The dark matter haloes of massive, relaxed galaxy clusters observed with Chandra

Abstract

We use the Chandra X-ray Observatory to study the dark matter haloes of 34 massive, dynamically relaxed galaxy clusters, spanning the redshift range 0.06 < z < 0.7. The observed dark matter and total mass (dark-plus-luminous matter) profiles can be approximated by the Navarro-Frenk-White (hereafter NFW) model for cold dark matter (CDM) haloes; for 80 per cent of the clusters, the NFW model provides a statistically acceptable fit. In contrast, the singular isothermal sphere model can, in almost every case, be completely ruled out. We observe a well-defined mass-concentration relation for the clusters with an intrinsic scatter in good agreement with the predictions from simulations. The slope of the mass-concentration relation, c M a vir /(1 + z) b with a = -0.45 0.12 at 95 per cent confidence, is steeper than the value a -0.1 predicted by CDM simulations for lower mass haloes. With the slope a included as a free fit parameter, the redshift evolution of the concentration parameter, b = 0.71 0.52 at 95 per cent confidence, is consistent with the same simulations (b 1). Fixing a -0.1 leads to an apparent evolution that is significantly slower, b = 0.30 0.49, although the goodness of fit in this case is significantly worse. Using a generalized NFW model, we find the inner dark matter density slope, , to be consistent with unity at 95 per cent confidence for the majority of clusters. Combining the results for all clusters for which the generalized NFW model provides a good description of the data, we measure = 0.88 0.29 at 95 per cent confidence, in agreement with CDM model predictions.