2013ApJ...770L..15Z

We present the first strong-lensing (SL) analysis of the galaxy cluster ACT-CL J0102-4915 (El Gordo), in recent HST/ACS images, revealing a prominent strong lens at a redshift of z = 0.87. This finding adds to the already-established unique properties of El Gordo: it is the most massive, hot, X-ray luminous, and bright Sunyaev-Zeldovich effect cluster at z ≳ 0.6, and the only "bullet"-like merging cluster known at these redshifts. The lens consists of two merging massive clumps, where, for a source redshift of z_s∼ 2, each clump exhibits only a small, separate critical area, with a total area of 0.69±0.11{squ}' over the two clumps. For a higher source redshift, z_s∼ 4, the critical curves of the two clumps merge together into one bigger and very elongated lens (axis ratio ≃ 5.5), enclosing an effective area of 1.44±0.22{squ}'. The critical curves continue expanding with increasing redshift so that for high-redshift sources (z_s ≳ 9) they enclose an area of ∼1.91±0.30{squ}' (effective θ_e≃ 46.''8±3.''7) and a mass of 6.09±1.04x10¹⁴ M_☉. According to our model, the area of high magnification (µ > 10) for such high-redshift sources is ≃1.2{squ}', and the area with µ > 5 is ≃2.3{squ}', making El Gordo a compelling target for studying the high-redshift universe. We obtain a strong lower limit on the total mass of El Gordo, ≳ 1.7x10¹⁵ M_☉ from the SL regime alone, suggesting a total mass of roughly M₂₀₀∼ 2.3x10¹⁵ M_☉. Our results should be revisited when additional spectroscopic and HST imaging data are available.