The mass–concentration relation in massive galaxy clusters at redshift ∼1

Mass and concentration of clusters of galaxies are related and evolving with redshift. We study the properties of a sample of 31 massive galaxy clusters at high redshift, 0.8 ∼ z ∼ 1.5, using weak and strong lensing observations. Concentration is a steep function of mass, c200 ∝ M−0.83±0.39200, with higher-redshift clusters being less concentrated. Mass and concentration from the stacked analysis, M200 = (4.1 ± 0.4) × 1014M/h and c200 = 2.3 ± 0.2, are in line with theoretical results extrapolated from the local universe. Clusters with signs of dynamical activity preferentially feature high concentrations. We discuss the possibility that the whole sample is a mix of two different kinds of haloes. Over-concentrated clusters might be accreting haloes out of equilibrium in a transient phase of compression, whereas less concentrated ones might be more relaxed.