Systematic errors in the determination of Hubble constant due to the asphericity and non-isothermality of clusters of galaxies

Abstract

Joint analyses on X-ray and Sunyaev-Zel'dovich (SZ) effect of a cluster of galaxies can give rise to an estimate on the angular diameter distance to the cluster. With the redshift information of the cluster, the Hubble constant $H_0$ can then be derived. Furthermore, such measurements on a sample of clusters with a range of redshift can potentially be used to discriminate different cosmological models. In this paper, we present statistical studies on the systematic errors in the determination of $H_0$ due to the triaxiality and non-isothermality of clusters of galaxies. Different from many other studies that assume artificially a specific distribution for the intracluster gas, we start from the triaxial model of dark matter halos obtained from numerical simulations. The distribution of the intracluster gas is then derived under the assumption of the hydrodynamic equilibrium. For the equation of state of the intracluster gas, both the isothermal and the polytropic cases are investigated. We run Monte Carlo simulations to generate samples of clusters according to the distributions of their masses, axial ratios, concentration parameters, as well as line-of-sight directions. To mimic observations, the estimation of the Hubble constant is done by fitting X-ray and SZ profiles of a triaxial cluster with the isothermal and spherical $\beta$-model. We find that for a sample of clusters with $M=10^{14}h^{-1}\hbox{M}_{\odot}$ and $z=0.1$, the value of the estimated $H_0$ is positively biased with $H_0^{peak}(estimated)\approx 1.05H_0(true)$ and $H_0^{ave}(estimated)\approx 1.05H_0(true)$ for the isothermal case. For the polytropic case with $\gamma=1.15$, the bias is rather large with $H_0^{peak}(estimated)\approx 1.35H_0(true)$ and $H_0^{ave}(estimated)\approx 3H_0(true)$. (abridged)