Measuring σ₈ with Cluster Lensing: Biases from Unrelaxed Clusters.

Smith, Graham P.; Edge, Alastair C.; Eke, Vincent R.; Nichol, Robert C.; Smail, Ian; Kneib, Jean-Paul

Abstract

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.

Citation

Smith, G. P., Edge, A. C., Eke, V. R., Nichol, R. C., Smail, I., & Kneib, J. (2003). Measuring σ₈ with Cluster Lensing: Biases from Unrelaxed Clusters. Astrophysical Journal, 590(2), L79-L82. https://doi.org/10.1086/376747

• Journal Article Type: Article
• Publication Date: 2003-06
• Journal: Astrophysical Journal
• Print ISSN: 0004-637X
• Electronic ISSN: 1538-4357
• Publisher: American Astronomical Society
• Peer Reviewed: Peer Reviewed
• Volume: 590
• Issue: 2
• Pages: L79-L82
• DOI: https://doi.org/10.1086/376747