Cosmology and astrophysics from relaxed galaxy clusters – II. Cosmological constraints

Abstract

We present cosmological constraints from measurements of the gas mass\nfraction, $f_{gas}$, for massive, dynamically relaxed galaxy clusters. Our data\nset consists of Chandra observations of 40 such clusters, identified in a\ncomprehensive search of the Chandra archive, as well as high-quality weak\ngravitational lensing data for a subset of these clusters. Incorporating a\nrobust gravitational lensing calibration of the X-ray mass estimates, and\nrestricting our measurements to the most self-similar and accurately measured\nregions of clusters, significantly reduces systematic uncertainties compared to\nprevious work. Our data for the first time constrain the intrinsic scatter in\n$f_{gas}$, $(7.4\\pm2.3)$% in a spherical shell at radii 0.8-1.2 $r_{2500}$,\nconsistent with the expected variation in gas depletion and non-thermal\npressure for relaxed clusters. From the lowest-redshift data in our sample we\nobtain a constraint on a combination of the Hubble parameter and cosmic baryon\nfraction, $h^{3/2}\\Omega_b/\\Omega_m=0.089\\pm0.012$, that is insensitive to the\nnature of dark energy. Combined with standard priors on $h$ and $\\Omega_b h^2$,\nthis provides a tight constraint on the cosmic matter density,\n$\\Omega_m=0.27\\pm0.04$, which is similarly insensitive to dark energy. Using\nthe entire cluster sample, extending to $z>1$, we obtain consistent results for\n$\\Omega_m$ and interesting constraints on dark energy:\n$\\Omega_\\Lambda=0.65^{+0.17}_{-0.22}$ for non-flat $\\Lambda$CDM models, and\n$w=-0.98\\pm0.26$ for flat constant-$w$ models. Our results are both competitive\nand consistent with those from recent CMB, SNIa and BAO data. We present\nconstraints on models of evolving dark energy from the combination of $f_{gas}$\ndata with these external data sets, and comment on the possibilities for\nimproved $f_{gas}$ constraints using current and next-generation X-ray\nobservatories and lensing data. (Abridged)\n