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 less than or similar to z less than or similar to 1.5, using weak and strong lensing observations. Concentration is a steep function of mass, c(200) M-200(-0.83) +/- 0.39, with higher redshift clusters being less concentrated. Mass and concentration from the stacked analysis, M-200 = (4.1 +/- 0.4) x 10(14) M-circle dot h(-1) and c(200) = 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. Overconcentrated clusters might be accreting haloes out of equilibrium in a transient phase of compression, whereas less concentrated ones might be more relaxed.