SLICE: SPT-CL J0546-5345 -- A prominent strong-lensing cluster at $z=1.07$

Abstract

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 $z_l\sim 0.2-0.7$, with only a few prominent examples at higher redshifts. Here we report a 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 $M_{200,c}=(7.95\pm 0.92)\times 10^{14}{M}_{\odot }$. Using recent JWST/NIRCam and archival HST 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 $\hat{I}{¸}_E=18.1\pm 1.8^{\prime \prime }$ for a source at $z_s=3$, and $\hat{I}{¸}_E=27.9\pm 2.8^{\prime \prime }$ for a source at $z_s=9$. The total projected mass within a $200$ kpc radius around the strong-lensing region is $M(<200\mathrm{kpc})=(1.9\pm 0.3)\times 10^{14}{M}_{\odot }$. 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 $\sim 3-4$ 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.

12 pages, 4 figures, 1 table, published in ApJL (August 29th 2025)