The gas mass fraction in local and Z > 0.7 galaxy clusters: Constraints on cosmology

We present a study of the baryonic fraction in galaxy clusters aimed at constraining the cosmological parameters Omega(m), Omega(Lambda) and the ratio between the pressure and density of the 'dark' energy, w. We use results on the gravitating mass profiles of a sample of nearby galaxy clusters observed with the BeppoSAX X-ray satellite (Ettori, De Grandi and Molendi, 2002) to set constraints on the dynamical estimate of Omega(m). We then analyze Chandra observations of a sample of eight distant clusters with redshift in the range 0.72 and 1.27 and evaluate the geometrical limits on the cosmological parameters Omega(m), Omega(Lambda) and w by requiring that the gas fraction remains constant with respect to the look- back time. By combining these two independent probability distributions and using a priori distributions on both Omega(b) and H-0 peaked around primordial nucleosynthesis and HST-Key Project results respectively, we obtain that, at 95.4 per cent level of confidence, (i) w < -0.54, (ii) Omega(m) = 0.34(-0.05)(+ 0.11), Omega(Lambda) = 1.50(-1.13)(+ 0.24) for w = -1 (corresponding to the case for a cosmological constant), and (iii) Omega(m) = 1 -Omega(Lambda) = 0.33(-0.05)(+ 0.06) for a flat Universe. This paper summarizes the work in press at Astronomy & Astrophysics by Ettori, Tozzi and Rosati (astro-ph/0211335).