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

Abstract

This is the second in a series of papers studying the astrophysics and cosmology of massive, dynamically relaxed galaxy clusters. The data set employed here consists of Chandra observations of 40 such clusters, identified in a comprehensive search of the Chandra archive for hot (kT ≳ 5舁keV), massive, morphologically relaxed systems, as well as high-quality weak gravitational lensing data for a subset of these clusters. Here we present cosmological constraints from measurements of the gas mass fraction,舁 fgas, for this cluster sample. By incorporating a robust gravitational lensing calibration of the X-ray mass estimates, and restricting our measurements to the most self-similar and accurately measured regions of clusters, we significantly reduce systematic uncertainties compared to previous work. Our data for the first time constrain the intrinsic scatter in fgas, 7.4 ± 2.3 per舁cent in a spherical shell at radii 0.8–1.2 r2500 (∼1/4 of the virial radius), consistent with the expected level of variation in gas depletion and non-thermal pressure for relaxed clusters. From the lowest redshift data in our sample, five clusters at | $z$ | < 0.16, we obtain a constraint on a combination of the Hubble parameter and cosmic baryon fraction, h3/2舁Ωb/Ωm = 0.089 ± 0.012, that is insensitive to the nature of dark energy. Combining this with standard priors on h and Ωbh2 provides a tight constraint on the cosmic matter density, Ωm = 0.27 ± 0.04, which is similarly insensitive to dark energy. Using the entire cluster sample, extending to | $z$ | > 1, we obtain consistent results for Ωm and interesting constraints on dark energy: | $\Omega _{\rm{\Lambda }}=0.65^{+0.17}_{-0.22}$ | for non-flat ΛCDM (cosmological constant) models, and | $w$ | = −0.98 ± 0.26 for flat models with a constant dark energy equation of state. Our results are both competitive and consistent with those from recent cosmic microwave background, Type Ia supernova and baryon acoustic oscillation data. We present constraints on more complex models of evolving dark energy from the combination of fgas data with these external data sets, and comment on the possibilities for improved fgas constraints using current and next-generation X-ray observatories and lensing data.