Precise Mass Determination of SPT-CL J2106-5844, the Most Massive Cluster at z > 1

We present a detailed high-resolution weak-lensing study of SPT-CL.J2106-5844 at z = 1.132, claimed to be the most massive system discovered at z > 1 in the South Pole Telescope Sunyaev-Zel'dovich survey. Based on the deep imaging data from the Advanced Camera for Surveys and Wide Field Camera 3 on board the Hubble Space Telescope, we find that the cluster mass distribution is asymmetric, composed of a main clump and a subclump similar to 640 kpc west thereof. The central clump is further resolved into two smaller northwestern and southeastern substructures separated by similar to 150 kpc. We show that this rather complex mass distribution is more consistent with the cluster galaxy distribution than a unimodal distribution as previously presented. The northwestern substructure coincides with the brightest cluster galaxy and the X-ray peak while the southeastern one agrees with the location of the peak in number density. These morphological features and the comparison with the X-ray emission suggest that the cluster might be a merging system. We estimate the virial mass of the cluster to be M-200c = 10.4(-3.0)(+3.3) +/- 1.0)x 10(14) M-circle dot, where the second error bar is the systematic uncertainty. Our result confirms that the cluster SPT-CL.J2106-5844 is indeed the most massive cluster at z > 1 known to date. We demonstrate the robustness of this mass estimate by performing a number of tests with different assumptions on the centroids, mass-concentration relations, and sample variance.