An improved analysis of the galaxy cluster ACT-CL J0102-4915 with new wide-field HST imaging data

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. El Gordo is undergoing a merging event showing two distinctive mass clumps and radio relics. El Gordo has also known to be the most massive system at z > 0.6 and a rare cluster for its redshift in the current ΛCDM cosmology. The previous gravitational lensing studies revealed a clear bimodal mass structure and its high mass estimates. However, their mass measurements relied on extrapolation because the previous HST observation did not extend to the virial radius of the cluster. In this talk, we present a more accurate mass estimate of the cluster using WL analysis from new wide-field HST imaging data. The new imaging data cover the ~3.5 × ~3.5 Mpc region centered on the cluster and enable us to detect WL signals beyond the virial radius. While confirming the binary mass structure and ~2σ dissociation between the southeastern mass peak and the X-ray cool core, we find that the new data yield a ~24% lower mass for the entire system compared with our previous WL study. The mass ratio of the two subclusters 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. We discuss the rarity of the cluster in the ΛCDM paradigm and the merger scenario.