Measuring the surface mass density ellipticity of redMaPPer galaxy clusters using weak-lensing

TL;DR

This study measures the average projected ellipticity of galaxy clusters using weak-lensing stacking, revealing dependencies on cluster properties and redshift, and comparing inner and outer mass distribution shapes.

Contribution

It provides new measurements of cluster ellipticity using weak-lensing with different proxies and analyzes its relation to redshift and richness, improving understanding of cluster shapes.

Findings

Inner region ellipticity: 0.21 ± 0.04

Higher ellipticity at higher redshifts

Outer region ellipticity varies with redshift

Abstract

In this work we study the shape of the projected surface mass density distribution of galaxy clusters using weak-lensing stacking techniques. In particular, we constrain the average aligned component of the projected ellipticity, $\epsilon$, for a sample of redMaPPer clusters ($0.1 \leq z < 0.4$). We consider six different proxies for the cluster orientation and measure $\epsilon$ for three ranges of projected distances from the cluster centres. The mass distribution in the inner region (up to $700\,$kpc) is better traced by the cluster galaxies with a higher membership probability, while the outer region (from $700\,$kpc up to $5\,$Mpc) is better traced by the inclusion of less probable galaxy cluster members. The fitted ellipticity in the inner region is $\epsilon = 0.21 \pm 0.04$, in agreement with previous estimates. We also study the relation between $\epsilon$ and the cluster mean redshift and richness. By splitting the sample in two redshift ranges according to the median redshift, we obtain larger $\epsilon$ values for clusters at higher redshifts, consistent with the expectation from simulations. In addition, we obtain higher ellipticity values in the outer region of clusters at low redshifts. We discuss several systematic effects that might affect the measured lensing ellipticities and their relation to the derived ellipticity of the mass distribution.