Cosmological constraints from X-ray observations of galaxy clusters

Chandra X-ray observations of rich, dynamically relaxed galaxy clusters allow the properties of the X-ray gas and the total gravitating mass to be determined precisely. By combining Chandra results on the X-ray gas mass fractions in clusters with independent measurements of the Hubble constant and the mean baryonic matter density of the universe, we obtain a tight constraint on the mean matter density of the universe, Omega(m), and an interesting constraint on the dark energy density, Omega(Lambda). Using these results, together with the observed local Xray luminosity function of the most X-ray luminous galaxy clusters, a massluminosity relation determined from Chandra and ROSAT X-ray data and weak gravitational lensing observations, and the mass function predicted by numerical simulations, we also obtain a precise constraint on the normalization of mass fluctuations in the nearby universe, us.