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

Abstract

We present constraints on the mean matter density, Omega_m, dark energy\ndensity, Omega_de, and the dark energy equation of state parameter, w, using\nChandra measurements of the X-ray gas mass fraction (fgas) in 42 hot (kT>5keV),\nX-ray luminous, dynamically relaxed galaxy clusters spanning the redshift range\n0.05<z<1.1. Using only the fgas data for the 6 lowest redshift clusters at\nz<0.15, for which dark energy has a negligible effect on the measurements, we\nmeasure Omega_m=0.28+-0.06 (68% confidence, using standard priors on the Hubble\nConstant, H_0, and mean baryon density, Omega_bh^2). Analyzing the data for all\n42 clusters, employing only weak priors on H_0 and Omega_bh^2, we obtain a\nsimilar result on Omega_m and detect the effects of dark energy on the\ndistances to the clusters at ~99.99% confidence, with Omega_de=0.86+-0.21 for a\nnon-flat LCDM model. The detection of dark energy is comparable in significance\nto recent SNIa studies and represents strong, independent evidence for cosmic\nacceleration. Systematic scatter remains undetected in the fgas data, despite a\nweighted mean statistical scatter in the distance measurements of only ~5%. For\na flat cosmology with constant w, we measure Omega_m=0.28+-0.06 and\nw=-1.14+-0.31. Combining the fgas data with independent constraints from CMB\nand SNIa studies removes the need for priors on Omega_bh^2 and H_0 and leads to\ntighter constraints: Omega_m=0.253+-0.021 and w=-0.98+-0.07 for the same\nconstant-w model. More general analyses in which we relax the assumption of\nflatness and/or allow evolution in w remain consistent with the cosmological\nconstant paradigm. Our analysis includes conservative allowances for systematic\nuncertainties. The small systematic scatter and tight constraints bode well for\nfuture dark energy studies using the fgas method. (Abridged)\n