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

Understand

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.