Weighing "el Gordo" with a precision scale: Hubble Space Telescope weak-lensing analysis of the merging galaxy cluster ACT-CL J0102-4915 at z = 0.87

We present a Hubble Space Telescope weak-lensing study of the merging galaxy cluster "El Gordo" (ACT-CL J0102-4915) at z = 0.87 discovered by the Atacama Cosmology Telescope (ACT) collaboration as the strongest Sunyaev-Zel'dovich decrement in its 1000 deg² survey. Our weak-lensing analysis confirms that ACT-CL J0102-4915 is indeed an extreme system consisting of two massive (≳ 10¹⁵ M each) subclusters with a projected separation of Mpc. This binary mass structure revealed by our lensing study is consistent with the cluster galaxy distribution and the dynamical study carried out with 89 spectroscopic members. We estimate the mass of ACT-CL J0102-4915 by simultaneously fitting two axisymmetric Navarro-Frenk-White (NFW) profiles allowing their centers to vary. We use only a single parameter for the NFW mass profile by enforcing the mass-concentration relation from numerical simulations. Our Markov-Chain-Monte-Carlo analysis shows that the masses of the northwestern (NW) and the southeastern (SE) components are and , respectively, where the quoted errors include only 1σ statistical uncertainties determined by the finite number of source galaxies. These mass estimates are subject to additional uncertainties (20%-30%) due to the possible presence of triaxiality, correlated/uncorrelated large scale structure, and departure of the cluster profile from the NFW model. The lensing-based velocity dispersions are and for the NW and SE components, respectively, which are consistent with their spectroscopic measurements (1290 ± 134 km s^-1 and 1089 ± 200 km s^-1, respectively). The centroids of both components are tightly constrained (4″) and close to the optical luminosity centers. The X-ray and mass peaks are spatially offset by 8″ ( kpc), which is significant at the 2σ confidence level. The mass peak, however, does not lead the gas peak in the direction expected if we are viewing the cluster soon after first core passage during a high speed merger. Under the assumption that the merger is happening in the plane of the sky, extrapolation of the two NFW halos to a radius Mpc yields a combined mass of . This extrapolated total mass is consistent with our two-component-based dynamical analysis and previous X-ray measurements, projecting ACT-CL J0102-4915 to be the most massive cluster at z > 0.6 known to date.