Constraints on dark energy from Chandra observations of the largest relaxed galaxy clusters

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 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 Λ cold dark matter (CDM) cosmology with the curvature Ω K included 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 −0.04 and 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.