Improved constraints on dark energy from Chandra Xray observations of the largest relaxed galaxy clusters
Abstract
We present constraints on the mean matter density, Ω_{m}, dark energy density, Ω_{DE}, and the dark energy equation of state parameter, w, using Chandra measurements of the Xray gas mass fraction (f_{gas}) in 42 hot (kT > 5keV), Xray 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 Ω_{m} = 0.28 +/ 0.06 (68 per cent confidence limits, using standard priors on the Hubble constant, H_{0}, and mean baryon density, Ω_{b}h^{2}). Analysing the data for all 42 clusters, employing only weak priors on H_{0} and Ω_{b}h^{2}, we obtain a similar result on Ω_{m} and a detection of the effects of dark energy on the distances to the clusters at ~99.99 per cent confidence, with Ω_{DE} = 0.86 +/ 0.21 for a nonflat ΛCDM model. The detection of dark energy is comparable in significance to recent type Ia 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 ~5 per cent. For a flat cosmology with a constant dark energy equation of state, we measure Ω_{m} = 0.28 +/ 0.06 and w = 1.14 +/ 0.31. Combining the f_{gas} data with independent constraints from cosmic microwave background and SNIa studies removes the need for priors on Ω_{b}h^{2} and H_{0} and leads to tighter constraints: Ω_{m} = 0.253 +/ 0.021 and w = 0.98 +/ 0.07 for the same constantw 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 w_{0} = 1.05 +/ 0.29 and w_{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 ~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 Ω_{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 Xray observatories, such as ConstellationX.
 Publication:

Monthly Notices of the Royal Astronomical Society
 Pub Date:
 January 2008
 DOI:
 10.1111/j.13652966.2007.12610.x
 arXiv:
 arXiv:0706.0033
 Bibcode:
 2008MNRAS.383..879A
 Keywords:

 cosmic microwave background;
 cosmological parameters;
 cosmology: observations;
 dark matter;
 distance scale;
 Xrays: galaxies: clusters;
 Astrophysics
 EPrint:
 Published in MNRAS. 20 pages, 11 figures. The data and analysis code (in the form of a patch to CosmoMC) are now available at http://www.stanford.edu/~drapetti/fgas_module/