A DETECTION OF DARK MATTER HALO ELLIPTICITY USING GALAXY CLUSTER LENSING IN THE SDSS

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 Navarro-Frenk-White (NFW) profile and find a projected, two-dimensional axis ratio for the dark matter of f = b/a = 0.48(-0.09)(+0.14) (1 sigma), and rule out f = 1 at 99.6% 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% in either direction.