On the Cluster Physics of Sunyaev-Zel'dovich and X-Ray Surveys. III. Measurement Biases and Cosmological Evolution of Gas and Stellar Mass Fractions

Gas masses tightly correlate with the virial masses of galaxy clusters, allowing for a precise determination of cosmological parameters by means of X-ray surveys. However, the gas mass fractions (f _gas) at the virial radius (R ₂₀₀) derived from recent Suzaku observations are considerably larger than the cosmic mean, calling into question the accuracy of cosmological parameters. Here, we use a large suite of cosmological hydrodynamical simulations to study measurement biases of f _gas. We employ different variants of simulated physics, including radiative gas physics, star formation, and thermal feedback by active galactic nuclei, which we show is able to arrest overcooling and to result in constant stellar mass fractions for redshifts z < 1. Computing the mass profiles in 48 angular cones, we find anisotropic gas and total mass distributions that imply an angular variance of f _gas at the level of 30%. This anisotropy originates from the recent formation epoch of clusters and from the strong internal baryon-to-dark-matter density bias. In the most extreme cones, f _gas can be biased high by a factor of two at R ₂₀₀ in massive clusters (M ₂₀₀ ~ 10¹⁵ M _⊙), thereby providing an explanation for high f _gas measurements by Suzaku. While projection lowers this factor, there are other measurement biases that may (partially) compensate. At R ₂₀₀, f _gas is biased high by 20% when assuming hydrostatic equilibrium masses, i.e., neglecting the kinetic pressure, and by another ~10%-20% due to the presence of density clumping. At larger radii, both measurement biases increase dramatically. While the cluster sample variance of the true f _gas decreases to a level of 5% at R ₂₀₀, the sample variance that includes both measurement biases remains fairly constant at the level of 10%-20%. The constant redshift evolution of f _gas within R ₅₀₀ for massive clusters is encouraging for using gas masses to derive cosmological parameters, provided the measurement biases can be controlled.