The structural and scaling properties of nearby galaxy clusters. I. The universal mass profile

We present the integrated mass profiles for a sample of ten nearby (z<~0.15), relaxed galaxy clusters, covering a temperature range of [2-9] keV, observed with XMM-Newton. The mass profiles were derived from the observed gas density and temperature profiles under the hypothesis of spherical symmetry and hydrostatic equilibrium. All ten mass profiles are well described by an NFW-type profile over the radial range from 0.01 to 0.5 R₂₀₀, where R₂₀₀ is the radius corresponding to a density contrast of 200 with respect to the critical density at the cluster redshift. A King model is inconsistent with these data. The derived concentration parameters and total masses are in the range c₂₀₀=4-6 and M₂₀₀=1.2× 10¹⁴-1.2 × 10¹⁵ M_⊙, respectively. Our qualitative and quantitative study of the mass profile shape shows, for the first time, direct and clear observational evidence for the universality of the total mass distribution in clusters. The mass profiles scaled in units of R₂₀₀ and M₂₀₀ nearly coincide, with a dispersion of less than 15% at 0.1 R₂₀₀. The c₂₀₀-M₂₀₀ relation is consistent with the predictions of numerical simulations for a ΛCDM cosmology, taking into account the measurement errors and expected intrinsic scatter. Our results provide further strong evidence in favour of the Cold Dark Matter cosmological scenario and show that dark matter collapse is well understood, at least down to the cluster scale.