Mass profiles and c − M_DM relation in X-ray luminous galaxy clusters

¹ INAF, Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
e-mail: stefano.ettori@oabo.inaf.it
² INFN, Sezione di Bologna, via le Berti Pichat 6/2, 40127 Bologna, Italy
³ INAF, IASF, via Bassini 15, 20133 Milano, Italy
⁴ Università degli Studi di Milano, Dip. di Fisica, via Celoria 16, 20133 Milano, Italy
⁵ Department of Physics and Astronomy, University of California, Irvine, CA 92697-4575, USA

Received: 24 June 2010
Accepted: 14 September 2010

Abstract

Context. Galaxy clusters represent valuable cosmological probes using tests that mainly rely on measurements of cluster masses and baryon fractions. X-ray observations represent one of the main tools for uncovering these quantities.

Aims. We aim to constrain the cosmological parameters Ω_m and σ₈ using the observed distribution of the both values of the concentrations and dark mass within R₂₀₀ and of the gas mass fraction within R₅₀₀.

Methods. We applied two different techniques to recover the profiles the gas and dark mass, described according to the Navarro, Frenk & White (1997, ApJ, 490, 493) functional form, of a sample of 44 X-ray luminous galaxy clusters observed with XMM-Newton in the redshift range 0.1−0.3. We made use of the spatially resolved spectroscopic data and of the PSF–deconvolved surface brightness and assumed that hydrostatic equilibrium holds between the intracluster medium and the gravitational potential. We evaluated several systematic uncertainties that affect our reconstruction of the X-ray masses.

Results. We measured the concentration c₂₀₀, the dark mass M₂₀₀ and the gas mass fraction in all the objects of our sample, providing the largest dataset of mass parameters for galaxy clusters in the redshift range 0.1 − 0.3. We confirm that a tight correlation between c₂₀₀ and M₂₀₀ is present and in good agreement with the predictions from numerical simulations and previous observations. When we consider a subsample of relaxed clusters that host a low entropy core, we measure a flatter c − M relation with a total scatter that is lower by 40 per cent. We conclude, however, that the slope of the c − M relation cannot be reliably determined from the fitting over a narrow mass range as the one considered in the present work. From the distribution of the estimates of c₂₀₀ and M₂₀₀, with associated statistical (15–25%) and systematic (5–15%) errors, we used the predicted values from semi-analytic prescriptions calibrated through N-body numerical runs and obtain (at 2σ level, statistical only) for the subsample of the clusters where the mass reconstruction has been obtained more robustly and for the subsample of the 11 more relaxed LEC objects. With the further constraint from the gas mass fraction distribution in our sample, we break the degeneracy in the σ₈ − Ω_m plane and obtain the best-fit values σ₈ ≈ 1.0 ± 0.2 (0.83 ± 0.1 when the subsample of the more relaxed objects is considered) and Ω_m = 0.26 ± 0.02.

Conclusions. Analysis of the distribution of the c₂₀₀ − M₂₀₀ − f_gas values represents a mature and competitive technique in the present era of precision cosmology, even though it needs more detailed analysis of the output of larger sets of cosmological numerical simulations to provide definitive and robust results.