Mock weak lensing analysis of simulated galaxy clusters: bias and scatter in mass and concentration
Abstract
(Abridged) We quantify the bias and scatter in galaxy cluster masses M200 and concentrations c derived from a mock weak gravitational lensing (WL) survey, and their effect on the cluster mass-concentration relation. For this, we simulate WL distortions on a population of background galaxies due to a large (approx. 3000) sample of galaxy cluster haloes extracted from the Millennium Simulation at z=0.2. While the normalisation of the derived M200-c relation agrees with the underlying true relation the slope is marginally too steep, owing to a bias in WL-derived concentrations that decreases from +11% for M200 < 10^14.1 Msun to -5% for M200 > 10^14.8 Msun. There is also a small bias in the derived mass (approx. -6%), whose magnitude depends only very weakly on true halo mass. Both the mass and concentration derived from WL show considerable scatter about their true values; this decreases slightly over the above mass range (from ~55% to ~30% for concentration and from ~35% to ~20% for mass). The scatter and bias in mass are shown to be largely due to inaccurate modelling of the matter distribution outside the virial radius (i.e., they reflect a departure of the true WL shear/matter distribution of the simulated clusters from the NFW distribution adopted in modelling the mock observations). We propose a very simple modification to the standard modelling methodology which can reduce the mass bias to the level +/-3%. Errors due to the intrinsic shape and shot noise of background galaxies are found to be only a minor contributor to the scatter for M200 > 10^{14.1} Msun. Orientation of the triaxial cluster haloes dominates the concentration scatter, while the bias in c (and its mass dependence) is due to substructure within the virial radius. Our results highlight both the need and possibility to properly account for bias (and scatter) in large samples of galaxy clusters studied with WL.Keywords
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