Publications

2026

2026

1. UNIONS-3500 Weak Lensing: I. A Galaxy Shape Catalogue in the Northern Sky

   F. Hervas-Peters, S. Guerrini, M. Kilbinger, and 15 more authors

   arXiv e-prints, May 2026

   Abs DOI arXiv Bib

   Weak gravitational lensing has become a widely used effect to characterise the dark-matter distribution on large scales in the Universe by measuring galaxy ellipticities and their statistical correlations. We present the first weak gravitational lensing catalogue for cosmic-shear cosmology of the Ultraviolet Near Infrared Optical Northern Survey (UNIONS). We analyse approximately 3 500 square degrees of sky area in the Northern Hemisphere, observed in the r-band by MegaCam on the Canada-France Hawai’i Telescope, achieving a median seeing of 0.7 arcsec. Starting from images calibrated for astrometry and photometry, we describe the steps from image processing to catalogue creation. These steps include masking, source detection and selection, star selection, point spread function (PSF) modelling, shape measurement, and calibration. We conduct extensive validation tests, particularly to assess and mitigate the leakage of PSF ellipticity into galaxy shapes. We demonstrate the robustness of the catalogue by investigating correlations between ellipticity and other observational variables as well as structural elements, such as observer-frame image positions and proximity to bright stars. The final galaxy catalogue contains 62 million galaxies, corresponding to an effective source density of 4.96 arcmin^-2. The ellipticity dispersion, commonly referred to as shape noise, is σ_∊= 0.27. Initiating the first major cosmological analysis by the UNIONS collaboration, this is the first in a series of five papers which cover the various aspects of a robust cosmic shear analysis. Two companion papers discuss the robustness of the catalogue, one through the level of B-mode contamination and another by producing and analysing dedicated image simulations for shear calibration, while the other two present cosmological results in real and harmonic space.

   @article{2026arXiv260513549H,
     adsnote = {Provided by the SAO/NASA Astrophysics Data System},
     adsurl = {https://ui.adsabs.harvard.edu/abs/2026arXiv260513549H},
     archiveprefix = {arXiv},
     author = {{Hervas-Peters}, F. and {Guerrini}, S. and {Kilbinger}, M. and {Baumont}, L. and {Guinot}, A. and {Daley}, C. and {Bonini}, C. and {Wittje}, A. and {Murray}, C. and {Goh}, L.~W.~K. and {Paradis}, A. and {Tersenov}, A. and {Hudson}, M.~J. and {Van Waerbeke}, L. and {Hildebrandt}, H. and {Fabbro}, S. and {Cuillandre}, J.-C. and {McConnachie}, A.~W.},
     doi = {10.48550/arXiv.2605.13549},
     eid = {arXiv:2605.13549},
     eprint = {2605.13549},
     journal = {arXiv e-prints},
     keywords = {Cosmology and Nongalactic Astrophysics},
     month = may,
     pages = {arXiv:2605.13549},
     primaryclass = {astro-ph.CO},
     title = {{UNIONS-3500 Weak Lensing: I. A Galaxy Shape Catalogue in the Northern Sky}},
     year = {2026}
   }

2. UNIONS-3500 Weak Lensing: III. 2D Cosmological Constraints in Configuration Space

   L. W. K. Goh, S. Guerrini, C. Daley, and 13 more authors

   arXiv e-prints, May 2026

   Abs DOI arXiv Bib

   We present the first cosmological constraints from the cosmic shear analysis of the UNIONS-3500 weak lensing galaxy catalogue in configuration space. The Ultraviolet Near Infrared Optical Northern Survey (UNIONS) is the largest and deepest photometric survey of the northern hemisphere to date, with the UNIONS-3500 catalogue using high-quality r-band imaging across 3500 deg2 of the sky. We perform a 2D cosmic shear analysis with a single tomographic bin, using the two-point correlation function (2PCF) statistic. Assuming a flat LCDM model, we obtain constraints on the clustering amplitude of S_8 = 0.831\^{+0.067}_{-0.078}, which is consistent with constraints from Planck CMB measurements and precedent cosmic shear results within 1sigma. We outline the construction of our cosmological inference pipeline, including the estimation of the source redshift distribution, shear calibration, and covariance matrix, and describe methodologies for the mitigation of systematic effects arising from PSF systematics and B-modes. We demonstrate that our results are robust to variations in analysis choices, including scale cuts, prior ranges, and nonlinear modelling. This paper is part of a coordinated release which collectively demonstrates the maturity and readiness of UNIONS to deliver competitive cosmological results, positioning it as a key stepping stone towards the forthcoming era of Stage IV weak lensing experiments.

   @article{2026arXiv260513547G,
     adsnote = {Provided by the SAO/NASA Astrophysics Data System},
     adsurl = {https://ui.adsabs.harvard.edu/abs/2026arXiv260513547G},
     archiveprefix = {arXiv},
     author = {{Goh}, L.~W.~K. and {Guerrini}, S. and {Daley}, C. and {Hervas-Peters}, F. and {Kilbinger}, M. and {Wittje}, A. and {Murray}, C. and {Fabbro}, S. and {Hildebrandt}, H. and {Hudson}, M.~J. and {van Waerbeke}, L. and {Wright}, A.~H. and {de Boer}, T. and {Cuillandre}, J.-C. and {Magnier}, E. and {McConnachie}, A.~W.},
     doi = {10.48550/arXiv.2605.13547},
     eid = {arXiv:2605.13547},
     eprint = {2605.13547},
     journal = {arXiv e-prints},
     keywords = {Cosmology and Nongalactic Astrophysics},
     month = may,
     pages = {arXiv:2605.13547},
     primaryclass = {astro-ph.CO},
     title = {{UNIONS-3500 Weak Lensing: III. 2D Cosmological Constraints in Configuration Space}},
     year = {2026}
   }

3. UNIONS-3500 Weak Lensing: IV. 2D cosmological constraints in harmonic space

   S. Guerrini, L. W. K. Goh, F. Hervas-Peters, and 14 more authors

   arXiv e-prints, May 2026

   Abs DOI arXiv Bib

   The Ultraviolet Near Infrared Optical Northern Survey (UNIONS) is a photometric survey in the northern sky. The quality of the data in the r band provides precise shape measurements to measure the growth of structures using cosmic shear. This work aims to constrain cosmological parameters using a harmonic-space estimator of the cosmic shear signal, known as pseudo-C_\ell, in a non-tomographic analysis. We perform our analysis in the context of the standard ΛCDM cosmology. We model astrophysical systematic effects such as baryonic feedback and intrinsic alignments of galaxies. We verify that the point spread function systematic contribution does not affect our results. We assess the impact of different scale cuts and modelling choices on the constraints. We find S_8 ≡σ_8 \sqrtΩ_\rm m/0.3 = 0.891^+0.057_-0.084, consistent at the 0.79 \,σ level with \textbackslashemph{Planck} and between 0.87 to 1.51 \,σ with other weak lensing surveys. Our results are robust to analysis choices, and we use lognormal simulations to assess the consistency between configuration and harmonic space results, finding a 2.18 \,σ agreement between the two statistics. The degeneracy between S_8 and the amplitude of the intrinsic alignment, A_\rm IA, sampled from a prior obtained from direct measurements, is one of the largest sources of uncertainty. This work is part of the first cosmological analysis of the UNIONS survey using cosmic shear and paves the way for future tomographic and 3 \times 2 point cross- correlation analyses, exploiting the unique overlap of UNIONS with deep spectroscopic surveys in the northern hemisphere.

   @article{2026arXiv260513543G,
     adsnote = {Provided by the SAO/NASA Astrophysics Data System},
     adsurl = {https://ui.adsabs.harvard.edu/abs/2026arXiv260513543G},
     archiveprefix = {arXiv},
     author = {{Guerrini}, S. and {Goh}, L.~W.~K. and {Hervas-Peters}, F. and {Daley}, C. and {Kilbinger}, M. and {Wittje}, A. and {Murray}, C. and {Baumont}, L. and {Fabbro}, S. and {Hildebrandt}, H. and {Hudson}, M.~J. and {van Waerbeke}, L. and {Wright}, A.~H. and {de Boer}, T. and {Cuillandre}, J.-C. and {Magnier}, E. and {McConnachie}, A.~W.},
     doi = {10.48550/arXiv.2605.13543},
     eid = {arXiv:2605.13543},
     eprint = {2605.13543},
     journal = {arXiv e-prints},
     keywords = {Cosmology and Nongalactic Astrophysics},
     month = may,
     pages = {arXiv:2605.13543},
     primaryclass = {astro-ph.CO},
     title = {{UNIONS-3500 Weak Lensing: IV. 2D cosmological constraints in harmonic space}},
     year = {2026}
   }

4. UNIONS-3500 Weak Lensing: II. B-mode validation for cosmic shear

   C. Daley, A. Guinot, S. Guerrini, and 9 more authors

   arXiv e-prints, Apr 2026

   Abs DOI arXiv Bib

   At Stage-III sensitivities, cosmic shear B modes unambiguously indicate systematic contamination and are often used to inform data selection and scale cuts for cosmological inference. We validate B modes for the Ultraviolet Near-Infrared Optical Northern Survey (UNIONS)-3500 (2894 deg^2, n_\mathrmeff ≈5.0 arcmin^-2) using three E/B-separable statistics: pure-mode correlation functions ξ_\pm^\mathrmB(θ), Complete Orthogonal Sets of E/B-mode Integrals (COSEBI) B-mode amplitudes B_n, and harmonic-space power spectra C_\ell^BB. For each statistic, we compute probability-to-exceed (PTE) values over a two- dimensional grid of scale-cut boundaries; our adopted cuts lie in broad stable regions of acceptable PTE. B-mode detections and PTE failures on initial catalog versions led us to investigate galaxy size cuts and stellar halo masking. After cuts, all three statistics pass the null test (minimum PTE = 0.18). Before scale cuts, we measure an oscillatory COSEBI B-mode pattern consistent with repeating additive shear bias, a detector-level effect seen across multiple Stage-III surveys including CFHTLenS, which used the same MegaCam camera; scale cuts that exclude the charge-coupled device (CCD) angular scale suppress it. Although these statistics probe the same two-point shear field, scale cuts in one do not map exactly onto cuts in another, because their respective filter functions weight angular scales differently. The most conservative validation therefore requires scale and sample selections that pass null tests across all frameworks simultaneously, an approach that applies directly to Stage-IV surveys where systematic errors dominate.

   @article{2026arXiv260403227D,
     adsnote = {Provided by the SAO/NASA Astrophysics Data System},
     adsurl = {https://ui.adsabs.harvard.edu/abs/2026arXiv260403227D},
     archiveprefix = {arXiv},
     author = {{Daley}, C. and {Guinot}, A. and {Guerrini}, S. and {Hervas-Peters}, F. and {Goh}, L.~W.~K. and {Murray}, C. and {Kilbinger}, M. and {Wittje}, A. and {Hudson}, M.~J. and {Hildebrandt}, H. and {van Waerbeke}, L. and {McConnachie}, A.~W.},
     doi = {10.48550/arXiv.2604.03227},
     eid = {arXiv:2604.03227},
     eprint = {2604.03227},
     journal = {arXiv e-prints},
     keywords = {Cosmology and Nongalactic Astrophysics, Instrumentation and Methods for Astrophysics},
     month = apr,
     pages = {arXiv:2604.03227},
     primaryclass = {astro-ph.CO},
     title = {{UNIONS-3500 Weak Lensing: II. B-mode validation for cosmic shear}},
     year = {2026}
   }

5. MNRAS

   Lensing the darkness: the matter density profile in cosmic voids from UNIONS

   Hunter L. Martin, Michael J. Hudson, Alex Woodfinden, and 14 more authors

   Monthly Notices of the Royal Astronomical Society, Mar 2026

   Abs DOI arXiv Bib

   We measure the distribution of matter contained within the emptiest regions of the Universe: cosmic voids. We use the large overlap between the Ultraviolet Near-Infrared Optical Northern Survey (UNIONS) and voids identified in the LOWZ and CMASS catalogues of the Baryon Oscillation Spectroscopic Survey (BOSS) to constrain the excess surface mass density of voids using weak lensing. We present and validate a novel method for computing the Gaussian component of the conventional weak lensing covariance, adapted for use with void studies. We detect the stacked weak lensing void density profile at the level, the most significant detection of void lensing from spectroscopically identified voids to date. We find that large and small voids have different matter density profiles, as expected from numerical studies of void profiles. This difference is significant at the level. Comparing the void profile to a measurement of the void─galaxy cross-correlation to test the linearity of the relationship between mass and light, we find good visual agreement between the two, and a galaxy bias factor of , consistent with other works. This work represents a promising detection of the lensing effect from underdensities, with the goal of promoting its development into a competitive cosmological probe.

   @article{2026MNRAS.546ag114M,
     adsnote = {Provided by the SAO/NASA Astrophysics Data System},
     adsurl = {https://ui.adsabs.harvard.edu/abs/2026MNRAS.546ag114M},
     archiveprefix = {arXiv},
     author = {{Martin}, Hunter L. and {Hudson}, Michael J. and {Woodfinden}, Alex and {Baumont}, Lucie and {de Boer}, Thomas and {Burger}, Pierre A. and {Elvin-Poole}, Jack and {Fabbro}, S{\'e}bastien and {Farrens}, Samuel and {Guerrini}, Sacha and {Guinot}, Axel and {Hervas-Peters}, Fabian and {Hildebrandt}, Hendrik and {Kilbinger}, Martin and {Morshed}, Magdy and {van Waerbeke}, Ludovic and {Wittje}, Anna},
     doi = {10.1093/mnras/stag114},
     eid = {stag114},
     eprint = {2507.13450},
     journal = {Monthly Notices of the Royal Astronomical Society},
     keywords = {gravitational lensing: weak, dark matter, large-scale structure of Universe, cosmology: observations, Cosmology and Nongalactic Astrophysics},
     month = mar,
     number = {4},
     pages = {stag114},
     primaryclass = {astro-ph.CO},
     title = {{Lensing the darkness: the matter density profile in cosmic voids from UNIONS}},
     volume = {546},
     year = {2026}
   }

2025

2025

1. Cluster properties as a function of dynamical state in the DESI Legacy x UNIONS surveys

   Syeda Lammim Ahad, Rashaad Reid, Charlie T. Mpetha, and 10 more authors

   arXiv e-prints, Dec 2025

   Abs DOI arXiv Bib

   We investigate how the dynamical state of galaxy clusters influences their galaxy populations and mass distributions. Using photometrically selected clusters from the DESI Legacy Imaging Survey cross-matched with the UNIONS galaxy shear catalogue, we classify clusters as evolved or evolving based on their rest- frame r-band magnitude gaps and stellar mass ratios between the brightest cluster galaxies (BCGs) and bright satellites. We measure the stellar mass functions, weak-lensing profiles, and radial number density and red-fraction profiles of stacked clusters in both subsamples. Evolved clusters exhibit more concentrated lensing profiles, bimodal stellar mass functions dominated by massive BCGs, and a deficit of intermediate-mass satellites, while evolving clusters show flatter central lensing signals and an excess of massive satellites. Applying the same selection to IllustrisTNG clusters reproduces these trends and links the observed differences to distinct mass accretion histories. These results demonstrate the close link between cluster galaxy populations and the overall dynamical state of their underlying dark matter halo.

   @article{2025arXiv251214636A,
     adsnote = {Provided by the SAO/NASA Astrophysics Data System},
     adsurl = {https://ui.adsabs.harvard.edu/abs/2025arXiv251214636A},
     archiveprefix = {arXiv},
     author = {{Ahad}, Syeda Lammim and {Reid}, Rashaad and {Mpetha}, Charlie T. and {Taylor}, James E. and {Hildebrandt}, Hendrik and {Hudson}, Michael J. and {Chambers}, Kenneth C. and {de Boer}, Thomas and {Guerrini}, Sacha and {Guinot}, Axel and {Gwyn}, Stephen and {Kilbinger}, Martin and {Van Waerbeke}, Ludovic},
     doi = {10.48550/arXiv.2512.14636},
     eid = {arXiv:2512.14636},
     eprint = {2512.14636},
     journal = {arXiv e-prints},
     keywords = {Astrophysics of Galaxies, Cosmology and Nongalactic Astrophysics},
     month = dec,
     pages = {arXiv:2512.14636},
     primaryclass = {astro-ph.GA},
     title = {{Cluster properties as a function of dynamical state in the DESI Legacy x UNIONS surveys}},
     year = {2025}
   }

2. AJ

   UNIONS: The Ultraviolet Near-infrared Optical Northern Survey

   Stephen Gwyn, Alan W. McConnachie, Jean-Charles Cuillandre, and 86 more authors

   Astronomical Journal, Dec 2025

   Abs DOI arXiv Bib

   The Ultraviolet Near-Infrared Optical Northern Survey (UNIONS) is a “collaboration of collaborations” that is using the Canada─France─Hawai’i Telescope, the Pan-STARRS telescopes, and the Subaru Observatory to obtain ugriz images of a core survey region of 6250 deg^2 of the northern sky. The 10\ensuremathσ point source depth of the data, as measured within a 2″ diameter aperture, are [u, g, r, i, z] = [23.7, 24.5, 24.2, 23.8, 23.3] in AB magnitudes. UNIONS is addressing some of the most fundamental questions in astronomy, including the properties of dark matter, the growth of structure in the Universe from the very smallest galaxies to large-scale structure, and the assembly of the Milky Way. It is set to become a major ground-based legacy survey for the northern hemisphere for the next decade, and it provides an essential northern complement to the static-sky science of the Vera C. Rubin Observatory’s Legacy Survey of Space and Time. UNIONS supports the core science mission of the Euclid space mission by providing the data necessary in the northern hemisphere for the calibration of the wavelength dependence of the Euclid point- spread function and derivation of photometric redshifts in the North Galactic Cap. This region contains the highest quality sky for Euclid, with low backgrounds from the zodiacal light, stellar density, extinction, and emission from Galactic cirrus. Here, we describe the UNIONS survey components, science goals, data products, and the current status of the overall program.

   @article{2025AJ....170..324G,
     adsnote = {Provided by the SAO/NASA Astrophysics Data System},
     adsurl = {https://ui.adsabs.harvard.edu/abs/2025AJ....170..324G},
     archiveprefix = {arXiv},
     author = {{Gwyn}, Stephen and {McConnachie}, Alan W. and {Cuillandre}, Jean-Charles and {Chambers}, Kenneth C. and {Magnier}, Eugene A. and {de Boer}, Thomas and {Hudson}, Michael J. and {Oguri}, Masamune and {Furusawa}, Hisanori and {Hildebrandt}, Hendrik and {Carlberg}, Raymond and {Ellison}, Sara L. and {Furusawa}, Junko and {Gavazzi}, Rapha{\"e}l and {Ibata}, Rodrigo and {Mellier}, Yannick and {Osato}, Ken and {Aussel}, H. and {Baumont}, Lucie and {Bayer}, Manuel and {Boulade}, Olivier and {C{\o}t{\'e}}, Patrick and {Chemaly}, David and {Daley}, Cail and {Duc}, Pierre-Alain and {Durret}, Florence and {Ellien}, A. and {Fabbro}, S{\'e}bastien and {Ferreira}, Leonardo and {Fitriana}, Itsna K. and {Le Floc'h}, Emeric and {Fudamoto}, Yoshinobu and {Gao}, Hua and {Goh}, L.~W.~K. and {Goto}, Tomotsugu and {Guerrini}, Sacha and {Guinot}, Axel and {H{\'e}nault-Brunet}, Vincent and {Hammer}, Francois and {Harikane}, Yuichi and {Hayashi}, Kohei and {Heesters}, Nick and {Ichikawa}, Kohei and {Kilbinger}, Martin and {Kuzma}, P.~B. and {Li}, Qinxun and {Liaudat}, Tob{\'\i}as I. and {Lin}, Chien-Cheng and {M{\"u}ller}, Oliver and {Martin}, Nicolas F. and {Matsuoka}, Yoshiki and {Medina}, Gustavo E. and {Miyatake}, Hironao and {Miyazaki}, Satoshi and {Mpetha}, Charlie T. and {Nagao}, Tohru and {Navarro}, Julio F. and {Niwano}, Masafumi and {Ogami}, Itsuki and {Okabe}, Nobuhiro and {Onoue}, Masafusa and {Paek}, Gregory S.~H. and {Parker}, Laura C. and {Patton}, David R. and {Peters}, Fabian Hervas and {Prunet}, Simon and {S{\'a}nchez-Janssen}, Rub{\'e}n and {Schultheis}, M. and {Sestito}, Federico and {Smith}, Simon E.~T. and {Starck}, J.-L. and {Starkenburg}, Else and {Stone}, Connor and {Storfer}, Christopher and {Suzuki}, Yoshihisa and {Erben}, T. and {Taibi}, Salvatore and {Thomas}, G.~F. and {Toba}, Yoshiki and {Uchiyama}, Hisakazu and {Valls-Gabaud}, David and {Venn}, Kim A. and {Van Waerbeke}, Ludovic and {Wainscoat}, Richard J. and {Wilkinson}, Scott and {Wittje}, Anna and {Yoshida}, Taketo and {Zhang}, TianFang and {Zhong}, Yuxing},
     doi = {10.3847/1538-3881/ae03ab},
     eid = {324},
     eprint = {2503.13783},
     journal = {Astronomical Journal},
     keywords = {Sky surveys, Weak gravitational lensing, Stellar streams, Galactic archaeology, 1464, 1797, 2166, 2178, Astrophysics - Astrophysics of Galaxies, Astrophysics - Instrumentation and Methods for Astrophysics},
     month = dec,
     number = {6},
     pages = {324},
     primaryclass = {astro-ph.GA},
     title = {{UNIONS: The Ultraviolet Near-infrared Optical Northern Survey}},
     volume = {170},
     year = {2025}
   }

3. OJAp

   Microlensing of lensed supernovae Zwicky & iPTF16geu: constraints on the lens galaxy mass slope and dark compact object fraction

   Nikki Arendse, Edvard Mörtsell, Luke Weisenbach, and 9 more authors

   The Open Journal of Astrophysics, Nov 2025

   Abs DOI arXiv Bib

   To date, only two strongly lensed type Ia supernovae (SNIa) have been discovered with an isolated galaxy acting as the lens: iPTF16geu and SN Zwicky. The observed image fluxes for both lens systems were inconsistent with predictions from a smooth macro lens model. A potential explanation for the anomalous flux ratios is microlensing: additional (de)magnification caused by stars and other compact objects in the lens galaxy. In this work, we combine observations of iPTF16geu and SN Zwicky with simulated microlensing magnification maps, leveraging their standardizable candle properties to constrain the lens galaxy mass slope, \ensuremathη, and the fraction of dark compact objects, . The resulting mass slopes are \ensuremathη=1.70\ensuremath\pm0.07 for iPTF16geu and \ensuremathη=1.81\ensuremath\pm0.10 for SN Zwicky. Our results indicate no evidence for a population of dark compact objects, placing upper limits at the 95% confidence level of for iPTF16geu and for SN Zwicky (for compact objects with masses above 0.02M\ensuremath⊙). Assuming a constant fraction of dark compact objects for both lensed SNe, we obtain . These results highlight the potential of strongly lensed SNIa to probe the innermost parts of lens galaxies and learn about compact matter.

   @article{2025OJAp....8E.166A,
     adsnote = {Provided by the SAO/NASA Astrophysics Data System},
     adsurl = {https://ui.adsabs.harvard.edu/abs/2025OJAp....8E.166A},
     archiveprefix = {arXiv},
     author = {{Arendse}, Nikki and {M{\"o}rtsell}, Edvard and {Weisenbach}, Luke and {Hayes}, Erin and {Thorp}, Stephen and {Dhawan}, Suhail and {Goobar}, Ariel and {Guerrini}, Sacha and {Hjortlund}, Jacob Osman and {Johansson}, Joel and {Lemon}, Cameron and {Al Zaif}, Abdullah},
     doi = {10.33232/001c.147126},
     eprint = {2501.01578},
     journal = {The Open Journal of Astrophysics},
     keywords = {Astrophysics of Galaxies, Cosmology and Nongalactic Astrophysics},
     month = nov,
     pages = {E166},
     primaryclass = {astro-ph.GA},
     title = {{Microlensing of lensed supernovae Zwicky \& iPTF16geu: constraints on the lens galaxy mass slope and dark compact object fraction}},
     volume = {8},
     year = {2025}
   }

4. MNRAS

   Cosmology from UNIONS weak lensing profiles of galaxy clusters

   C. T. Mpetha, J. E. Taylor, Y. Amoura, and 12 more authors

   Monthly Notices of the Royal Astronomical Society, Oct 2025

   Abs DOI arXiv Bib

   Cosmological information is encoded in the structure of galaxy clusters. In Universes with less matter and larger initial density perturbations, clusters form earlier and have more time to accrete material, leading to a more extended infall region. Thus, measuring the mean mass distribution in the infall region provides a novel cosmological test. The infall region is largely insensitive to baryonic physics and provides a cleaner structural test than other measures of cluster assembly time, such as concentration. We consider cluster samples from three publicly available galaxy cluster catalogues: the Spectroscopic Identification of eROSITA Sources (SPIDERS) catalogue, the X-ray and Sunyaev─Zeldovich effect selected clusters in the meta- catalogue M2C, and clusters identified in the Dark Energy Spectroscopic Instrument (DESI) Legacy Imaging Survey. Using a preliminary shape catalogue from the Ultraviolet Near Infrared Optical Northern Survey (UNIONS), we derive excess surface mass density profiles for each sample. We then compare the mean profile for the DESI Legacy sample, which is the most complete, to predictions from a suite of simulations covering a range of and , obtaining constraints of and . We also measure mean (comoving) splashback radii for SPIDERS, M2C, and DESI Legacy Imaging Survey clusters of , , and cMpc , respectively. Performing this analysis with the final UNIONS shape catalogue and the full sample of spectroscopically observed clusters in DESI, we can expect to improve on the best current constraints from cluster abundance studies by a factor of two or more.

   @article{2025MNRAS.543.1393M,
     adsnote = {Provided by the SAO/NASA Astrophysics Data System},
     adsurl = {https://ui.adsabs.harvard.edu/abs/2025MNRAS.543.1393M},
     archiveprefix = {arXiv},
     author = {{Mpetha}, C.~T. and {Taylor}, J.~E. and {Amoura}, Y. and {Haggar}, R. and {de Boer}, T. and {Guerrini}, S. and {Guinot}, A. and {Peters}, F. Hervas and {Hildebrandt}, H. and {Hudson}, M.~J. and {Kilbinger}, M. and {Liaudat}, T. and {McConnachie}, A. and {Van Waerbeke}, L. and {Wittje}, A.},
     doi = {10.1093/mnras/staf1538},
     eprint = {2501.09147},
     journal = {Monthly Notices of the Royal Astronomical Society},
     keywords = {gravitational lensing: weak, methods: observational, galaxies: clusters: general, galaxies: groups: general, galaxies: haloes, cosmological parameters, Cosmology and Nongalactic Astrophysics},
     month = oct,
     number = {2},
     pages = {1393-1409},
     primaryclass = {astro-ph.CO},
     title = {{Cosmology from UNIONS weak lensing profiles of galaxy clusters}},
     volume = {543},
     year = {2025}
   }

5. ApJ

   Unions with UNIONS: Using Galaxy─Galaxy Lensing to Probe Galaxy Mergers

   Isaac Cheng, Jack Elvin-Poole, Michael J. Hudson, and 13 more authors

   Astrophysical Journal, Oct 2025

   Abs DOI arXiv Bib

   We use galaxy─galaxy lensing to investigate how the dark matter (DM) haloes and stellar content of galaxies with 0.012 \ensuremath≤ z \ensuremath≤ 0.32 and 10\ensuremat h≤log10(M\ensuremath⋆/M\ensuremath⊙)\ensu remath≤12 change as a result of the merger process. To this end, we construct two samples of galaxies obtained from the Ultraviolet Near Infrared Optical Northern Survey, comprising 1623 postmergers and \ensuremath∼30,000 nonmerging controls, that live in low-density environments to use as our lenses. These samples are weighted to share the same distributions of stellar mass, redshift, and geometric mean distance to a galaxy’s three nearest neighbors to ensure differences in the lensing signal are due to the merger process itself. We do not detect a statistically significant difference in the excess surface density profile of postmergers and nonmerging controls with current data. Fitting haloes composed of a pointlike stellar mass component and an extended DM structure described by a Navarro─Frenk─White profile to the lensing measurements yields, for both samples, halo masses of M_halo \ensuremath∼ 4 \texttimes 10^12 M_\ensuremath⊙ and a moderately negative correlation between M_halo and concentration c. This allows us to rule out, at the 95% confidence level, merger-induced starbursts in which more than 60% of the stellar mass is formed in the burst. The application of our methods to upcoming surveys that are able to provide samples \ensuremath∼10\texttimes larger than our current catalog is expected to detect the weak-lensing signatures of mergers and further constrain their properties.

   @article{2025ApJ...992..171C,
     adsnote = {Provided by the SAO/NASA Astrophysics Data System},
     adsurl = {https://ui.adsabs.harvard.edu/abs/2025ApJ...992..171C},
     archiveprefix = {arXiv},
     author = {{Cheng}, Isaac and {Elvin-Poole}, Jack and {Hudson}, Michael J. and {Barr{\'e}}, Ruxin and {Ellison}, Sara L. and {Bickley}, Robert W. and {de Boer}, Thomas J.~L. and {Fabbro}, S{\'e}bastien and {Ferreira}, Leonardo and {Guerrini}, Sacha and {Hervas Peters}, Fabian and {Hildebrandt}, Hendrik and {Kilbinger}, Martin and {McConnachie}, Alan W. and {van Waerbeke}, Ludovic and {Wittje}, Anna},
     doi = {10.3847/1538-4357/ae03b8},
     eid = {171},
     eprint = {2502.00584},
     journal = {Astrophysical Journal},
     keywords = {Weak gravitational lensing, Galaxy evolution, Galaxy dark matter halos, 1797, 594, 1880, Astrophysics of Galaxies},
     month = oct,
     number = {2},
     pages = {171},
     primaryclass = {astro-ph.GA},
     title = {{Unions with UNIONS: Using Galaxy─Galaxy Lensing to Probe Galaxy Mergers}},
     volume = {992},
     year = {2025}
   }

6. A&A

   Galaxy─point spread function correlations as a probe of weak-lensing systematics with UNIONS data

   Sacha Guerrini, Martin Kilbinger, Hubert Leterme, and 6 more authors

   Astronomy and Astrophysics, Aug 2025

   Abs DOI arXiv Bib

   Context. Weak gravitational lensing requires precise measurements of galaxy shapes and therefore accurate knowledge of the point spread function (PSF) model. The latter can be a source of systematics that affect the shear two-point correlation function. A key aspect of weak-lensing analysis is the forecasting of the systematics due to the PSF. Aims. Correlation functions of galaxies and the PSF, the so-called \ensuremathρ and \ensuremathτ statistics, are used to evaluate the level of systematics coming from the PSF model and PSF corrections and contributing to the two-point correlation function used to perform cosmological inference. Our goal is to introduce a fast and simple method to estimate this level of systematics and to assess its agreement with state-of- the-art approaches. Methods. We introduce a new way to estimate the covariance matrix of \ensuremathτ statistics using analytical expressions. The covariance allows us to estimate parameters directly related to the level of systematics associated with the PSF and provides us with a tool to validate the PSF model used in a weak-lensing analysis. We applied these methods to data from the Ultraviolet Near-Infrared Optical Northern Survey (UNIONS). Results. We show that semi-analytical covariance yields results comparable to those obtained by using covariances obtained from simulations or jackknife resampling. The approach requires less computation time and is therefore well suited to rapid comparison of the systematic level obtained from different catalogues. We also show how one can break degeneracies between parameters with a redefinition of the \ensuremathτ statistics. Conclusions. The methods developed in this work will be useful tools in the analysis of current weak-lensing data but also of Stage IV surveys such as Euclid, LSST, and Roman. They provide fast and accurate diagnostics on PSF systematics that are crucial in the context of cosmic shear studies.

   @article{2025A&A...700A.215G,
     adsnote = {Provided by the SAO/NASA Astrophysics Data System},
     adsurl = {https://ui.adsabs.harvard.edu/abs/2025A&A...700A.215G},
     archiveprefix = {arXiv},
     author = {{Guerrini}, Sacha and {Kilbinger}, Martin and {Leterme}, Hubert and {Guinot}, Axel and {Wang}, Jingwei and {Peters}, Fabian Hervas and {Hildebrandt}, Hendrik and {Hudson}, Michael J. and {McConnachie}, Alan},
     doi = {10.1051/0004-6361/202453512},
     eid = {A215},
     eprint = {2412.14666},
     journal = {Astronomy and Astrophysics},
     keywords = {methods: statistical, cosmological parameters, cosmology: observations, large-scale structure of Universe, Cosmology and Nongalactic Astrophysics},
     month = aug,
     pages = {A215},
     primaryclass = {astro-ph.CO},
     title = {{Galaxy─point spread function correlations as a probe of weak-lensing systematics with UNIONS data}},
     volume = {700},
     year = {2025}
   }

7. A&A

   UNIONS: A direct measurement of intrinsic alignment with BOSS/eBOSS spectroscopy

   Fabian Hervas Peters, Martin Kilbinger, Romain Paviot, and 12 more authors

   Astronomy and Astrophysics, Jul 2025

   Abs DOI arXiv Bib

   Context. During their formation, galaxies are subject to tidal forces, which create correlations between their shapes and the large- scale structure of the Universe, known as intrinsic alignment. This alignment is a source of contamination for cosmic-shear measurements as we need to disentangle correlations induced by external lensing effects from those intrinsically present in galaxies. Aims. We constrained the amplitude of intrinsic alignment and test models by making use of the overlap between the Ultraviolet Near-Infrared Optical Northern Survey (UNIONS) covering 3500 deg^2 and spectroscopic data from the Baryon Oscillation Spectroscopic Survey (BOSS/eBOSS). By comparing our results to measurements from other lensing surveys on the same spectroscopic tracers, we can test the reliability of these estimates. Methods. We measured projected correlation functions between positions and ellipticities, which we modelled with perturbation theory to constrain the commonly used non-linear alignment model and its higher order expansion. We computed an analytical covariance matrix and validated it using jackknife estimates. Results. Using the non-linear alignment model, we obtained a 13\ensuremathσ detection with CMASS galaxies, a 3\ensuremathσ detection with LRGs, and a detection compatible with the null hypothesis for ELGs. We tested the tidal alignment and tidal torque model. This is a higher order alignment model that we found to be in good agreement with the non-linear alignment prediction and for which we were able to constrain the second-order parameters. We demonstrate the strong scaling of our intrinsic alignment amplitude with luminosity. We also demonstrate that the UNIONS sample is robust against systematic contributions, particularly concerning the point spread function (PSF) biases. We reached a reasonable agreement when comparing our measurements to other lensing samples for the same spectroscopic samples. We take this agreement as an indication that direct measurements of intrinsic alignment are mature for stage IV priors.

   @article{2025A&A...699A.201H,
     adsnote = {Provided by the SAO/NASA Astrophysics Data System},
     adsurl = {https://ui.adsabs.harvard.edu/abs/2025A&A...699A.201H},
     archiveprefix = {arXiv},
     author = {{Hervas Peters}, Fabian and {Kilbinger}, Martin and {Paviot}, Romain and {Baumont}, Lucie and {Russier}, Elisa and {Zhang}, Ziwen and {Murray}, Calum and {Pettorino}, Valeria and {de Boer}, Thomas and {Fabbro}, S{\'e}bastien and {Guerrini}, Sacha and {Hildebrandt}, Hendrik and {Hudson}, Michael J. and {Van Waerbeke}, Ludovic and {Wittje}, Anna},
     doi = {10.1051/0004-6361/202453442},
     eid = {A201},
     eprint = {2412.01790},
     journal = {Astronomy and Astrophysics},
     keywords = {cosmological parameters, large-scale structure of Universe, Cosmology and Nongalactic Astrophysics},
     month = jul,
     pages = {A201},
     primaryclass = {astro-ph.CO},
     title = {{UNIONS: A direct measurement of intrinsic alignment with BOSS/eBOSS spectroscopy}},
     volume = {699},
     year = {2025}
   }

8. A&A

   Euclid: I. Overview of the Euclid mission

   Euclid Collaboration, Y. Mellier, Abdurro’uf, and 197 more authors

   Astronomy and Astrophysics, May 2025

   Abs DOI arXiv Bib

   The current standard model of cosmology successfully describes a variety of measurements, but the nature of its main ingredients, dark matter and dark energy, remains unknown. Euclid is a medium- class mission in the Cosmic Vision 2015─2025 programme of the European Space Agency (ESA) that will provide high-resolution optical imaging, as well as near-infrared imaging and spectroscopy, over about 14 000 deg^2 of extragalactic sky. In addition to accurate weak lensing and clustering measurements that probe structure formation over half of the age of the Universe, its primary probes for cosmology, these exquisite data will enable a wide range of science. This paper provides a high- level overview of the mission, summarising the survey characteristics, the various data-processing steps, and data products. We also highlight the main science objectives and expected performance.

   @article{2025A&A...697A...1E,
     adsnote = {Provided by the SAO/NASA Astrophysics Data System},
     adsurl = {https://ui.adsabs.harvard.edu/abs/2025A&A...697A...1E},
     archiveprefix = {arXiv},
     author = {{Euclid Collaboration} and {Mellier}, Y. and {Abdurro'uf} and {Acevedo Barroso}, J.~A. and {Ach{\'u}carro}, A. and {Adamek}, J. and {Adam}, R. and {Addison}, G.~E. and {Aghanim}, N. and {Aguena}, M. and {Ajani}, V. and {Akrami}, Y. and {Al-Bahlawan}, A. and {Alavi}, A. and {Albuquerque}, I.~S. and {Alestas}, G. and {Alguero}, G. and {Allaoui}, A. and {Allen}, S.~W. and {Allevato}, V. and {Alonso-Tetilla}, A.~V. and {Altieri}, B. and {Alvarez-Candal}, A. and {Alvi}, S. and {Amara}, A. and {Amendola}, L. and {Amiaux}, J. and {Andika}, I.~T. and {Andreon}, S. and {Andrews}, A. and {Angora}, G. and {Angulo}, R.~E. and {Annibali}, F. and {Anselmi}, A. and {Anselmi}, S. and {Arcari}, S. and {Archidiacono}, M. and {Aric{\`o}}, G. and {Arnaud}, M. and {Arnouts}, S. and {Asgari}, M. and {Asorey}, J. and {Atayde}, L. and {Atek}, H. and {Atrio-Barandela}, F. and {Aubert}, M. and {Aubourg}, E. and {Auphan}, T. and {Auricchio}, N. and {Aussel}, B. and {Aussel}, H. and {Avelino}, P.~P. and {Avgoustidis}, A. and {Avila}, S. and {Awan}, S. and {Azzollini}, R. and {Baccigalupi}, C. and {Bachelet}, E. and {Bacon}, D. and {Baes}, M. and {Bagley}, M.~B. and {Bahr-Kalus}, B. and {Balaguera-Antolinez}, A. and {Balbinot}, E. and {Balcells}, M. and {Baldi}, M. and {Baldry}, I. and {Balestra}, A. and {Ballardini}, M. and {Ballester}, O. and {Balogh}, M. and {Ba{\~n}ados}, E. and {Barbier}, R. and {Bardelli}, S. and {Baron}, M. and {Barreiro}, T. and {Barrena}, R. and {Barriere}, J.-C. and {Barros}, B.~J. and {Barthelemy}, A. and {Bartolo}, N. and {Basset}, A. and {Battaglia}, P. and {Battisti}, A.~J. and {Baugh}, C.~M. and {Baumont}, L. and {Bazzanini}, L. and {Beaulieu}, J.-P. and {Beckmann}, V. and {Belikov}, A.~N. and {Bel}, J. and {Bellagamba}, F. and {Bella}, M. and {Bellini}, E. and {Benabed}, K. and {Bender}, R. and {Benevento}, G. and {Bennett}, C.~L. and {Benson}, K. and {Bergamini}, P. and {Bermejo-Climent}, J.~R. and {Bernardeau}, F. and {Bertacca}, D. and {Berthe}, M. and {Berthier}, J. and {Bethermin}, M. and {Beutler}, F. and {Bevillon}, C. and {Bhargava}, S. and {Bhatawdekar}, R. and {Bianchi}, D. and {Bisigello}, L. and {Biviano}, A. and {Blake}, R.~P. and {Blanchard}, A. and {Blazek}, J. and {Blot}, L. and {Bosco}, A. and {Bodendorf}, C. and {Boenke}, T. and {B{\"o}hringer}, H. and {Boldrini}, P. and {Bolzonella}, M. and {Bonchi}, A. and {Bonici}, M. and {Bonino}, D. and {Bonino}, L. and {Bonvin}, C. and {Bon}, W. and {Booth}, J.~T. and {Borgani}, S. and {Borlaff}, A.~S. and {Borsato}, E. and {Bose}, B. and {Botticella}, M.~T. and {Boucaud}, A. and {Bouche}, F. and {Boucher}, J.~S. and {Boutigny}, D. and {Bouvard}, T. and {Bouwens}, R. and {Bouy}, H. and {Bowler}, R.~A.~A. and {Bozza}, V. and {Bozzo}, E. and {Branchini}, E. and {Brando}, G. and {Brau-Nogue}, S. and {Brekke}, P. and {Bremer}, M.~N. and {Brescia}, M. and {Breton}, M.-A. and {Brinchmann}, J. and {Brinckmann}, T. and {Brockley-Blatt}, C. and {Brodwin}, M. and {Brouard}, L. and {Brown}, M.~L. and {Bruton}, S. and {Bucko}, J. and {Buddelmeijer}, H. and {Buenadicha}, G. and {Buitrago}, F. and {Burger}, P. and {Burigana}, C. and {Busillo}, V. and {Busonero}, D. and {Cabanac}, R. and {Cabayol-Garcia}, L. and {Cagliari}, M.~S. and {Caillat}, A. and {Caillat}, L. and {Calabrese}, M. and {Calabro}, A. and {Calderone}, G. and {Calura}, F. and {Camacho Quevedo}, B. and {Camera}, S. and {Campos}, L. and {Ca{\~n}as-Herrera}, G. and {Candini}, G.~P. and {Cantiello}, M. and {Capobianco}, V. and {Cappellaro}, E. and {Cappelluti}, N. and {Cappi}, A. and {Caputi}, K.~I. and {Cara}, C. and {Carbone}, C. and {Cardone}, V.~F. and {Carella}, E. and {Carlberg}, R.~G. and {Carle}, M. and {Carminati}, L. and {Caro}, F. and {Carrasco}, J.~M. and {Carretero}, J. and {Carrilho}, P. and {Carron Duque}, J. and {Carry}, B.},
     doi = {10.1051/0004-6361/202450810},
     eid = {A1},
     eprint = {2405.13491},
     journal = {Astronomy and Astrophysics},
     keywords = {instrumentation: detectors, instrumentation: spectrographs, space vehicles: instruments, telescopes, surveys, cosmology: observations, Cosmology and Nongalactic Astrophysics, Astrophysics of Galaxies, Instrumentation and Methods for Astrophysics},
     month = may,
     pages = {A1},
     primaryclass = {astro-ph.CO},
     title = {{Euclid: I. Overview of the Euclid mission}},
     volume = {697},
     year = {2025}
   }

2024

2024

1. Phys. Rev. D

   Probing a scale dependent gravitational slip with galaxy strong lensing systems

   Sacha Guerrini and Edvard Mörtsell

   Physical Review D, Jan 2024

   Abs DOI arXiv Bib

   Observations of galaxy-scale strong gravitational lensing systems enable unique tests of departures from general relativity at the kilo- to megaparsec scale. In this work, the gravitational slip parameter \ensuremathγ_PN, measuring the amplitude of a hypothetical fifth force, is constrained using 130 elliptical galaxy lens systems. We implement a lens model with a power-law total mass density and a deprojected De Vaucouleurs luminosity density, favored over a power-law luminosity density. To break the degeneracy between the lens velocity anisotropy \ensuremathβ and the gravitational slip, we introduce a new prior on the velocity anisotropy based on recent dynamical data. For a constant gravitational slip, we find \ensuremathγ_PN=0.9 0_-0.14^+0.18 in agreement with general relativity at the 68% confidence level. Introducing a Compton wavelength \ensuremathλ_g, effectively screening the fifth force at small and large scales, the best fit is obtained for \ensuremathλ_g\ensuremath∼0.2 Mpc and \ensuremathγ_PN=0.7 7_-0.14^+0.25. A local minimum is found at \ensuremathλ_g\ensuremath∼100 Mpc and \ensuremathγ_PN=0.5 6_-0.35^0.45. We conclude that there is no evidence in the data for a significant departure from general relativity and that using accurate assumptions and having good constraints on the lens galaxy model is key to ensure reliable constraints on the gravitational slip.

   @article{2024PhRvD.109b3533G,
     adsnote = {Provided by the SAO/NASA Astrophysics Data System},
     adsurl = {https://ui.adsabs.harvard.edu/abs/2024PhRvD.109b3533G},
     archiveprefix = {arXiv},
     author = {{Guerrini}, Sacha and {M{\"o}rtsell}, Edvard},
     doi = {10.1103/PhysRevD.109.023533},
     eid = {023533},
     eprint = {2309.11915},
     journal = {Physical Review D},
     keywords = {Astrophysics - Cosmology and Nongalactic Astrophysics, General Relativity and Quantum Cosmology},
     month = jan,
     number = {2},
     pages = {023533},
     primaryclass = {astro-ph.CO},
     title = {{Probing a scale dependent gravitational slip with galaxy strong lensing systems}},
     volume = {109},
     year = {2024}
   }