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A. M. C. Le Brun

Publications and source records attributed to A. M. C. Le Brun.

At least 19 recordsLinked to original sources
arXiv2026

Euclid Quick Data Release (Q1). Data processing for intracluster light measurements

The Euclid survey provides images with extraordinary potential for studies of the low-surface-brightness Universe, including intracluster light (ICL). However, to realise this promise, the data must be carefully treated to minimise background variations while preserving the signal from diffuse structures. In this work, we describe the post-processing and measurement procedures used for studies of ICL with Euclid. We demonstrate that we are able to substantially reduce background variations without resorting to a small-scale background subtraction. By injecting simulated clusters into the real images, we show that our approach preserves the faint outskirts of extended sources. On scales of several arcminutes, we can reliably detect surface-brightness levels around 27.5 mag arcsec$^{-2}$ in $Y_\textrm{E}$, $J_\textrm{E}$, $H_\textrm{E}$, and 29 mag arcsec$^{-2}$ in $I_\textrm{E}$. As a result, we can obtain accurate measurements of the ICL in individual clusters from the local Universe out to z~0.8 with uncertainties on the order of 10% in surface brightness, 10 degrees in position angle, and 0.1 in ellipticity. Our ability to measure the diffuse light with Euclid is thus set to meet expectations and revolutionise our understanding of the ICL.

astro-ph.GA↗
arXiv2026

Euclid Quick Data Release (Q1). A systematic study of the intracluster light fraction at 0.2 < z < 1.5

We present a homogeneous analysis of the intracluster light (ICL) in approximately 300 galaxy clusters spanning $0.2 10^{10}\,M_\odot$). The inferred ICL fraction also depends systematically on wavelength, while remaining largely insensitive to the adopted outer integration radius of 200-600 kpc. These results highlight the importance of homogeneous ICL analyses and constrain the evolution of diffuse stellar components in clusters.

astro-ph.GA↗
arXiv2026

Euclid Quick Data Release (Q1). The metallicity of the intracluster light

Colour provides key information to constrain the stellar population of the intracluster light (ICL), the diffuse light within galaxy clusters. We derive radial colour profiles ($Y_\mathrm{E}-H_\mathrm{E}$, $J_\mathrm{E}-H_\mathrm{E}$, $I_\mathrm{E}-H_\mathrm{E}$) of the brightest central galaxy (BCG) and ICL for 168 galaxy clusters from the Dark Energy Survey redMaPPer catalogue within the redshift range $0.1 < z < 0.75$ and halo mass $M_{\rm 200m} > 10^{14.05}M_\odot/h$, within the Euclid Quick Data Release (Q1). By stacking the light profiles measured using elliptical annuli on the BCG and ICL images, we reliably measure the ICL colour out to approximately 300 kpc, over which we find a colour gradient, with the ICL becoming bluer than the BCG out to approximately 150 kpc. Using stellar population synthesis (SPS) models, we find that near-infrared (NIR) colours are sensitive to metallicity, but not age. Thus, we measure a metallicity profile in the BCG and ICL that extends out to 300 kpc for a statistical sample for the first time. While the colours, and the detection of a colour and metallicity gradient, are robust, we note that our metallicity measurement is dependent upon our choice of both SPS model and colour. We find that the metallicity of the BCG is super-solar [$\log_{10} (Z/Z_\odot) > 0.1$], while the ICL is metal-poor [$\log_{10}(Z/Z_\odot)\approx -0.5$], placing the first statistical constraint on the metallicity of the ICL out to large radii. These results support our current understanding of BCG and ICL formation: BCGs form predominantly through mergers with massive galaxies or the cores of satellite galaxies, while the ICL grows via the tidal stripping of lower-metallicity stars from the outskirts of massive satellite galaxies.

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arXiv2026

Euclid Bypass end-to-end simulations for spectroscopic galaxy clustering performance assessment

We present a simulation framework for Euclid galaxy clustering measurements that reproduces the essential behaviour of the spectroscopic reduction chain, bypassing the computation required by full pixel-level simulations. The framework comprises two complementary tools, FastSpec, which simulates 1-dimensional slitless spectra with a simplified instrument and noise model, and Pypelid, which computes detection probabilities on input catalogues, accounting for the survey dithering sequence and spatially varying noise. We develop calibrated prescriptions for the survey selection function, redshift errors, and redshift interloper populations. Applied to the Euclid Wide Survey, the baseline selection yields a redshift purity of 74% and a sample completeness of 45%, which improves with the use of Euclid photometric colours. The redshift error distribution is found to have a Gaussian core of width $σ_z = 9.4 \times 10^{-4}$, with no significant trend over $0.9 < z < 1.8$. Emission lines [OIII]5008 and [SIII]9530 misidentified as H$α$ contribute a few to ten per cent depending on redshift, while catastrophic redshift measurement failures dominate at low signal-to-noise ratio. Simulating the Data Release 1 footprint, we find that complex survey selection effects can be corrected with a forward-modelled random catalogue, recovering the monopole and quadrupole of the galaxy 2-point correlation function. The simplified noise model does not include residual contamination from overlapping spectra, or other artefacts seen in early Euclid data; this will be revisited with DR1 data. The framework provides an efficient route for characterising the Euclid spectroscopic selection function at the precision required for galaxy clustering science, and is the basis for systematic error assessments in Euclid galaxy clustering analyses.

astro-ph.CO↗
arXiv2026

Euclid: Impact of halo mass-conversion model assumptions on galaxy cluster number-count analyses

The large catalogues of galaxy clusters expected from the Euclid survey will enable cosmological analyses of cluster number counts that require accurate cosmological model predictions. One possibility is to use parametric fits calibrated against $N$-body simulations, that capture the cosmological parameter dependence of the halo mass function. Several studies have shown that this can be obtained through a calibration against haloes with spherical masses defined at the virial overdensity. In contrast, if different mass definitions are used for the HMF and the scaling relation, a mapping between them is required. Here, we investigate the impact of such a mapping on the cosmological parameter constraints inferred from galaxy cluster number counts. Using synthetic data from $N$-body simulations, we show that the standard approach, which relies on assuming a concentration-mass relation, can introduce significant systematic bias. In particular, depending on the mass definition and the relation assumed, this can lead to biased constraints at more than 2$σ$ level. In contrast, we find that in all the cases we have considered, the mass conversion based on the halo sparsity statistics result in a systematic bias smaller than the statistical error.

astro-ph.CO↗
arXiv2026

Euclid preparation CIX. Baryon acoustic oscillation analysis of photometric galaxy clustering in configuration space

With about 1.5 billion galaxies expected to be observed, the very large number of objects in the Euclid photometric survey will allow for precise studies of galaxy clustering from a single survey, over a large range of redshifts $0.2 < z < 2.5$. In this work, we use photometric redshifts to extract the baryon acoustic oscillation signal (BAO) from the Flagship galaxy mock catalogue with a tomographic approach to constrain the evolution of the Universe and infer its cosmological parameters. We measured the two-point angular correlation function in 13 redshift bins. A template-fitting approach was applied to the measurement to extract the shift of the BAO peak through the transverse Alcock-Paczynski parameter $α$. A joint analysis of all redshift bins was performed to constrain $α$ at the effective redshift $z_\mathrm{eff}=0.77$ with MCMC and profile likelihood techniques. We also extracted one $α_i$ parameter per redshift bin to quantify its evolution as a function of time. From these 13 $α_i$, which are directly proportional to the ratio $D_\mathrm{A}/r_\mathrm{d}$, we constrain the product $h\,r_\mathrm{d}$ and $Ω_\mathrm{m}$. From the joint analysis, we constrain $D_\mathrm{A}/r_\mathrm{d}=10.764^{+0.0839}_{-0.0849}$, a three-fold improvement over current constraints from the Dark Energy Survey. As expected, the constraining power in the analysis of each redshift bin is lower, with an uncertainty ranging from $\pm\,1.23$ to $\pm\,0.289$. From these results, we constrain $Ω_\mathrm{m}=0.296^{+0.074}_{-0.059}$ and $h\,r_\mathrm{d}=99.35^{+4.29}_{-4.55}\,\mathrm{Mpc}$. We quantify the influence of analysis choices such as the template, scale cuts, redshift bins, and systematic effects such as redshift-space distortions, over our constraints, both at the level of the extracted $α_i$ parameters and at the level of cosmological inference.

astro-ph.CO↗
arXiv2026

Euclid preparation. Testing analytic models of galaxy intrinsic alignments in the Euclid Flagship simulation

We model intrinsic alignments (IA) in Euclid's Flagship simulation to investigate its impact on Euclid's weak lensing signal. Our IA implementation in the Flagship simulation takes into account photometric properties of galaxies as well as their dark matter host halos. We compare simulations against theory predictions, determining the parameters of two of the most widely used IA models: the Non Linear Alignment (NLA) and the Tidal Alignment and Tidal Torquing (TATT) models. We measure the amplitude of the simulated IA signal as a function of galaxy magnitude and colour in the redshift range $0.1<z<2.1$. We find that both NLA and TATT can accurately describe the IA signal in the simulation down to scales of $6$-$7 \,h^{-1}\,$Mpc. We measure alignment amplitudes for red galaxies comparable to those of the observations, with samples not used in the calibration procedure. For blue galaxies, our constraints are consistent with zero alignments in our first redshift bin $0.1 < z < 0.3$, but we detect a non-negligible signal at higher redshift, which is, however, consistent with the upper limits set by observational constraints. Additionally, several hydrodynamical simulations predict alignment for spiral galaxies, in agreement with our findings. Finally, the evolution of alignment with redshift is realistic and comparable to that determined in the observations. However, we find that the commonly adopted redshift power-law for IA fails to reproduce the simulation alignments above $z=1.1$. A significantly better agreement is obtained when a luminosity dependence is included, capturing the intrinsic luminosity evolution with redshift in magnitude-limited surveys. We conclude that the Flagship IA simulation is a useful tool for translating current IA constraints into predictions for IA contamination of Euclid-like samples.

astro-ph.CO↗
arXiv2026

Euclid: Disky titans - surprisingly high star formation activity and gas content in two brightest group galaxies at z~0.75

We present the discovery of two disky titans in the first data release of the Euclid satellite. These sources are massive (M>10^11 Msun) star-forming (SFR~20 Msun/yr) discs located in strong over-densities at intermediate redshift (z~0.75). They represent an small fraction of the massive galaxies in over-dense regions (just four candidates in more than 20 deg2 analysed in this study), and their existence is puzzling considering the abundance of passive and bulge-dominated sources commonly found at the centre of groups and clusters at low redshift. Firstly, our analysis shows that these objects are located in massive groups (Mh~10^13.8 Msun), where rapid accretion of cold gas should be prevented from the formation of a static hot halo. Despite this, a millimetre follow-up with NOEMA shows significant cold gas reservoirs (M_h2~10^10.3 Msun) within these sources. Secondly, our morphological analysis shows the presence of a massive and passive bulge in these galaxies, which is expected to stabilise the disc against fragmentation thereby suppressing further star formation. However, these sources lie on the Schmidt-Kennicutt relation or even slightly above. Building on these observations, we propose a scenario where these disky titans are the product of a merger-induced rejuvenation episode, in which the most massive galaxy of a group accretes cold gas from another member and briefly restarts star-formation. Such scenario is supported by a comparison with the TNG300 simulation and easily explains the surviving of star-formation activity in massive galaxies in over-dense environments as temporary stages in a more complex evolution. More in general, our study showcases the ability of Euclid to find rare objects thanks to the unprecedented statistics offered by its surveys and the scientific potential residing in the synergy between Euclid and other facilities observing at longer wavelengths.

astro-ph.GA↗
arXiv2026

Euclid preparation. XCV. Cosmology Likelihood for Observables in Euclid (CLOE). 2. Code implementation

We provide a description of the code implementation and structure of Cosmology Likelihood for Observables in Euclid (CLOE), developed by members of the Euclid Consortium. CLOE is a modular Python code for computing the theoretical predictions of cosmological observables and evaluating them against state-of-the-art data from galaxy surveys such as Euclid in a unified likelihood. This primarily includes the core observables of weak gravitational lensing, photometric galaxy clustering, galaxy-galaxy lensing, and spectroscopic galaxy clustering, but also extended probes such as the clusters of galaxies and cross-correlations of galaxy positions and shapes with the cosmic microwave background. While CLOE has been developed to serve as the unified framework for the parameter inferences in Euclid, it has general capabilities that can serve the broader cosmological community. It is different from other comparable cosmological tools in that it is written entirely in Python, performs the full likelihood calculation, and includes both photometric and spectroscopic observables. We focus on the primary probes of Euclid and describe the overall code structure, rigorous code development practices, extensive documentation, unique features, speed optimization, and future development plans. CLOE is publicly available at https://github.com/cloe-org/cloe.

astro-ph.CO↗
arXiv2026

Euclid preparation XCVII. Cosmology Likelihood for Observables in Euclid (CLOE). 4: Validation and performance

The Euclid satellite will provide data on the clustering of galaxies and on the distortion of their measured shapes, which can be used to constrain and test the cosmological model. However, the increase in precision places strong requirements on the accuracy of the theoretical modelling for the observables and of the full analysis pipeline. In this paper, we investigate the accuracy of the calculations performed by the Cosmology Likelihood for Observables in Euclid (CLOE), a software able to handle both the modelling of observables and their fit against observational data for both the photometric and spectroscopic surveys of Euclid, by comparing the output of CLOE with external codes used as benchmark. We perform such a comparison on the quantities entering the calculations of the observables, as well as on the final outputs of these calculations. Our results highlight the high accuracy of CLOE when comparing its calculation against external codes for Euclid observables on an extended range of operative cases. In particular, all the summary statistics of interest always differ less than $0.1\,σ$ from the chosen benchmark, and CLOE predictions are statistically compatible with simulated data obtained from benchmark codes. The same holds for the comparison of correlation function in configuration space for spectroscopic and photometric observables.

astro-ph.CO↗
arXiv2026

Euclid Quick Data Release (Q1): Disc breaks highlight the structural evolution of galaxies through time

Changes in the slope of the radial surface brightness profiles of disc galaxies, or disc breaks, encode valuable information about the processes driving disc formation and growth. Until now, due to the resolution and depth required, statistical studies of break types have been limited mostly to the local Universe. We aim to study the structural properties and redshift evolution of disc breaks in a large and statistically representative sample of galaxies out to redshift z=1 using the superb depth and resolution of Euclid. We develop an automatic pipeline to extract and analyse surface brightness profiles of 8748 disc galaxies selected from the Euclid Q1 dataset. The pipeline includes advanced masking, surface brightness profile extraction, integrated photometry and structural parameter measurements, and piecewise modelling to classify galaxies into Types I (pure exponential), Type II (down-bending), Type III (up-bending), and composite break (e.g., Type~II+III, Type~II+II, etc) profiles. We quantify structural parameters such as scale lengths, break radii, and surface brightness levels, and analyse their distributions and redshift dependence. Our automatic method produces reliable masks and deep surface brightness profiles, and classifies galaxy profiles with an estimated accuracy of 70%. Using a subsample of 4385 galaxies with reliable classifications, we find that the most common profiles are Types II, II+III, I, and III. We find substantial evolution in the fraction of disc breaks with time. Type II profiles dominate at low redshift (50%), while Types III and II+III profiles increase in frequency with lookback time, reaching 30% each at z~1. Our study provides the first statistically robust measurement of the evolution of disc break types over 5 Gyr. The observed trends support a scenario in which secular processes replace environmental influences as galaxies mature.

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arXiv2026

Euclid: Quick Data Release (Q1) -- Optical hotspots in powerful radio galaxies

The hotspots of powerful radio galaxies are the sites of high-energy particle acceleration, but radio observations give us limited information on the electron energy distributions that they generate, which affects our overall understanding of their physics and energetics. Optical synchrotron counterparts to radio galaxy hotspots, which probe high electron energies, have been known for many years, but have only been studied in a small number of objects selected as being very bright in the radio. By combining Low-Frequency Array (LOFAR) observations with the Euclid Q1 data release in the Euclid Deep Field North, we identify 77 powerful radio galaxies with hotspots and show that a small but significant fraction of them, 7--15\%, may have detected infrared and optical counterparts to their hotspots. The hotspot spectral energy distributions in our clearest cases are consistent with particle acceleration up to (but not beyond) 1\,TeV in energy on the assumption of field strengths close to equipartition, whereas non-detected hotspots either do not reach such high electron energies or would have to have very different electron energy spectra or significantly sub-equipartition magnetic field strengths. Two new candidate optical hotspots are clearly detected matching high-resolution LOFAR data; one is remarkable as both the highest-redshift optical hotspot known to date ($z \approx 1.6$) and also the first optical detection of the hotspot of the inner lobe of a restarting source. We discuss approaches to performing a large-scale study of hotspots with the full wide-area Euclid surveys, particularly when combined with the international baselines of LOFAR, enabling for the first time a statistical analysis of high-energy particle acceleration in hotspots.

astro-ph.HE↗
arXiv2026

Euclid: Galaxy cluster detection through the weak lensing effect - algorithm assessment and selection

Weak gravitational lensing offers a powerful way to detect galaxy clusters by directly tracing their total matter content. Its effectiveness increases with the density of background galaxies, a requirement that is now being met by the high sensitivity and wide coverage of current large-area surveys. A prime example is the Euclid telescope, which will map approximately 14 000 deg2 of the sky, measuring the shapes of billions of galaxies and opening a meaningful window for detecting galaxy clusters uniquely through their weak lensing signal. We present the results of nine galaxy cluster detection algorithms in a blind challenge, using 1200 deg2 of Euclid-like weak lensing observations from the DEMNUni-Cov simulations. The performance of these methods was assessed by matching their detections to known synthetic clusters with signal-to-noise ratios greater than 2, satisfying the z-M selection cut, and adopting two different matching procedures. The purpose of the challenge was to identify and improve strategies for galaxy cluster detection via weak lensing for the upcoming Euclid data releases. We pre-selected four methods based on their individual performance and complementarity. Each pre-selected method adopts a distinct approach: AMICO-WL uses an optimal filtering technique, DoG employs Gaussian filtering, O21 relies on aperture-mass filtering, and W234 applies multi-scale wavelet filtering. Together, the results of these methods can be merged to enhance the processing of Euclid data. Individually, these algorithms reach approximately 10% completeness for a mean purity of 90%, while their combination leads to roughly a two-fold improvement in overall performance, exceeding 70% completeness for low-redshift, high-mass clusters. When extrapolating the results of this work to Euclid Data Release 1, we expect to detect approximately 2500 galaxy clusters via weak gravitational lensing.

astro-ph.CO↗
arXiv2026

Euclid Quick Data Release (Q1) Euclid spectroscopy of quasars. 2. Physical properties from spectral fitting

A substantial volume of spectroscopic data has become available with the Euclid Data Release (Q1), which represents a unique opportunity to study quasars (QSOs) in the Euclid Deep Fields, provide spectroscopic measurements, and estimates of their physical properties. We present results from a spectroscopic analysis of QSOs with HE < = 22.5 using Q1. We use external catalogues from surveys such as the Dark Energy Spectroscopic Instrument (DESI), Gaia, Wide-field Infrared Survey Explorer (WISE), and the QUasars as BRIght beacons for Cosmology in the Southern hemisphere (QUBRICS) for the QSO selection and redshift determination, totaling 5489 QSOs. We provide measurements of the line fluxes, full widths at half-maximum, and respective uncertainties for the emission lines of Euclid spectra based on fitting statistica and signal-to-noise ratio (S/N). Of the total 5489 QSOs, 5387 are successfully fitted. We find that 45% of the total number of successfully fitted spectra are free of problematic artefacts allowing for trustworthy measurements. Our final Euclid QSO sample spans a redshift range of 0.01 < = z < = 4.8 and shows a median spectral index of αλ = - 1.17. Finally, we estimate black hole masses with single epoch methods for 1213 sources, using the He I λ10830, Pa β, H α, H β, and Mg II emission lines, obtainning a mean logarithmc mass of log10 (MBH / Msolar) = 8.7 and mean Edigton ratio of λEdd = 0.34. The catalogue with the physical properties of this sample together with the software developed for the analysis is available online for scientific exploitation.

astro-ph.GA↗
arXiv2026

Euclid: Data Release 1 (DR1) -- Fornax-7, an ultra-faint companion to the Fornax dwarf spheroidal galaxy? A remote star cluster or dwarf satellite of the Fornax dSph

We report the discovery of an ultra-faint stellar system in the vicinity of the Fornax dwarf spheroidal (dSph) galaxy in Euclid DR1. This system, which we designate Fornax-7, is located $1.1$ deg from the centre of the Fornax dSph at about $145$ kpc, corresponding to a projected distance of $\sim 2.8$ kpc assuming the distance of the Fornax dSph. We characterise Fornax-7 using constraints from its colour-magnitude diagram (CMD), luminosity function (LF), and integrated optical colours from ground-based data. These observables are modelled jointly through a forward-modelling framework to constrain its distance modulus, stellar population properties, and total stellar mass. Assuming a low metallicity of $-2.2 \leq {\rm [M/H]} \leq -2.0$, similar to known ultra-faint systems around the Milky Way and the LMC, we derive a distance modulus consistent with that of the Fornax dSph. We estimate an age of $(10.4\pm1.9)$ Gyr and infer a stellar mass of $170^{+50}_{-62}$ solar masses. Relaxing the metallicity constraint and allowing for a wider range that covers the metallicity of the known old Fornax globular clusters (GCs), we find a metal-poor population but with a higher metallicity ${\rm [M/H]}=-1.4\pm0.3$, while the age, distance modulus, and mass remain mostly the same. These results suggest a likely physical association with the Fornax dSph. Fornax-7 may represent either an extremely remote star cluster associated with Fornax, a dwarf satellite of Fornax (i.e., a "satellite of a satellite"), or an independent ultra-faint satellite in the outer halo of the Milky Way. In all cases, it constitutes an exceptional system probing the lowest-mass regime of galaxy and star cluster formation, and the structure of dwarf galaxy haloes. Deeper photometric and spectroscopic follow-up observations are required to confirm both the nature of the system and its dynamical connection to Fornax.

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arXiv2026

Euclid: Quick Data Release (Q1) -- Exploring the detailed visual morphology of galaxies in clusters

Galaxy clusters provide unique laboratories for studying environmental effects on galaxy evolution. The morphology--density ($T$--$Σ$) and morphology--cluster-centric radius ($T$--$R$) relations trace how galaxy morphology is influenced by the environment, but previous studies at intermediate redshifts have been limited in both sample size and radial coverage. We use the Euclid Quick Data Release 1 (Q1) visual morphology catalogue to measure the $T$--$Σ$ and $T$--$R$ relations for smooth, featured-or-disc, and barred galaxies in known clusters at $0.2\leq z\leq 0.5$, extending this analysis to large cluster-centric distances ($3 R_{500c}$) and studying their dependence on stellar mass. Using photometric redshifts and stellar masses provided by Euclid, we identify 1754 cluster members within $1.5 R_{500c}$, distributed across 71 clusters. We classify the identified galaxies as smooth, featured-or-disc, or barred using the predicted vote fractions provided by the Zoobot deep learning foundation model in the Q1 visual morphology catalogue. We confirm the $T$--$Σ$ relation in all the stellar mass ranges studied, with a stronger influence of the cluster environment on galaxy morphology in the densest parts of the cluster and closer to the cluster centre. Beyond $1.5 R_{500c}$, the morphological segregation weakens, with featured-or-disc galaxies overtaking smooth galaxies at the lowest densities, consistent with the growing contribution of field interlopers in the cluster outskirts. For barred galaxies, we find a tentative decline of the bar fraction toward lower densities that is most pronounced for the most massive galaxies. In conclusion, our results show that the cluster environment drives a progressive transformation of galaxy morphology, with the loss of disc structure becoming more pronounced towards the cluster core, where environmental processes act most efficiently.

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arXiv2026

Euclid. A two-point correlation approach to diagnosing star-related systematics in the Euclid spectroscopic survey

The Euclid spectroscopic survey will measure galaxy clustering with unprecedented precision, requiring stringent control of observational and instrumental systematics. Star-related effects may contaminate spectroscopic images through photometric persistence and imperfect masking of stars. We characterized their impact on galaxy clustering analyses, focusing on angular features identifiable in the data. We used angular and spatial auto- and cross-correlation statistics. For galaxies, we used mock spectroscopic catalogues from the EuclidLargeMocks in a 330 deg2 region of the Euclid Wide Survey (EWS). For stars, we used Gaia and 2MASS catalogues in the same area. To simulate photometric persistence, we implemented a simplified detector-level model calibrated on spectroscopic measurements and varied its strength to introduce different interloper fractions. For stellar masking, we modelled inconsistencies between the mask applied to the data and to the random catalogue. We measured star-star, galaxy-galaxy, and star-galaxy angular correlation functions using the Landy--Szalay estimator, and quantified deviations from the expected null star-galaxy correlation. We also evaluated the large-scale impact of such systematics through the three-dimensional two-point correlation function (2PCF). Photometric persistence produces a characteristic feature in the star-galaxy angular cross-correlation at scales of order 100", corresponding to the Euclid dithering pattern and grism dispersion geometry, and a spurious positive signal approximately constant up to 1°. Star-galaxy cross-correlation can detect residual persistence at contamination levels as low as 10% in a Data Release 1 spectroscopic catalogue. In contrast, stellar-mask mismatches produce strong small-scale angular signatures but have negligible impact on the large-scale 2PCF under realistic EWS conditions.

astro-ph.CO↗
arXiv2026

Euclid preparation. The shape of halo profiles in $Λ$CDM and non-standard cosmologies

We study the shape of three-dimensional and projected dark-matter halo profiles extracted from cosmological $N$-body simulations in $Λ$CDM and non-standard cosmologies, using the \texttt{DUSTGRAIN-PF} and \texttt{DEMNUni} suites. The models considered include massive neutrinos, $f(\mathcal{R})$ gravity, and dynamical dark energy. By comparing density, mass, velocity-dispersion, and excess-surface-density profiles up to $5\,r_{500{\rm c}}$, we quantify the differential imprint of non-standard physics on halo structure in view of \textit{Euclid} cluster WL studies. Our main analysis is performed at $z=1.1$, a high-redshift regime where the weak-lensing signal-to-noise starts to degrade, providing a conservative stress test for detectability; for \texttt{DUSTGRAIN-PF} we additionally analyse $z=0.5$ and $z=0.3$ snapshots. In low-mass haloes ($M_{\rm 200c}<7\times10^{13}\,M_\odot$), $f(\mathcal{R})$ gravity produces deviations of order $10\,\%$ in projected and three-dimensional profiles, especially in the outskirts where screening is less efficient. Massive neutrinos partially reduce this signal, reflecting the competition between free streaming and fifth-force-enhanced growth. Dynamical dark energy and massive-neutrino cosmologies generally induce smaller, few-percent deviations, with the largest effects again found in low-mass haloes and at large radii. Under simplified assumptions for \Euclid WL, detecting such profile differences at $z=1.1$ requires stacks of $\sim10^5$ haloes, while a few thousands haloes may be sufficient at $z\lesssim0.5$. This further calls for the need of integrating such precise modelling of non-standard effects -- along with other observational effects -- in any likelihood involving \textit{Euclid} WL masses to avoid non-negligible systematic biases. Concentration--mass relations show weaker cosmology dependence, typically at the $\sim5\,\%$ level. [...]

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