Reconciling extremely different concentration-mass relations

The concentration-mass relations proposed by Prada et al. (2012) and by Duffy et al. (2008) on the scales of galaxy clusters show some of the largest discrepancies among all the works present in literature. This is surprising because they are both derived from the analysis of dark-matter halos forming in ΛCDM simulations with similar set-ups. With the help of analytic models and numerical simulations we investigate the origin of this discrepancy focusing on the procedures used to derive the concentrations (circular velocity ratios versus density profile fitting) and on the selection criteria used to bin the halos (binning in maximum circular velocity versus binning in mass). We find that both steps of the analysis have a large impact on the resulting c −M relation. In particular, we show that the two c−M relations can be entirely reconciled, if we account for these methodological differences. Our analysis demonstrates that the concentration estimates are sensitive to the largely different radial scales probed by a particular measurement method. This implies that concentrations derived with different techniques (both in observations and in simulations) must be compared over the same radial range