2004MNRAS.353..457A

We present constraints on the mean dark energy density, Ω_X and dark energy equation of state parameter, w_X, based on Chandra measurements of the X-ray gas mass fraction in 26 X-ray luminous, dynamically relaxed galaxy clusters spanning the redshift range 0.07 < z < 0.9. Under the assumption that the X-ray gas mass fraction measured within r₂₅₀₀is constant with redshift and using only weak priors on the Hubble constant and mean baryon density of the Universe, we obtain a clear detection of the effects of dark energy on the distances to the clusters, confirming (at comparable significance) previous results from Type Ia supernovae studies. For a standard Λ cold dark matter (CDM) cosmology with the curvature Ω_Kincluded as a free parameter, we find Ω_Λ= 0.94^+0.21_–0.23 (68 per cent confidence limits). We also examine extended XCDM dark energy models. Combining the Chandra data with independent constraints from cosmic microwave background experiments, we find Ω_X= 0.75±0.04, Ω_m= 0.26^+0.06_–0.04and w_X=-1.26±0.24. Imposing the prior constraint w_X> -1, the same data require w_X< -0.7 at 95 per cent confidence. Similar results on the mean matter density and dark energy equation of state parameter, Ω_m= 0.24±0.04 and w_X=-1.20^+0.24_–0.28, are obtained by replacing the cosmic microwave background data with standard priors on the Hubble constant and mean baryon density and assuming a flat geometry.