New constraints on dark energy from the observed growth of the most X-ray luminous galaxy clusters

We present constraints on the mean matter density, Ωm, the normalization of the density fluctuation power spectrum, σ8, and the dark-energy equation-of-state parameter, w, obtained from measurements of the X-ray luminosity function of the largest known galaxy clusters at redshifts z < 0.7, as compiled in the Massive Cluster Survey (MACS) and the local BCS and REFLEX galaxy cluster samples. Our analysis employs an observed mass–luminosity relation, calibrated by hydrodynamical simulations, including corrections for non-thermal pressure support and accounting for the presence of intrinsic scatter. Conservative allowances for all known systematic uncertainties are included, as are standard priors on the Hubble constant and mean baryon density. We find Ωm = 0.28 +0.11 −0.07 and σ8 = 0.78 +0.11 −0.13 for a spatially flat, cosmological-constant model, and Ωm = 0.24 +0.15 −0.07, σ8 = 0.85 +0.13 −0.20 and w = −1.4 +0.4 −0.7 for a flat, constant-w model (marginalized 68 per cent confidence intervals). Our findings constitute the first determination of the dark-energy equation of state from measurements of the growth of cosmic structure in galaxy clusters, and the consistency of our result with w = −1 lends additional support to the cosmological-constant model. Future work improving our understanding of redshift evolution and observational biases affecting the mass– X-ray luminosity relation have the potential to significantly tighten these constraints. Our results are consistent with those from recent analyses of type Ia supernovae, cosmic microwave background anisotropies, the X-ray gas mass fraction of relaxed galaxy clusters, baryon acoustic oscillations and cosmic shear. Combining the new Xray luminosity function data with current supernova, cosmic microwave background and cluster gas fraction data yields the improved constraints Ωm = 0.269 ± 0.016, σ8 = 0.82± 0.03 and w = −1.02± 0.06.