching, Germany

Received: 17 October 2008
Accepted: 20 November 2008

Abstract

Context. We study the properties of the X-ray surface brightness profiles in a sample of galaxy clusters that were observed with Chandra and have emission detectable with a signal-to-noise ratio higher than 2 per radial bin at a radius beyond R₅₀₀ ≈ 0.7 R₂₀₀.

Aims. Our study aims to measure the slopes in both the X-ray surface brightness and gas density profiles in the outskirts of massive clusters. These constraints are compared with similar results obtained from observations and numerical simulations of the temperature and dark-matter density profiles with the intention of presenting a consistent picture of the outer regions of galaxy clusters.

Methods. We extract the surface brightness profiles of 52 X-ray luminous galaxy clusters at from X-ray exposures obtained with Chandra. These objects, which are of both high X-ray surface brightness and high redshift, allow us to use Chandra either in ACIS-I or even ACIS-S configuration to survey the cluster outskirts. We estimate R₂₀₀ using both a β-model that reproduces the surface brightness profiles and scaling relations from the literature. The two methods converge to comparable values. We determine the radius, R_S2N, at which the signal-to-noise ratio is higher than 2, and select the objects in the sample that satisfy the criterion . For the eleven selected objects, we model by a power-law function the behaviour of to estimate the slope at several characteristic radii expressed as a fraction of R₂₀₀.

Results. We measure a consistent steepening of the profile moving outward from , where an average slope of -3.6 (σ = 0.8) is estimated. At R₂₀₀, we evaluate a slope of -4.3 (σ = 0.9) that implies a slope in the gas density profile of ≈-2.6 and a predicted mean value of the surface brightness in the 0.5–2 keV band of 2 10^-12 erg s^-1 cm^-2 deg^-2.

Conclusions. Combined with estimates of the outer slope of the gas temperature profile and expectations about the dark matter distribution, these measurements lie well within the physically allowed regions, allowing us to describe properly how X-ray luminous clusters behave out to the virial radius.