The XMM-Newton Ω project

We study the gas mass fraction behavior in distant galaxy clusters observed within the XMM-Newton Ω project. The typical gas mass fraction f_gas shape of high redshift galaxy clusters follows the global shape inferred at low redshift quite well, once scaled appropriately: the gas mass fraction increases with radius and flattens outward. This result is consistent with the simple picture in which clusters essentially form by gravitational collapse, leading to self similar structures for both the dark and baryonic matter. However, we find that the mean gas profile in distant clusters shows some differences to local ones, indicating a departure from strict scaling. Assuming an Einstein-de Sitter cosmology, we find a slight deficit of gas in the central part of high-z clusters. This result is consistent with the observed evolution in the luminosity-temperature relation. We quantitatively investigate this departure from scaling laws by comparing f_gas from a sample of nearby galaxy clusters (Vikhlinin et al. 1999) to our eight high-z clusters. Within the local sample, a moderate but clear variation of the amplitude of the gas mass fraction with temperature is found, a trend that weakens in the outer regions. Taking into account these variations with radius and temperature, the apparent scaled gas mass fractions in our distant clusters still systematically differ from local clusters. This reveals that the gas fraction does not strictly follow a scaling law with redshift. This provides clues to understand the redshift evolution of the L-T relation whose origin is probably due to non-gravitational processes during cluster formation. An important implication of our results is that the gas fraction evolution, a test of the cosmological parameters, can lead to biased values when applied at radii smaller than the virial radius. From our XMM clusters, as well as {Chandra} clusters in the same redshift range, the apparent gas fraction at the virial radius obtained by extrapolation of the inner gas profile is consistent with a non-evolving universal value in a high matter density model while in a concordance model, high redshift clusters show an apparent higher f_gas at the virial radius than local clusters.