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 \raise 0.817 pt \hbox { $ \scriptstyle \sim$ } 1000 deg ^ { 2 } survey .
Our weak-lensing analysis confirms that ACT-CL J0102 - 4915 is indeed an extreme system consisting of two massive ( \gtrsim 10 ^ { 15 } M _ { \sun } each ) subclusters with a projected separation of \raise 0.817 pt \hbox { $ \scriptstyle \sim$ } 0.7 h _ { 70 } ^ { -1 } 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 ( MCMC ) analysis shows that the masses of the northwestern ( NW ) and the southeastern ( SE ) components are M _ { 200 c } = ( 1.38 \pm 0.22 ) \times 10 ^ { 15 } h _ { 70 } ^ { -1 } M _ { \sun } and ( 0.78 \pm 0.20 ) \times 10 ^ { 15 } h _ { 70 } ^ { -1 } M _ { \sun } , respectively , where the quoted errors include only 1 \sigma 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 1133 _ { -61 } ^ { +58 } ~ { } \mbox { km } ~ { } \mbox { s } ^ { -1 } and 1064 _ { -66 } ^ { +62 } ~ { } \mbox { km } ~ { } \mbox { s } ^ { -1 } for the NW and SE components , respectively , which are consistent with their spectroscopic measurements ( 1290 \pm 134 ~ { } \mbox { km } ~ { } \mbox { s } ^ { -1 } and 1089 \pm 200 ~ { } \mbox { km } ~ { } \mbox { s } ^ { -1 } , respectively ) .
The centroids of both components are tightly constrained ( \raise 0.817 pt \hbox { $ \scriptstyle \sim$ } 4 \arcsec ) and close to the optical luminosity centers .
The X-ray and mass peaks are spatially offset by \raise 0.817 pt \hbox { $ \scriptstyle \sim$ } 8 \arcsec ( \raise 0.817 pt \hbox { $ \scriptstyle \sim$ } 100 h _ { 70 } ^ { -1 } kpc ) , which is significant at the \raise 0.817 pt \hbox { $ \scriptstyle \sim$ } 2 \sigma 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 r _ { 200 a } = 2.4 h _ { 70 } ^ { -1 } Mpc yields a combined mass of M _ { 200 a } = ( 3.13 \pm 0.56 ) \times 10 ^ { 15 } h _ { 70 } ^ { -1 } M _ { \sun } .
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 .