On the over-concentration problem of strong lensing clusters

TL;DR

This paper investigates the apparent over-concentration of strong lensing galaxy clusters by analyzing their shapes and orientations, finding that most align with theoretical expectations, while some outliers may be due to unlikely filamentary structures.

Contribution

It provides a detailed analysis of cluster shapes and orientations, clarifying the discrepancy between observations and predictions in strong lensing clusters.

Findings

Nearly half of the clusters are outliers, possibly due to unlikely filamentary structures.

Most clusters are consistent with N-body simulation predictions of mild triaxiality and orientation bias.

The study offers a method to infer intrinsic shapes from lensing data.

Abstract

Lambda cold dark matter paradigm predicts that galaxy clusters follow an universal mass density profile and fit a well defined mass-concentration relation, with lensing clusters being preferentially triaxial haloes elongated along the line of sight. Oddly, recent strong and weak lensing analyses of clusters with a large Einstein radius suggested those haloes to be highly over-concentrated. Here, we investigate what intrinsic shape and orientation an halo should have to account for both theoretical predictions and observations. We considered a sample of 10 strong lensing clusters. We first measured their elongation assuming a given mass-concentration relation. Then, for each cluster we found the intrinsic shape and orientation which are compatible with the inferred elongation and the measured projected ellipticity. We distinguished two groups. The first one (nearly one half) seems to be composed of outliers of the mass-concentration relation, which they would fit only if they were characterised by a filamentary structure extremely elongated along the line of sight, that is not plausible considering standard scenarios of structure formations. The second sample supports expectations of N-body simulations which prefer mildly triaxial lensing clusters with a strong orientation bias.