Alexandre Refregier

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Refregier

First Name

Alexandre

ORCID

0000-0003-3416-9317

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03928 - Refregier, Alexandre / Refregier, Alexandre

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Publications 1 - 10 of 20

Karabo: A versatile SKA observation simulation framework
Sharma , Rohit; Felix , Simon; Machado Poletti Valle, Luis; et al. (2026)
Astronomy and Computing
Karabo is a versatile Python-based software framework simplifying research with radio astronomy data. It bundles existing software packages into a coherent whole to improve the ease of use of its components. Karabo includes useful abstractions, like strategies to scale and parallelize typical workloads or science-specific Python modules. The framework includes functionality to access datasets and mock observations to study the Square Kilometre Array (SKA) instruments and their expected accuracy. SKA will address problems in a wide range of fields of astronomy. We demonstrate the application of Karabo relevant to some of the SKA science cases from HI intensity mapping, simulation of the radio surveys, radio source detection, the epoch of re-ionization and heliophysics. We discuss the capabilities, scalabilities and challenges of simulating large radio datasets in the context of SKA.
Euclid II. The VIS instrument
Euclid Collaboration; Cropper, Mark S.; Refregier, Alexandre; et al. (2025)
Astronomy & Astrophysics
This paper presents the specification, design, and development of the Visible Camera (VIS) on the European Space Agency’s Euclid mission. VIS is a large optical-band imager with a field of view of 0.54 deg$^2$ sampled at 0''.1 with an array of 609 Megapixels and a spatial resolution of 0''.18. It will be used to survey approximately 14 000 deg$^2$ of extragalactic sky to measure the distortion of galaxies in the redshift range z = 0.1–1.5 resulting from weak gravitational lensing, one of the two principal cosmology probes leveraged by Euclid. With photometric redshifts, the distribution of dark matter can be mapped in three dimensions, and the extent to which this has changed with look-back time can be used to constrain the nature of dark energy and theories of gravity. The entire VIS focal plane will be transmitted to provide the largest images of the Universe from space to date, specified to reach m$_{AB}$ ≥ 24.5 with a signal-to-noise ratio S/N ≥ 10 in a single broad I$_E$ ≃ (r + i + z) band over a six-year survey. The particularly challenging aspects of the instrument are the control and calibration of observational biases, which lead to stringent performance requirements and calibration regimes. With its combination of spatial resolution, calibration knowledge, depth, and area covering most of the extra-Galactic sky, VIS will also provide a legacy data set for many other fields. This paper discusses the rationale behind the conception of VIS and describes the instrument design and development, before reporting the prelaunch performance derived from ground calibrations and brief results from the in-orbit commissioning. VIS should reach fainter than m$_{AB}$ = 25 with S/N ≥ 10 for galaxies with a full width at half maximum of 0''.3 in a 1''.3 diameter aperture over the Wide Survey, and m$_{AB}$ ≥ 26.4 for a Deep Survey that will cover more than 50 deg$^2$. The paper also describes how the instrument works with the Euclid telescope and survey, and with the science data processing, to extract the cosmological information.
GalSBI-SPS: A stellar population synthesis-based galaxy population model for cosmology and galaxy evolution applications
Tortorelli , Luca; Fischbacher, Silvan; Grün , Daniel; et al. (2025)
Astronomy & Astrophysics
Context. Next-generation photometric and spectroscopic galaxy surveys will enable unprecedented tests of the concordance cosmological model and of galaxy formation and evolution. Fully exploiting their potential requires a precise understanding of the selection effects on galaxies and biases on measurements of their properties, which are required, above all, for accurate estimates of redshift distributions. The forward-modelling of galaxy surveys offers a powerful framework to simultaneously recover galaxy redshift distributions and characterise the observed galaxy population. Aims. We present GalSBI-SPS, a new stellar population synthesis (SPS)-based galaxy population model developed for cosmological and galaxy evolution studies. The model generates realistic galaxy catalogues, which we use to forward-model Hyper-Suprime Cam (HSC) observations in the COSMOS field. Methods. GalSBI-SPS samples the physical properties of galaxies from analytical parametrisations informed by GAMA, DEVILS, and literature data, it computes galaxy magnitudes with the generative SED package ProSpect, and it simulates HSC images in the COSMOS field with UFig. We measured photometric properties consistently in real data and simulations. We compared redshift distributions and photometric and physical properties to observations and to those from the phenomenological GalSBI model. Results. GalSBI-SPS reproduces the observed g, r, i, z, y magnitude, colour, and size distributions down to i ≤ 23 with good accuracy. Median differences in magnitudes and colours remain below 0.14 mag, with the model covering the full colour space spanned by HSC data. Galaxy sizes are overestimated by ∼0.2⁰⁰ on average and some tension exists in the g − r colour distribution, but the latter is comparable to that seen in the phenomenological GalSBI model. Redshift distributions show a mild positive offset (0.01 . ∆z̄ . 0.08) in the mean. GalSBI-SPS qualitatively reproduces the stellar mass–star formation rate and size–stellar mass relations seen in COSMOS2020 data. Conclusions. GalSBI-SPS provides a realistic, survey-independent description of the galaxy population at a Stage-III-like depth using only literature-based parameters. Its predictive power is expected to improve significantly when constrained against deep observed data using simulation-based inference, thereby providing accurate redshift distributions that satisfy the stringent requirements set by Stage IV surveys.
Euclid: Early Release Observations – Programme overview and pipeline for compact- and diffuse-emission photometry
Cuillandre, Jean-Charles; Bertin, Emmanuel; Bolzonella, Micol; et al. (2025)
Astronomy & Astrophysics
The Euclid Early Release Observations (ERO) showcase Euclid’s capabilities in advance of its main mission by targeting 17 astronomical objects, including galaxy clusters, nearby galaxies, globular clusters, and star-forming regions. A total of 24 hours of observing time was allocated in the early months of operation, and the scientific community was engaged through an early public data release. We describe the development of the ERO pipeline to create visually compelling images while simultaneously meeting the scientific demands within months of launch by leveraging a pragmatic data-driven development strategy. The pipeline’s key requirements are to preserve the image quality and to provide flux calibration and photometry for compact and extended sources. The pipeline’s five pillars are removal of instrumental signatures, astrometric calibration, photometric calibration, image stacking, and the production of science-ready catalogues for both the VIS and NISP instruments. We report a point spread function (PSF) with a full width at half maximum of 0''.16 in the optical I$_E$-band and 0''.49 in the near-infrared (NIR) bands Y$_E$, J$_E$, and H$_E$. Our VIS mean absolute flux calibration is accurate to about 1%, and the accuracy is 10% for NISP due to a limited calibration set; both instruments have considerable colour terms for individual sources. The median depth is 25.3 and 23.2 AB mag with a signal-to-noise ratio (S/N) of ten for galaxies, while it is 27.1 and 24.5 AB mag at an S/N of five for point sources for VIS and NISP, respectively. Euclid’s ability to observe diffuse emission is exceptional due to its extended PSF nearly matching a pure diffraction halo, the best ever achieved by a wide-field high-resolution imaging telescope. Euclid offers unparalleled capabilities for exploring the low-surface brightness (LSB) Universe across all scales, providing high precision within a wide field of view (FoV), and opening a new observational window in the NIR. Median surface-brightness levels of 29.5 and 27.9, AB mag arcsec$^{-2}$ are achieved for VIS and NISP, respectively, for detecting a 10'' × 10'' extended feature at the 1 σ level.
Tuning the cosmic instrument: robust cosmology through combined probes
Reeves, Alexander; Nicola, Andrina; Refregier, Alexandre (2025)
Journal of Cosmology and Astroparticle Physics
Euclid: Early Release Observations – Globular clusters in the Fornax galaxy cluster, from dwarf galaxies to the intracluster field
Saifollahi, Teymoor; Voggel, Karina; Lançon, Ariane; et al. (2025)
Astronomy & Astrophysics
We present an analysis of Euclid observations of a 0.6 deg$^2$ field in the central region of the Fornax galaxy cluster that were acquired during the performance verification phase. With these data, we investigated the potential of Euclid to identify globular clusters (GCs) at 20 Mpc and validated the search methods using artificial GCs and known GCs within the field from the literature. Our analysis of artificial GCs injected into the data shows that Euclid’s data in the I$_E$ band is 80% complete at about I$_E$ ∼ 26.0 mag (M$_V$ ∼ -5.0 mag), and it resolves GCs as small as r$_h$ = 2.5 pc. In the I$_E$ band, we detected more than 95% of the known GCs from previous spectroscopic surveys and GC candidates of the ACS Fornax Cluster Survey, of which more than 80% are resolved. We identify more than 5000 new GC candidates within the field of view down to I$_E$ = 25.0 mag, about 1.5 mag fainter than the typical GC luminosity function turn-over magnitude, and we investigated their spatial distribution within the intracluster field. We then focused on the GC candidates around dwarf galaxies and investigated their numbers, stacked luminosity distribution, and stacked radial distribution. While the overall GC properties are consistent with those in the literature, we found an interesting over-representation of relatively bright candidates within a small number of relatively GC-rich dwarf galaxies. Our work confirms the capabilities of Euclid data in detecting GCs and separating them from foreground and background contaminants at a distance of 20 Mpc, particularly for low GC-count systems such as dwarf galaxies.
GalSBI: phenomenological galaxy population model for cosmology using simulation-based inference
Fischbacher, Silvan; Kacprzak, Tomasz; Tortorelli, Luca; et al. (2025)
Journal of Cosmology and Astroparticle Physics
We present GalSBI, a phenomenological model of the galaxy population for cosmological applications using simulation-based inference. The model is based on analytical parametrizations of galaxy luminosity functions, morphologies and spectral energy distributions. Model constraints are derived through iterative Approximate Bayesian Computation, by comparing Hyper Suprime-Cam deep field images with simulations which include a forward model of instrumental, observational and source extraction effects. We developed an emulator trained on image simulations using a normalizing flow. We use it to accelerate the inference by predicting detection probabilities, including blending effects and photometric properties of each object, while accounting for background and PSF variations. This enables robustness tests for all elements of the forward model and the inference. The model demonstrates excellent performance when comparing photometric properties from simulations with observed imaging data for key parameters such as magnitudes, colors and sizes. The redshift distribution of simulated galaxies agrees well with high-precision photometric redshifts in the COSMOS field within 1.5σ for all magnitude cuts. Additionally, we demonstrate how GalSBI's redshifts can be utilized for splitting galaxy catalogs into tomographic bins, highlighting its potential for current and upcoming surveys. GalSBI is fully open-source, with the accompanying Python package, galsbi (https://cosmo-docs.phys.ethz.ch/galsbi/), offering an easy interface to quickly generate realistic, survey-independent galaxy catalogs.
Euclid: Early Release Observations – Dwarf galaxies in the Perseus galaxy cluster
Marleau, Francine R.; Cuillandre, Jean-Charles; Cantiello, Michele; et al. (2025)
Astronomy & Astrophysics
We make use of the unprecedented depth, spatial resolution, and field of view of the Euclid Early Release Observations (EROs) of the Perseus galaxy cluster to detect and characterise the dwarf galaxy population in this massive system. Using a dedicated annotation tool, the Euclid high-resolution VIS and combined VIS+Near Infrared Spectrometer and Photometer (NISP) colour images were visually inspected and dwarf galaxy candidates were identified. Their morphologies, the presence of nuclei, and their globular cluster (GC) richness were visually assessed richness were visually assessed, complementing an automatic detection of the GC candidates. Structural and photometric parameters, including Euclid filter colours, were extracted from two-dimensional fitting. Based on this analysis, a total of 1100 dwarf candidates were found across the image; 606 of these appear to be new identifications. The majority (96%) are classified as dwarf ellipticals, 53% are nucleated, 26% are GC-rich, and 6% show disturbed morphologies. A relatively high fraction of galaxies, 8%, are categorised as ultra diffuse galaxies. The majority of the dwarfs follow the expected scaling relations of galaxies. Globally, the GC specific frequency, S$_N$, of the Perseus dwarf candidates is intermediate between those measured in the Virgo and Coma clusters. While the dwarf candidates with the largest GC counts are found throughout the Euclid field of view, the dwarfs located around the east–west strip, where most of the brightest cluster members are found, exhibit higher S$_N$ values on average. The spatial distribution of the dwarfs, GCs, and intracluster light show a main iso-density and isophotal centre displaced to the west of the bright galaxy light distribution. The ERO imaging of the Perseus cluster demonstrates the unique capability of Euclid to concurrently detect and characterise large samples of dwarf galaxies, their nuclei, and their GC systems, allowing us to construct a detailed picture of the formation and evolution of galaxies over a wide range of mass scales and environments.
Euclid: Early Release Observations – Deep anatomy of nearby galaxies
Hunt, Leslie K.; Annibali, Francesca; Cuillandre, Jean-Charles; et al. (2025)
Astronomy & Astrophysics
Euclid is poised to make significant advances in the study of nearby galaxies in the Local Universe. Here we present a first look at six galaxies observed for the Nearby Galaxy Showcase as part of the Euclid Early Release Observations acquired between August and November, 2023. These targets, three dwarf galaxies (Holmberg II, IC 10, and NGC 6822) and three spirals (IC 342, NGC 2403, and NGC 6744), range in distance from about 0.5 Mpc to 8.8 Mpc. We first assess the surface brightness depths in the stacked Euclid images, and confirm previous estimates in 100 arcsec$^2$ regions for Visible Camera (VIS) of 1σ limits of 30.5 mag arcsec$^{-2}$, but find deeper than previous estimates for Near-Infrared Spectrometer and Photometer (NISP) with 1σ = 29.2–29.4 mag arcsec$^{-2}$. By combining Euclid H$_E$, Y$_E$, and I$_E$ into RGB images, we illustrate the large field of view (FoV) covered by a single reference observing sequence (ROS), together with exquisite detail on scales of <1–4 parsecs in these nearby galaxies. Our analysis of radial surface brightness and color profiles demonstrates that the photometric calibration of Euclid is consistent with what is expected for galaxy colors according to stellar synthesis models. We perform standard source-selection techniques for stellar photometry, and find approximately 1.3 million stars across the six galaxy fields. After subtracting foreground stars and background galaxies, and applying a color and magnitude selection, we extract stellar populations of different ages for the six galaxies. The resolved stellar photometry obtained with Euclid allows us to constrain the star-formation histories of these galaxies, which we do by disentangling the distributions of young stars and asymptotic giant branch and red giant branch stellar populations. We finally examine two galaxies individually for surrounding systems of dwarf galaxy satellites and globular cluster populations. Our analysis of the ensemble of dwarf satellites around NGC 6744 recovers all the previously known dwarf satellites within the Euclid FoV, and also confirms the satellite nature of a previously identified candidate, dw1909m6341, a nucleated dwarf spheroidal at the end of a spiral arm. Our new census of the globular clusters around NGC 2403 yields nine new star-cluster candidates, eight of which exhibit colors indicative of evolved stellar populations. In summary, our first investigation of six “showcase” galaxies demonstrates that Euclid is a powerful probe of stellar structure and stellar populations in nearby galaxies, and will provide vastly improved statistics on dwarf satellite systems and extragalactic globular clusters in the local Universe, among many other exciting results.
The cosmological analysis of DES 3x2pt data from the Effective Field Theory of Large-Scale Structure
D'Amico, Guido; Refregier, Alexandre; Senatore, Leonardo; et al. (2025)
arXiv
We analyze the Dark Energy Survey (DES) Year 3 data using predictions from the Effective Field Theory of Large-Scale Structure (EFTofLSS). Specifically, we fit three two-point observables (3××2pt), galaxy clustering, galaxy-galaxy lensing, and cosmic shear, using the one-loop expressions for the projected angular correlation functions. We validate our pipeline against numerical simulations and we check for several internal consistencies before applying it to the observational data. Fixing the spectral tilt and the baryons abundance, we measure S8=0.833±0.032S8=0.833±0.032, Ωm=0.272±0.022Ωm=0.272±0.022, and h=0.773±0.049h=0.773±0.049, to about 3.8%3.8%, 8.1%8.1%, and 6.3%6.3%, at 68%68%CL, respectively. Our results are consistent at the ∼1.5−2σ∼1.5−2σ level with those from Planck and the BOSS full-shape analyses, as well as with those from DES collaboration 3××2pt analysis combined with a Big-Bang Nucleosynthesis prior and a Planck prior on nsns. The shift in the posterior compared to DES collaboration results highlights the impact of modeling, scale cuts, and choice of prior. The theory code and likelihood used for our analyses, \texttt{PyFowl}, is made publicly available.

Publications 1 - 10 of 20

Alexandre Refregier

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