The XMM − Newton Ω project : III . Gas mass fraction shape in high redshift clusters

We study the gas mass fraction behavior in distant galaxy clu sters observed within the XMM − Newton Ω project. The typical gas mass fraction fgas shape of high redshift galaxy clusters follows the global sh pe inferred at low redshift quite well, once scaled appropriately : the gas mass fraction incr eases with radius and flattens outward. This result is consis te t with the simple picture in which clusters essentially form b y gravitational collapse, leading to self similar structur es for both the dark and baryonic matter. However, we find that the mean ga s profile in distant clusters shows some di fferences to local ones, indicating a departure from strict scaling. Assuming a Einstein-de Sitter cosmology, we find a slight deficit of ga s in the central part of highz clusters. This result is consistent with the observed evolu tion in the luminosity-temperature relation. We quantitatively investigate this departure from scaling la ws by comparingfgasfrom a sample of nearby galaxy clusters (Vikhlinin, Forman & Jones, 1999) to our eight high-z clusters. Within th e local sample, a moderate but clear variation of the amplitu de of the gas mass fraction with temperature is found, a trend that we kens in the outer regions. Taking into account these vari ations with radius and temperature, the apparent scaled gas mass fr actions in our distant clusters still systematically di ffer from local clusters. This reveals that the gas fraction does not strict ly follow a scaling law with redshift. This provides clues to understand the redshift evolution of theL−T relation whose origin is probably due to non-gravitational processes during cluster formation. An important implication of our results is that the gas fract ion evolution, a test of the cosmological parameters, can le d to biased values when applied at radii smaller than the virial r adius. From ourXMM 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 c onsistent with a non-evolving universal value in a high matter density model while in a concordance, model high redshift clusters s how an apparent higher fgas at the virial radius than to local clusters.