Improved constraints on dark energy from Chandra X-ray observations of the largest relaxed galaxy clusters

We present constraints on the mean matter density, Omega m, dark energy density, Omega(DE), and the dark energy equation of state parameter, omega, using Chandra measurements of the X-ray gas mass fraction (f(gas)) in 42 hot (kT > 5 keV), X-ray luminous, dynamically relaxed galaxy clusters spanning the redshift range 0.05 < z < 1. 1. Using only the f(gas) data for the six lowest redshift clusters at z < 0.15, for which dark energy has a negligible effect on the measurements, we measure Omega(m) = 0.28 +/- 0.06 (68 per cent confidence limits, using standard priors on the Hubble constant, H-0, and mean baryon density, Omega(b) h(2)). Analysing the data for all 42 clusters, employing only weak priors on H-0 and Omega(b) h(2), we obtain a similar result on Omega(m) and a detection of the effects of dark energy on the distances to the clusters at similar to 99.99 per cent confidence, with Omega(DE) = 0.86 +/- 0.21 for a non-flat Lambda CDM model. The detection of dark energy is comparable in significance to recent type la supernovae (SNIa) studies and represents strong, independent evidence for cosmic acceleration. Systematic scatter remains undetected in the f(gas) data, despite a weighted mean statistical scatter in the distance measurements of only similar to 5 per cent. For a flat cosmology with a constant dark energy equation of state, we measure Omega(m), = 0.28 +/- 0.06 and w = -1.14 +/- 0.3 1. Combining the f(gas) data with independent constraints from cosmic microwave background and SNIa studies removes the need for priors on Omega(b) h(2) and H-0 and leads to tighter constraints: Omega(m), = 0.253 +/- 0.021 and omega = -0.98 +/- 0.07 for the same constant-omega model. Our most general analysis allows the equation of state to evolve with redshift. Marginalizing over possible transition redshifts 0.05 < Z(t) < 1, the combined f(gas) + CMB + SNIa data set constrains the dark energy equation of state at late and early times to be omega(0) = -1.05 +/- 0.29 and omega(et) = -0.83 +/- 0.46, respectively, in agreement with the cosmological constant paradigm. Relaxing the assumption of flatness weakens the constraints on the equation of state by only a factor of similar to 2. Our analysis includes conservative allowances for systematic uncertainties associated with instrument calibration, cluster physics and data modelling. The measured small systematic scatter, tight constraint on Omega(m) and powerful constraints on dark energy from the f(gas) data bode well for future dark energy studies using the next generation of powerful X-ray observatories, such as Constellation-X.