Gas mass fraction from XMM-Newton and Chandra high redshift clusters and   its use as a cosmological test
  We investigate the cosmological test based on the evolution of the gas fraction in X-ray galaxy clusters and the stability of the cosmological parameters derived from it. Using a sample of distant clusters observed by XMM-Newton and Chandra, and comparing their gas fraction at different radii to the gas fraction observed for nearby clusters, we have determined the likelihood functions for $\Omega_m$ in a flat universe and the confidence contours in the $\Omega_m-\Omega_{\Lambda}$ plane. Results obtained at the virial radius point to a high matter density Universe, while for inner radii the $\Omega_m$ parameter obtained tends to decrease, reaching values compatible with the concordance model. The analysis allows us to conclude that this test provides ambiguous results due to the complex structure of the ICM that induces a dependence of the gas fraction with temperature, radius, and redshift which cannot be accounted by a self similar argument expected in pure gravitational heating of the ICM. The use of gas fraction in X-ray clusters to constrain cosmological parameters seems therefore to be compromised until a better understanding of the ICM physics and the ability to obtain observations of better quality up to the virial radius are achieved. 
Luis Ferramacho (LATT), Alain Blanchard (LATT)