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Durham University

Department of Physics

Staff profile

Publication details for Professor Ian Smail

Smith, Graham P., Edge, Alastair C., Eke, Vincent R., Nichol, Robert C., Smail, Ian & Kneib, Jean-Paul (2003). Measuring σ8 with Cluster Lensing: Biases from Unrelaxed Clusters. Astrophysical Journal 590(2): L79-L82.

Author(s) from Durham


We use gravitational lens models and X-ray spectral analysis of 10 X-ray-luminous galaxy clusters at z sime 0.2 to study the impact of cluster substructure on attempts to normalize the matter power spectrum. We estimate that unrelaxed clusters are 30% hotter than relaxed clusters, causing σ8 to be overestimated by 20% if the cluster selection function is not accounted for. This helps to explain the wide range in σ8 derived from different techniques, σ8 ~ 0.6-1, and offers a physically motivated explanation for some of the discrepancy. We identify two further systematics: (1) the extrapolation of small field-of-view mass measurements to cluster virial radii and (2) the projection of three-dimensional mass estimates from n-body simulations to match two-dimensional observables. We quantify these effects and estimate from the current data that σ8 = 0.75 ± 0.05(statistical) ± 0.15(systematic), where the systematic error reflects the extrapolation and projection uncertainties, and we assume ΩM = 0.3 and ΩΛ = 0.7. All three systematics (substructure, extrapolation, and projection) are fundamental to future cluster-based measurements of σ8 regardless of the techniques employed. We identify gravitational lensing as the tool of choice for such studies because a combination of strong and weak lensing offers the most direct route to control the systematics and thus achieve an unbiased comparison between observation and theory.