Measuring H₀ using X-ray and SZ effect observations of dynamically relaxed galaxy clusters

Abstract

We use a sample of 14 massive, dynamically relaxed galaxy clusters to constrain the Hubble constant, H₀, by combining X-ray and Sunyaev–Zel'dovich (SZ) effect signals measured with Chandra, Planck, and Bolocam. This is the first such analysis to marginalize over an empirical, data-driven prior on the overall accuracy of X-ray temperature measurements, while our restriction to the most relaxed, massive clusters also minimizes astrophysical systematics. For a cosmological-constant model with Ω_m = 0.3 and Ω_Λ = 0.7, we find H₀ = 67.3^(+21.3)_(−13.3) kms⁻¹ Mpc⁻¹⁠, limited by the temperature calibration uncertainty (compared to the statistically limited constraint of H₀ = 72.3^(+7.6)_(−7.6) kms⁻¹ Mpc⁻¹⁠). The intrinsic scatter in the X-ray/SZ pressure ratio is found to be 13 ± 4 per cent (10 ± 3 per cent when two clusters with significant galactic dust emission are removed from the sample), consistent with being primarily due to triaxiality and projection. We discuss the prospects for reducing the dominant systematic limitation to this analysis, with improved X-ray calibration and/or precise measurements of the relativistic SZ effect providing a plausible route to per cent level constraints on H₀.

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