arXiv subjects

Gavin Leroy

Publications and source records attributed to Gavin Leroy.

At least 19 recordsLinked to original sources

arXiv2026

3D dark matter map of the JWST COSMOS-Web field using diffusion-based generative priors

Mapping the three-dimensional distribution of dark matter is key to tracing the growth of cosmic structure. Two-dimensional weak lensing mass maps recover its projected distribution, but they blend structures at different distances along the line of sight. We reconstruct the three-dimensional matter distribution in the 0.54 deg$^2$ COSMOS-Web field using deep James Webb Space Telescope imaging, with an effective source density of 121 galaxies arcmin$^{-2}$ We combine a physical weak lensing forward model with a diffusion-based Bayesian generative prior learned from cosmological simulations, yielding a posterior distribution of the matter field across 20 lens planes to $z \simeq 2$, doubling the redshift reach of previous Hubble Space Telescope-based 3D reconstructions and approaching cosmic noon, an epoch of rapid galaxy growth. The reconstruction separates structures that overlap in projection and recovers non-Gaussian matter structure across a range of spatial scales, including over-densities associated with independently identified galaxy clusters that were not detected in earlier weak lensing maps. Posterior sampling quantifies the range of three-dimensional matter distributions allowed by the data, enabling a probabilistic visualization of the cosmic web while propagating uncertainties in its inferred structure. Null and simulation-based tests show that the recovered structures are supported by the lensing signal and are consistent with expectations from the fiducial $Λ$CDM cosmology. These results extend three-dimensional weak lensing mapping into the high-redshift, non-Gaussian regime and establish a route towards probabilistic mapping of the evolving cosmic web with current wide-field lensing surveys.

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arXiv2026

Lensing in the Blue IV: The First Weak Gravitational Lensing Maps from the Stratosphere

The Super-Pressure Balloon-borne Imaging Telescope (SuperBIT) completed its science flight in April 2023, obtaining imaging data for 30 galaxy clusters. In this work, we present the first weak-lensing convergence maps from these observations, using six clusters selected from the 30 targets. We construct background-galaxy catalogs using SuperBIT's three-band photometry and a color-color-based method to separate foreground and background galaxies, and use the calibrated galaxy shapes presented in Lensing in the Blue III to reconstruct the weak-lensing convergence field using Kaiser-Squires inversion. We compare the convergence signal-to-noise maps for the six clusters with independent tracers, including X-ray emission and red-sequence cluster-member number-density maps. These results demonstrate the first successful weak-lensing mass mapping with imaging from a sub-orbital astronomical observatory.

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arXiv2026

X-ray Characterization of Galaxy Groups and Hickson Groups in COSMOS-Web to z~3.8 I. X-ray Luminosity Function Evolution

Galaxy groups are the dominant halo environment in the Universe and a sensitive laboratory for feedback and gas-cycling physics, yet a uniform, intermediate-redshift X-ray characterization of the group population has been limited by the availability of deep, wide-area imaging. We present a uniform X-ray characterization of the COSMOS-Web galaxy group population: 1,678 groups (CW-All) identified with the AMICO algorithm applied to the JWST-selected COSMOS-Web photometric catalog ($0.08 \lesssim z \lesssim 3.7$), and a parallel sample of 993 Hickson compact groups (CW-HCG; $0.08 \lesssim z \lesssim 3.82$) over $0.45~\mathrm{deg}^2$. We apply a uniform aperture-photometry X-ray pipeline to the combined Chandra and XMM-Newton COSMOS mosaic, measuring fluxes, luminosities, temperatures, and halo masses and radii for every group, together with detections, upper limits, and quality flags. We detect X-ray emission from 499 CW-All groups (29.7\%) and 332 CW-HCG groups (33.4\%) at SNR~$\geq 2.0$, and present the X-ray luminosity function in four redshift bins. The characteristic luminosity $L^\star$ rises monotonically by $\sim$1.9--2.1~dex from the lowest to highest redshift bin in both catalogs, a trend that persists under two independent cross-check binning schemes and is not explained by mass or richness selection of the detected population. A targeted search for diffuse X-ray emission from the AmicOne protocluster candidate at $2.50 \lesssim z < 3.09$ finds no statistically significant signal at the current depth. We publicly release the X-ray measurements and derived properties for both catalogs.

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arXiv2026

SLICE: Unveiling the Multi-Component Merging Core of SPT-CLJ1150-2805 Through Strong-Lensing Mass Modelling

We present a new strong-lensing and X-ray mass model of the merging galaxy cluster SPT-CLJ1150-2805 ($z=0.383$), combining JWST NIRCam, HST, Chandra, and VLT/MUSE data to robustly decompose its collisionless and collisional components. Exploiting JWST, we expand the multiple-image catalogue with 66 new images (30 comprising 6 new systems). Our baseline model uses 118 spectroscopically confirmed images from 20 systems, achieving an image-plane root-mean-square offset of $0.56''$. An expanded model incorporating 31 additional photometric images from 8 additional systems yields a near-identical mass distribution, confirming excellent model stability. We resolve a complex multi-axis post-merger system requiring five cluster-scale dark matter haloes, including a bimodal core with distinct haloes coincident with each brightest cluster galaxy. The remaining mass components are independently supported by kinematics of 291 cluster members ($σ_v=1960^{+84}_{-92}$ km s$^{-1}$), which reveal two new galaxy substructures - the northernmost of which we tentatively identify as the baryonic counterpart to our fifth dark matter halo. The cluster is an extraordinarily efficient lens, possessing an effective Einstein radius $θ_E=42.41^{+0.05}_{-0.04}{''}$ and an enclosed mass $M(<θ_E)=3.35^{+0.04}_{-0.03}\times10^{14}M_\odot$ for a source at $z_s=2$. For a high-redshift source at $z_s=9$, it produces a magnification cross-section of $A(>10)\simeq0.15$ arcmin$^2$ and $A(>30)\simeq0.020$ arcmin$^2$. This global lensing power surpasses all Hubble Frontier Fields clusters. In a regime where systematic uncertainties between mass models dominate high-redshift magnification errors, the stability of our reconstruction sets this lens apart. This combination of extreme lensing power and a robust mass model establishes SPT-CLJ1150-2805 as a premier cosmic telescope for high-redshift studies.

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arXiv2026

SMART : Spherical Mass ApeRture Toolkit

The aperture mass filter is a widely used tool in weak lensing studies. While previous surveys have typically relied on the flat-sky approximation, forthcoming Stage IV surveys such as Euclid will cover sufficiently large areas to require the construction of curved-sky aperture mass maps. In this paper, we extend the shear-based aperture mass formalism to the sphere and introduce a software package for a fast and precise computation of spherical aperture mass maps from the shear field, defined directly on the celestial sphere. Unlike existing approaches, our method does not rely on planar projections and does not require the reconstruction of convergence maps. The software provides four different implementations, all of which are independent of the choice of aperture mass filter function. Starting from the brute-force approach, whose computational cost renders it impractical for future surveys covering more than 10,000 deg2, we investigate alternative galaxy-grouping strategies to improve computational efficiency. We demonstrate speed-up factors of up to 40 at a resolution of NSIDE = 4096 while achieving significantly higher precision than existing state-of-the-art methods. In particular, we show that the spherical shear-based implementations using a pixel-based approach outperforms the existing methods in terms of both precision and computational time. Finally, we show that, for weak-lensing cluster detection, the cluster sample produced using the spherical shear-based aperture mass method consistently outperforms those obtained using projected shear-based approaches in the purity-completeness plane, while simplifying the resulting selection function. Our spherical mass aperture software, called SMART, is made available on GitHub.

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arXiv2026

Radiation damage to the Hubble Space Telescope has been several years out of phase with the Solar cycle

As well as obtaining beautiful images of the Universe, the Hubble Space Telescope's CCD detectors are sensitive radiation dosimeters that have been monitored in Low Earth Orbit for more than 24 years. The rate of radiation damage they received has varied over each Solar cycle, but several years out of phase with the appearance of sunspots or coronal mass ejections. We investigate functional forms that successfully fit the time series of damage to telescopes elsewhere in the Solar system. We obtain remarkably accurate fits to Hubble data but with physically absurd parameter values. During image post-processing, such fits can be used empirically, to correct more than 99.5% of the radiation damage's effect on image quality. However, fits to the time series with physically reasonable parameters produce worse performance. Our results highlight the diversity of radiation environments in different parts of our Solar system, and the complexity of Low Earth Orbit in particular. Our results also motivate continued monitoring of radiation damage to currently operational spacecraft, to more reliably predict the rate of degradation in (and useful lifespan of) future missions.

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arXiv2026

Multi-sensor fusion for fine-guidance and milliarcsecond-level attitude estimation of balloon-borne telescope

Balloon-borne telescopes rely on fine-guidance systems to achieve milliarcsecond image stability despite residual disturbances from the balloon environment. In these systems, the Fast Steering Mirror (FSM) stabilizes the image in two focal-plane axes, but leaves systematic, field-dependent residual motion induced by boresight roll. This effect, referred to as roll leakage, becomes more important for wider fields of view. In this work, roll leakage is characterized using data from the 2023 Superpressure Balloon-borne Imaging Telescope (SuperBIT) science flight. SuperBIT is a 0.5-m near-ultraviolet to near-infrared telescope that demonstrated milliarcsecond-level image stability during its 45-night science flight. We find that passive focal-plane star-camera measurements correlate strongly with independent roll measurements across a large set of science exposures, showing that boresight roll frequently drives residual focal-plane motion. We then develop a simulation framework combining optical ray tracing, asynchronous guide-star measurements, estimation, and FSM control to study this behavior. The framework is used to compare single-star and multi-star guidance architectures under realistic flight disturbances. For the SuperBIT geometry, we find that multi-star estimation reduces average roll-induced science-field image motion by 31.8%, increasing to 77.4% for a representative geometry of GigaBIT, SuperBIT's planned larger-aperture successor. These results motivate further investigation of multi-star fine-guidance architectures for GigaBIT.

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arXiv2026

The impact of cosmic filaments on starburst galaxies across cosmic times

Cosmological simulations suggest that various galaxy properties depend on their location within the cosmic web. Yet direct observational evidence of the dependence of star formation activity on distance to filaments remains scarce and is missing at z>1. We investigate how starburst, main-sequence (MS), and quenched galaxies are distributed with respect to cosmic web filaments, and how this distribution evolves with redshift. We first use the SIMBA cosmological simulation to predict the redshift evolution of the mean distance to the closest filament from z=3 to z=0 for different galaxy populations after removing stellar-mass dependencies. We then measure the corresponding signal in the COSMOS field, using COSMOS2020 and COSMOS-Web data, where accurate photometric redshifts enable reconstruction of the projected cosmic web from z=2 to z=0.5, and starbursts are identified through far-infrared spectral energy distribution fitting. In agreement with the results from SIMBA, starburst galaxies are found closer to filaments at z>1 and at larger distances at z<1, MS galaxies occupy intermediate environments with little evolution, and quenched galaxies show progressively shorter distances to filaments toward low redshift, with a crossing between starburst and MS populations around z~1. In COSMOS-Web, the relative evolution in the average distance to filaments between starburst and MS galaxies is detected at a significance level of at least 5σ. We show that a minimal toy model in which the only environmental ingredient is the sSFR-filament distance modulation measured in simulations is sufficient to reproduce the observed differential evolution of the average filament distance between starburst and MS galaxies. These results show that the imprint of large-scale environmental effects on the star formation activity of galaxies, predicted by simulations, is detectable from z=2 down to z=0.5.

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arXiv2026

COSMOS-Web: Galaxy Size and Surface Brightness Evolution at Rest-Frame 1.22 $μ$m Since $z=3$

We present the evolution of galaxy size and surface brightness in the rest-frame $J$ band (1.22 $μ$m), tracing the stellar mass distribution, over $0.5 \leq z \leq 3$, using a sample of 15,420 galaxies with stellar masses $M_\star=10^{10}$-$10^{11.5}\ M_{\odot}$ from the JWST COSMOS-Web survey. The rest-frame $J$-band effective radius ($R_{e,J}$) is obtained from previous measurements and mapped from the available JWST/NIRCam filters, while the surface brightness ($μ_J$) is corrected for dust extinction and cosmological dimming. At a characteristic mass of $M_\star = 5 \times 10^{10}\ M_{\odot}$, star-forming galaxies exhibit a size evolution of $R_{e,J} \propto (1+z)^β$ with $β= -0.92 \pm 0.04$, falling between previously reported shallower and steeper measurements. Quiescent galaxies evolve more rapidly, with $β= -1.34 \pm 0.05$, consistent with earlier studies. Among star-forming galaxies, lower-mass systems ($10^{10}$ to $10^{10.5}\ M_{\odot}$) show slower ($β=-0.66\pm0.02$) size evolution compared to their higher-mass counterparts. Furthermore, the surface brightness brightens toward higher redshifts, scaling as $μ_J \propto -2.5 \log(1+z)^γ$. We find $γ= 3.07 \pm 0.08$ for star-forming galaxies and $γ= 3.70 \pm 0.08$ for quiescent galaxies. We also find that massive star-forming galaxies ($M_\star > 10^{10.5}\ M_{\odot}$) exhibit similar $μ_J$ values at fixed redshift, independent of mass. Finally, we demonstrate that the observed surface brightness evolution is driven by the combined evolution of galaxy luminosity and size.

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arXiv2026

COSMOS-Web galaxy groups: Evolution of red sequence and quiescent galaxy fraction

We investigate the redshift and group richness dependence of the quiescent fraction and red-sequence (RS) parameters in COSMOS galaxy groups from z=0 to z=3.7. We analyzed the deep and well-characterized sample of groups detected with AMICO in the COSMOS(-Web) field. Our study of the quiescent galaxy population is based on a machine-learning classification tool based on rest-frame magnitudes. The algorithm learns from several traditional methods to estimate the probability of a galaxy being quiescent, achieving high precision and recall. Starting from this classification, we computed quiescent galaxy fractions within groups via two methods: one based on the membership probabilities provided by AMICO, which rely on an analytical model, and another using a model-independent technique. We then detected the RS by estimating the ridgeline position using photometric data, followed by sigma clipping to remove outliers. This analysis was performed using both rest-frame and observed-frame magnitudes with rest-frame matching. We compared the results from both approaches and investigated the $z$ and richness dependence of the RS parameters. We found that the quiescent galaxy population in groups builds up steadily from z=1.5-2 across all richnesses, with faster and earlier growth in the richest groups. The first galaxies settle onto the RS ridgeline by $z \sim 2$, consistent with current evolutionary scenarios. Notably, we reported a rare overdensity of quiescent galaxies at z=3.4, potentially one of the most distant early RSs observed. Extending our study to X-rays, we found that X-ray faint groups have, on average, lower quiescent fractions than X-ray bright ones, likely reflecting their typical location in filaments. Leveraging the broad wavelength coverage of COSMOS2025, we traced RS evolution over $\sim 12$ Gyr, finding no significant trends in either slope or scatter of the ridgeline.

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arXiv2026

SLICE -- Combining Strong Lensing and X-ray in AC 114. Further Insights into the Merger Scenario

AC114 is a historically significant galaxy cluster, being one of the first strong lensing clusters detected from the ground in the early 1990s, prior to the launch of the HST. Despite this early prominence, no detailed lensing analyses have been carried out for more than fifteen years. We here study this cluster using JWST imaging obtained as part of the SLICE program, complemented by archival HST and X-ray observations. JWST data reveal ten new multiply imaged systems and enable the identification of conjugate substructures in several of the sixteen systems, significantly increasing the number of strong lensing constraints. Using these data, we construct a parametric mass model with Lenstool and extend it by explicitly incorporating the Chandra data in a combined strong lensing+X-ray fit. Our best-fit model reproduces the multiple images with an RMS of 0.4" while simultaneously matching the X-ray data. The dark matter distribution is unimodal and centered on the brightest cluster galaxy, with a large core radius of 83+-5kpc, consistent with values reported in other strong lensing clusters. The strong lensing constraints require the inclusion of an external shear component which position angle points unambiguously towards a nearby (~1Mpc), well defined mass concentration at the same redshift in the North-West, for which we propose the naming AC114b. The spatial coverage of the XMM-Newton data encompasses the whole structure, allowing us to probe the X-ray properties of the companion cluster and the thermodynamics of AC114, providing evidence for a major merger, in line with previous signatures seen in Chandra, radio and optical spectroscopic data. Our results shed new light on the merging scenario, revealing a major merger caught in a late post-collisional phase, where AC114 is the dominant system and Ac114b has likely been stripped of its hot gas.

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arXiv2026

Lensing in the Blue III: Weak Lensing Shape Catalogs of 30 Merging Galaxy Clusters

We present the weak gravitational lensing dataset from the Super-pressure Balloon-Borne Imaging Telescope (SuperBIT), which imaged 30 galaxy clusters during its 45 night flight in April to May 2023. SuperBIT is a first-of-its-kind balloon-borne imaging telescope that achieved near diffraction-limited observations in near-space conditions above 98% of the Earth's atmosphere. We use the metacalibration algorithm to obtain calibrated galaxy shapes for our target clusters and several calibration fields, enabling unbiased reconstruction of the weak-lensing signal. We employ several diagnostics throughout the pipeline, including assessments of point-spread function (PSF) modeling residuals and their impact on weak-lensing measurements, as well as tests for correlations between galaxy shapes and measured galaxy and PSF properties. To assess the multiplicative shear bias of the pipeline, we analyze a parallel set of simulated images that incorporate the real observing conditions from the flight, including measured SuperBIT PSFs, observed sky backgrounds, and detector noise, yielding a bias of $(1.1 \pm 7.8)$~per~cent.

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arXiv2026

A Kiloparsec-Scale Stellar Cavity in the Center of Abell402-BCG May be Caused by Dynamic Interactions with an Ultramassive Black Hole

We present new observations from JWST NIRCam that reveal a striking kpc-wide cavity in the stellar distribution of the central galaxy in the cluster Abell402. Supporting data from HST allow us to rule out extinction due to dust as an explanation and, instead, suggest that this is a localized depression in the stellar density field corresponding to ~2x10^9 Msun in missing stars within a volume of 0.5kpc^3. On larger scales, both the JWST and HST data show evidence for a 2.2kpc flattened core in the stellar distribution (on which the smaller-scale cavity is superimposed), which implies the presence of a central ultra-massive black hole with M_BH = 6 +/- 4 x10^10 Msun. We report evidence for a mid-IR-bright point source at one edge of the cavity, suggesting that this black hole is actively accreting. MUSE spectroscopy reveal that this source is a LINER AGN and that there is a second candidate AGN on the opposite side of the cavity with a relative velocity of 370km/s -- if real, this implies the presence of a kpc-separation dual AGN with a total binary mass of 6 +/- 2 x10^10 Msun, which would make this the most massive binary black hole system discovered to date. We propose that this unique stellar cavity is the result of a short-lived dynamical interaction between at least one supermassive black hole and the background stellar density field, caused either by three-body scattering during binary hardening or the induction of a dipole instability in the stellar density field.

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arXiv2026

COSMOS-Web: Estimating Physical Parameters of Galaxies Using Self-Organizing Maps

The COSMOS-Web survey, with its unparalleled combination of multiband data, notably, near-infrared imaging from JWST's NIRCam (F115W, F150W, F277W, and F444W), provides a transformative dataset down to $\sim28$ mag (F444W) for studying galaxy evolution. In this work, we employ Self-Organizing Maps (SOMs), an unsupervised machine learning method, to estimate key physical parameters of galaxies -- redshift, stellar mass, star formation rate (SFR), specific SFR (sSFR), and age -- directly from photometric data out to $z=3.5$. SOMs efficiently project high-dimensional galaxy color information onto 2D maps, showing how physical properties vary among galaxies with similar spectral energy distributions. We first validate our approach using mock galaxy catalogs from the HORIZON-AGN simulation, where the SOM accurately recovers the true parameters, demonstrating its robustness. Applying the method to COSMOS-Web observations, we find that the SOM delivers robust estimates despite the increased complexity of real galaxy populations. Performance metrics ($σ_{\mathrm{NMAD}}$ typically between $0.1$--$0.3$, and Pearson correlation between $0.7$ and $0.9$) confirm the precision of the method, with $\sim$ $70\%$ of predictions within 1$σ$ dex of reference values. Although redshift estimation in COSMOS-Web remains challenging (median $σ_{\mathrm{NMAD}} = 0.04$), the overall success of the highlights its potential as a powerful and interpretable tool for galaxy parameter estimation. A key advance of this work is the use of JWST/NIRCam photometry, particularly the F444W band, which enhances SOM training and allows more accurate estimation of stellar mass, SFR, and age compared to previous studies using IRAC/Spitzer filters.

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arXiv2026

An ultra-high-resolution map of (dark) matter

Ordinary matter-including particles such as protons and neutrons-accounts for only about one sixth of all matter in the Universe. The rest is dark matter, which does not emit or absorb light but plays a fundamental role in galaxy and structure evolution. Because it interacts only through gravity, one of the most direct probes is weak gravitational lensing: the deflection of light from distant galaxies by intervening mass. Here we present an extremely detailed, wide-area weak-lensing mass map, covering 0.77 deg x 0.70 deg, using high-resolution imaging from the James Webb Space Telescope (JWST) as part of the COSMOS-Web survey. By measuring the shapes of 129 galaxies per square arcminute-many independently in the F115W and F150W bands-we achieve an angular resolution of 1.00 +/- 0.01 arcmin. Our map has more than twice the resolution of earlier Hubble Space Telescope maps, revealing how dark and luminous matter co-evolve across filaments, clusters, and under-densities. It traces mass features out to z ~ 2, including the most distant structure at z ~ 1.1. The sensitivity to high-redshift lensing constrains galaxy environments at the peak of cosmic star formation and sets a high-resolution benchmark for testing theories about the nature of dark matter and the formation of large-scale cosmic structure

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arXiv2025

Transformational astrophysics and exoplanet science with Habitable Worlds Observatory's High Resolution Imager

Habitable Worlds Observatory (HWO) will be NASA's flagship space telescope of the 2040s, designed to search for life on other planets and to transform broad areas of astrophysics. NASA are seeking international partners, and the UK is well-placed to lead the design and construction of its imaging camera - which is likely to produce the mission's most visible public impact. Early participation in the mission would return investment to UK industry, and bring generational leadership for the UK in space science, space technology, and astrophysics.

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arXiv2025

Radiation damage to the Hubble Space Telescope during two Solar cycles, and correction of Charge Transfer Inefficiency using ArCTIc

From 2002 to 2025, the Hubble Space Telescope's Advanced Camera for Surveys has suffered in the harsh radiation environment above the protection of the Earth's atmosphere. We track the degradation of its image quality, as Solar protons and galactic cosmic rays have damaged its photosensitive charge-coupled device (CCD) imaging sensors. The rate of damage in low Earth orbit is modulated by $18.5^{+4.5}_{-0.5}$ per cent during an 11 year Solar cycle, peaking $430^{+11}_{-5}$ days after Solar minimum as recorded in the number of sunspots. The type of damage is consistent with defects in the silicon lattice that have all stabilised into one of three configurations. We also present the open-source Algorithm for Charge Transfer Inefficiency correction (ArCTIc) v7. This models the (instantaneous or gradual) capture of photoelectrons into lattice defects, and their release after (a discrete set or continuum of) characteristic time delays, which creates spurious trailing in an image. Calibrated using the trailing of hot pixels, and applied during post-processing of astronomical images, ArCTIc can correct 99.5% of Charge Transfer Inefficiency trailing averaged over the camera's lifetime, and 99.9% of trailing in the worst-affected recent data.

astro-ph.IM↗

arXiv2025

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

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 $z_{l}\sim0.2-0.7$, 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 $θ_{E}= 18.1 \pm 1.8 ''$ for a source at $z_{s}=3$, and $θ_{E}= 27.9 \pm 2.8 ''$ for a source at $z_{s}=9$. The total projected mass within a $200$ kpc radius around the strong-lensing region is $M(<200\,\mathrm{kpc}) = (1.9 \pm 0.3) \times 10^{14}\,M_{\odot}$. 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.

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