Why weak lensing cluster shapes are insensitive to self-interacting dark matter

TL;DR

Weak lensing cluster shapes are nearly indistinguishable between CDM and SIDM models due to projection effects and sensitivity to outer regions, limiting their use in testing dark matter properties.

Contribution

The paper demonstrates that weak lensing measurements of cluster shapes cannot effectively differentiate between CDM and SIDM, highlighting the limitations of lensing in probing dark matter nature.

Findings

Weak lensing shapes are similar for CDM and SIDM clusters.

Projection effects diminish differences in 3D shapes.

Weak lensing is most sensitive to outer cluster regions.

Abstract

We investigate whether the shapes of galaxy clusters inferred from weak gravitational lensing can be used as a test of the nature of dark matter. We analyse mock weak lensing data, with gravitational lenses extracted from cosmological simulations run with two different dark matter models (CDM and SIDM). We fit elliptical NFW profiles to the shear fields of the simulated clusters. Despite large differences in the distribution of 3D shapes between CDM and SIDM, we find that the distributions of weak-lensing-inferred cluster shapes are almost indistinguishable. We trace this information loss to two causes. Firstly, weak lensing measures the shape of the projected mass distribution, not the underlying 3D shape, and projection effects wash out some of the difference. Secondly, weak lensing is most sensitive to the projected shape of clusters, on a scale approaching the virial radius (~ 1.5 Mpc), whereas SIDM shapes differ most from CDM in the inner halo. We introduce a model for the mass distribution of galaxy clusters where the ellipticity of the mass distribution can vary with distance to the centre of the cluster. While this mass distribution does not enable weak lensing data to distinguish between CDM and SIDM with cluster shapes (the ellipticity at small radii is poorly constrained by weak lensing), it could be useful when modelling combined strong and weak gravitational lensing of clusters.