2025ApJ...990L..25A

Massive galaxy clusters act as prominent strong lenses. Due to a combination of observational biases, cluster evolution, and lensing efficiency, most of the known cluster lenses lie typically at z_l ∼ 0.2-0.7, with only a few prominent examples at higher redshifts. Here we report the first strong-lensing analysis of the massive galaxy cluster SPT-CL J0546-5345 at a redshift z_l = 1.07. This cluster was first detected through the Sunyaev-Zel'dovich effect, with a high estimated mass for its redshift of M_200,c = (7.95 ± 0.92) × 10¹⁴M_⊙. Using recent JWST/NIRCam and archival Hubble Space Telescope imaging, we identify at least 10 secure and 6 candidate sets of multiply imaged background galaxies, which we use to constrain the mass distribution in the cluster. We derive effective Einstein radii of θ_E = 18$\mathop{.}\limits^{\unicode{x02033}}$1 ± 1$\mathop{.}\limits^{\unicode{x02033}}$8 for a source at z_s = 3 and θ_E = 27$\mathop{.}\limits^{\unicode{x02033}}$9 ± 2$\mathop{.}\limits^{\unicode{x02033}}$8 for a source at z_s = 9. The total projected mass within a 200 kpc radius around the strong-lensing region is M (<200 kpc) = (1.9 ± 0.3) × 10¹⁴M_⊙. While our results rely on photometric redshifts warranting spectroscopic follow-up, this central mass resembles that of the Hubble Frontier Fields clusters-although SPT-CL J0546-5345 is observed when the Universe was ∼3-4 Gyr younger. Amongst the multiply imaged sources, we identify a hyperbolic-umbilic-like configuration, and, thanks to its point-like morphology, a possible active galactic nucleus (AGN). If confirmed spectroscopically, it will add to just a handful of other quasars and AGN known to be multiply lensed by galaxy clusters.