A highly elongated prominent lens at z = 0.87: First strong-lensing analysis of El Gordo

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 zs ∼ 2, each clump exhibits only a small, separate critical area, with a total area of 0.69 ± 0.11□′ over the two clumps. For a higher source redshift, zs ∼ 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□′. The critical curves continue expanding with increasing redshift so that for high-redshift sources (zs ≳ 9) they enclose an area of ∼1.91 ± 0.30□′ (effective θ e ≃ 46.″8 ± 3.″7) and a mass of 6.09 ± 1.04 × 10¹⁴ M . According to our model, the area of high magnification (μ > 10) for such high-redshift sources is ≃1.2□′, and the area with μ > 5 is ≃2.3□′, 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, ≳ 1.7 × 10¹⁵ M from the SL regime alone, suggesting a total mass of roughly M ₂₀₀ ∼ 2.3 × 10¹⁵ M. Our results should be revisited when additional spectroscopic and HST imaging data are available.