Direct measurement of dark matter halo ellipticity from two-dimensional lensing shear maps of 25 massive clusters

We present new measurements of dark matter distributions in 25 X-ray luminous clusters by making a full use of the two-dimensional (2D) weak lensing signals obtained from high-quality Subaru/Suprime-Cam imaging data. Our approach to directly compare the measured lensing shear pattern with elliptical model predictions allows us to extract new information on the mass distributions of individual clusters, such as the halo ellipticity and mass centroid. We find that these parameters on the cluster shape are little degenerate with cluster mass and concentration parameters. By combining the 2D fitting results for a subsample of 18 clusters, the elliptical shape of dark matter haloes is detected at 7\sigma significance level. The mean ellipticity is found to be e = 0.46 \pm 0.04 (1\sigma), which is in excellent agreement with the standard collisionless CDM model prediction. The mass centroid can be constrained with a typical accuracy of ~20" (~50 kpc/h) in radius for each cluster with some significant outliers, enabling to assess one of the most important systematic errors inherent in the stacked cluster weak lensing technique, the mass centroid uncertainty. In addition, the shape of the dark mass distribution is found to be only weakly correlated with that of the member galaxy distribution. We carefully examine possible sources of systematic errors in our measurements, finding none of them to be significant. Our results demonstrate the power of high-quality imaging data for exploring the detailed spatial distribution of dark matter (Abridged).