| Abstract
| - We present a study of the baryonic fraction in galaxy clusters aimed at constraining the cosmological parameters $\Omega_{\rm m}$, $\Omega_{\Lambda}$ 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 et al. 2002) to set constraints on the dynamical estimate of $\Omega_{\rm m}$. 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 $\Omega_{\rm m}$, $\Omega_{\Lambda}$ 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 $\Omega_{\rm b}$ and H0 peaked around primordial nucleosynthesis and HST-Key Project results respectively, we obtain that, at 95.4 per cent level of confidence, (i) $w < -0.49$, (ii) $\Omega_{\rm m} = 0.34^{+0.11}_{-0.05}$, $\Omega_{\Lambda} = 1.30^{+0.44}_{-1.09}$ for $w=-1$ (corresponding to the case for a cosmological constant), and (iii) $\Omega_{\rm m} = 1-\Omega_{\Lambda} = 0.33^{+0.07}_{-0.05}$ for a flat Universe. These results are in excellent agreement with the cosmic concordance scenario which combines constraints from the power spectrum of the Cosmic Microwave Background, the galaxy and cluster distribution, the evolution of the X-ray properties of galaxy clusters and the magnitude-redshift relation for distant type Ia supernovae. By combining our results with the latter method we further constrain $\Omega_{\Lambda} =0.94^{+0.30}_{-0.30}$ and $w < -0.89$ at the $2 \sigma$ level.
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