Comparison of hydrostatic and dynamical masses of distant X-ray luminous galaxy clusters

Abstract

The main goal of this work is to compare the results of three dynamical mass estimators to the X-ray hydrostatic values, focussing on massive galaxy clusters at intermediate redshifts $z\sim 0.3$. We estimated dynamical masses with the virial theorem, the Jeans equation, and the caustic method using wide-field VIMOS spectroscopy. We investigated the role of colour selection and the impact of substructures on the dynamical estimators. The Jeans and caustic methods give consistent results, whereas the virial theorem leads to masses $\sim 15\%$ larger. The Jeans, caustic, and virial masses are respectively $\sim 20\%$, $\sim 30\%$, and $\sim 50\%$ larger than the hydrostatic values. Large scatters of $\gtrsim 50\%$ are mainly due to the two outliers RXCJ0014 and RXCJ1347; excluding the latter increases the mass ratios by $\sim 10\%$, giving a fractional mass bias significant at $\gtrsim 2\sigma$. We found a correlation between the dynamical-to-hydrostatic mass ratio and two substructure indicators, suggesting a bias in the dynamical measurements. The velocity dispersions of blue galaxies are $\sim 15\%$ ($\sim 25\%$ after removing the substructures) larger than that of the red-sequence galaxies; using the latter leads to dynamical masses $\sim 10\%-15\%$ smaller. Discarding the galaxies part of substructures reduces the masses by $\sim 15\%$; the effect is larger for the more massive clusters, owing to a higher level of substructures. After the substructure analysis, the dynamical masses are in perfect agreement with the hydrostatic values and the scatters around the mean ratios are divided by $\sim 2$.

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