Constraints on dark energy from<i>Chandra</i>observations of the largest relaxed galaxy clusters

We present constraints on the mean dark energy density, Omega_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 r_2500 is 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 LambdaCDM cosmology with the curvature Omega_K included as a free parameter, we find Omega_Lambda=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 Omega_X=0.75\pm0.04, Omega_m=0.26^{+0.06}_{-0.04} and w_X=-1.26\pm0.24. 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, Omega_m=0.24\pm0.04 and w_X=-1.20^{+0.24}_{-0.28}, are obtained by replacing the CMB data with standard priors on the Hubble constant and mean baryon density and assuming a flat geometry.