A Detection of Dark Matter Halo Ellipticity using Galaxy Cluster Lensing in SDSS

TL;DR

This study uses SDSS galaxy cluster lensing data to measure dark matter halo ellipticity, finding it to be significantly non-spherical and consistent with galaxy distribution ellipticity.

Contribution

It provides the first direct measurement of dark matter halo ellipticity using gravitational lensing in SDSS clusters, confirming non-sphericity.

Findings

Dark matter halos have an axis ratio of approximately 0.48.

Spherical halos (axis ratio=1) are ruled out at 99.6% confidence.

Galaxy distribution ellipticity matches dark matter halo ellipticity.

Abstract

We measure the ellipticity of isolated clusters of galaxies in the Sloan Digital Sky Survey (SDSS) using gravitational lensing. We stack the clusters, rotating so that the major axes of the ellipses determined by the positions of cluster member galaxies are aligned. We exclude the signal from the central 0.5 h^-1 Mpc to avoid problems with stacking alignment and cluster member contamination. We fit an elliptical NFW profile and find an axis ratio for the dark matter of f = b/a = 0.48+0.14-0.09 (1 sigma), and rule out f=1 at 99.6 per cent confidence thus ruling out a spherical halo. We find that the ellipticity of the cluster galaxy distribution is consistent with being equal to the dark matter ellipticity. The results are similar if we change the isolation criterion by 50 per cent in either direction.