Head-to-Toe Measurement of El Gordo: Improved Analysis of the Galaxy Cluster ACT-CL J0102-4915 with New Wide-field Hubble Space Telescope Imaging Data

Abstract: We present an improved weak-lensing (WL) study of the high$-z$ $(z=0.87)$ merging galaxy cluster ACT-CL J0102-4915 (El Gordo) based on new wide-field Hubble Space Telescope (HST) imaging data. The new imaging data cover the 3.5$\times$3.5 Mpc region centered on the cluster and enable us to detect WL signals beyond the virial radius, which was not possible in previous studies. We confirm the binary mass structure consisting of the northwestern (NW) and southeastern (SE) subclusters and the 2$\sigma$ dissociation between the SE mass peak and the X-ray cool core. We obtain the mass estimates of the subclusters by simultaneously fitting two Navarro-Frenk-White (NFW) halos without employing mass-concentration relations. The masses are $M_{200c}^{NW} = 9.9^{+2.1}_{-2.2} \times 10^{14} M_{\sun}$ and $M_{200c}^{SE} = 6.5^{+1.9}_{-1.4} \times 10^{14} M_{\sun}$ for the NW and SE subclusters, respectively. The mass ratio is consistent with our previous WL study but significantly different from the previous strong lensing results. This discrepancy is attributed to the use of extrapolation in strong lensing studies because the SE component possesses a higher concentration. By superposing the two best-fit NFW halos, we determine the total mass of El Gordo to be $M_{200c} = 2.13^{+0.25}_{-0.23} \times 10^{15} M_{\sun}$, which is 23% lower than our previous WL result [$M_{200c} =(2.76\pm0.51) \times 10^{15} M_{\sun}$]. Our updated mass is a more direct measurement since we are not extrapolating to $R_{200c}$ as in all previous studies. The new mass is compatible with the current $\Lambda$CDM cosmology.