Showing 1–50 of 329 results for author: Zacchei, A

LiteBIRD Mission Overview after Mission Reformation

Authors: LiteBIRD Collaboration, K. Aizawa, H. Akamatsu, R. Akizawa, E. Allys, A. Anand, D. Audley, J. Aumont, S. Azzoni, C. Baccigalupi, M. Ballardini, A. J. Banday, G. Barbieri Ripamonti, R. B. Barreiro, N. Bartolo, S. Basak, M. Bersanelli, A. Besnard, D. Blinov, F. Bouchet, F. Boulanger, N. Brancadori, T. Brinckmann, E. Calabrese, P. Campeti , et al. (150 additional authors not shown)

Abstract: LiteBIRD is a JAXA-led space mission designed to produce all-sky microwave polarization maps. Its primary science goal is to test representative inflationary models by measuring the cosmic microwave background $B$-mode polarization generated by primordial gravitational waves, while also providing new insights into cosmology, particle physics, and astrophysics. The mission concept has been updated… ▽ More LiteBIRD is a JAXA-led space mission designed to produce all-sky microwave polarization maps. Its primary science goal is to test representative inflationary models by measuring the cosmic microwave background $B$-mode polarization generated by primordial gravitational waves, while also providing new insights into cosmology, particle physics, and astrophysics. The mission concept has been updated following the reformation activities initiated after the Mission Definition Review in 2024. The current concept preserves the central scientific objectives, while simplifying the payload configuration: a single telescope covers 12 frequency bands with band centers spanning 40 to 402~GHz, corresponding to an optical coverage of 34--448~GHz. The telescope is a cross-Dragone reflector with a 500~mm aperture diameter, cooled to approximately 5~K and coupled to transition-edge-sensor bolometer arrays operated at 0.1~K. LiteBIRD will observe from a Lissajous orbit around the Sun--Earth L2 point during a nominal 3-year survey. More specifically, the primary scientific objective is to achieve total uncertainty in the tensor-to-scalar ratio of $δr < 0.002$ (68\% C.L.), including contributions from foreground residuals, statistical uncertainties, instrumental systematics, and margin contingency. The corresponding map-noise requirements are specified separately for the low-, mid-, and high-frequency ranges over the reionization and recombination multipole ranges. This sensitivity makes LiteBIRD unique not only for inflationary science but also for a broad range of scientific investigations probing the history of both the early and late Universe, as well as for astrophysical processes, including Galactic science. This paper summarizes the scientific objectives, updated payload and instrument concepts, observation strategy, and ground segment plans. △ Less

Submitted 28 July, 2026; originally announced July 2026.

Comments: 11 pages, 3 figures, 2 tables. SPIE Astronomical Telescopes + Instrumentation 2026

Euclid Quick Data Release (Q1). Searching for radio-selected \Euclid-dark galaxies in the EDF-N

Authors: Euclid Collaboration, M. Giulietti, I. Prandoni, L. Bisigello, G. Rodighiero, M. Talia, F. Gentile, G. Girardi, M. Bondi, L. Wang, A. La Marca, P. A. C. Cunha, R. Hill, A. Abghari, D. Scott, G. Zamorani, G. A. Mamon, A. Lapi, M. Behiri, G. Santhosh, H. J. A. Rottgering, R. Gilli, R. Scaramella, G. Gandolfi, S. Andreon , et al. (286 additional authors not shown)

Abstract: We present and investigate the properties of a sample of radio-selected, Euclid-dark galaxies, identified from LOFAR HBA observations at 144 MHz within the Euclid Deep Field-North (EDF-N). Starting from radio sources lacking optical counterparts in previous surveys, but detected with Spitzer/IRAC, we identified 166 galaxies with no emission at a more than $3σ$ level in Euclid Quick Release 1 (Q1)… ▽ More We present and investigate the properties of a sample of radio-selected, Euclid-dark galaxies, identified from LOFAR HBA observations at 144 MHz within the Euclid Deep Field-North (EDF-N). Starting from radio sources lacking optical counterparts in previous surveys, but detected with Spitzer/IRAC, we identified 166 galaxies with no emission at a more than $3σ$ level in Euclid Quick Release 1 (Q1) images, and no matches in the Euclid Q1 catalogue. To minimise contamination from nearby sources, we selected a sub-sample of 88 isolated galaxies. By exploiting multi-band images and catalogues available for the EDF-N, we inferred the physical properties of our sample via SED fitting. The resulting redshift distribution spans $0.4 \leq z_\mathrm{ph} \leq 5.0$. We used recent sub-arcsecond imaging from the International LOFAR Telescope to constrain the nature of the compact radio emission through brightness temperature estimates. By combining this information with the radio excess relative to the infrared/radio correlation (IRRC), we searched for possible active galactic nuclei (AGN) activity. Approximately 40% of our sources show evidence of AGN activity. The Euclid-dark sources detected in the far-infrared are consistent with a population of heavily obscured, massive star-forming galaxies with high star formation rates. Their location above the star-forming main sequence is consistent with similar near-infrared-dark galaxy populations reported in the literature. We also performed a UV-to-radio median stacking analysis, finding that the two subsamples exhibit similar global physical properties and differ primarily in their radio emission. These preliminary results indicate that the wide area covered by Euclid enables the identification of a higher fraction of systems in which intense star formation and AGN activity coexist, likely capturing a key phase of galaxy--black hole co-evolution. △ Less

Submitted 8 July, 2026; originally announced July 2026.

Comments: Paper accepted by A&A, 20 pages, 14 figures, 4 tables

Euclid Quick Data Release (Q2) -- The Euclid Galactic Bulge Survey

Authors: J. -P. Beaulieu, E. Bachelet, M. Gilles, C. Ranc, E. N. Rektsini, E. Kerins, M. T. Penny, H. Verma, S. Mottet, R. Vavrek, V. Bozza, J. -C. Cuillandre, M. Chang, L. R. Bedin, K. Kuijken, C. Laigle, M. Libralato, I. McDonald, R. Nakajima, V. Popoff, J. Rhodes, E. Thygesen, S. Awan, J. E. Davies, A. M. Di Giorgio , et al. (151 additional authors not shown)

Abstract: The Euclid Quick Data Release Q2 provides an unprecedented deep, wide-field, high-angular-resolution view of the inner Galactic bulge through the Euclid Galactic Bulge Survey (EGBS). Nine contiguous fields covering 4.8 deg$^2$ were observed with the VIS instrument over 24 hours in March 2025. Unlike the nominal Euclid survey strategy, the EGBS employed 16 dithered exposures of 400 s each, correspo… ▽ More The Euclid Quick Data Release Q2 provides an unprecedented deep, wide-field, high-angular-resolution view of the inner Galactic bulge through the Euclid Galactic Bulge Survey (EGBS). Nine contiguous fields covering 4.8 deg$^2$ were observed with the VIS instrument over 24 hours in March 2025. Unlike the nominal Euclid survey strategy, the EGBS employed 16 dithered exposures of 400 s each, corresponding to a total integration time of 1.8 hours per field. Dedicated calibration observations were obtained to derive point spread functions (PSFs). The primary objective of the EGBS is gravitational microlensing, with high-angular-resolution imaging enabling lens-mass measurements to better than 10%. The survey provides an optical reference dataset for previously discovered microlensing events and future observations of the Galactic bulge. The Q2 data were processed with the VIS-PF pipeline. The non-standard observing strategy required dedicated calibration products, while the extreme stellar density necessitated modifications to the astrometric calibration and cosmic-ray detection procedures. These improvements have since been incorporated into later pipeline versions. We describe the motivation, observing strategy, data processing, products, and first results of the EGBS. Astrometric residuals with respect to the Gaia DR3 catalogue are 5.5 mas in right ascension and 4.4 mas in declination across the survey area, while the photometric zero points are calibrated to 1.5%. The Q2 release includes calibrated single-dither images, dedicated PSF models, and photometric catalogues containing approximately 45 million detected sources per dither down to AB magnitude 26. Although incomplete because of extreme crowding, these products provide a robust astrometric and photometric foundation for future exploitation of the dataset. △ Less

Submitted 24 June, 2026; originally announced June 2026.

Comments: 21 pages, 19 figures, submitted for publication to Astronomy and Astrophysics. (abridged abstract)

Euclid Quick Data Release (Q1): The geometry of dark matter halos from extragalactic streams

Authors: Euclid Collaboration, N. Starkman, J. Nibauer, S. Pearson, S. Wu, M. Walmsley, L. Necib, J. Bovy, F. R. Marleau, E. Sola, D. Scott, B. Altieri, S. Andreon, N. Auricchio, C. Baccigalupi, M. Baldi, A. Balestra, S. Bardelli, P. Battaglia, A. Biviano, M. Bolzonella, E. Branchini, M. Brescia, J. Brinchmann, S. Camera , et al. (273 additional authors not shown)

Abstract: Wide-field surveys like Euclid mark a new era of extragalactic stellar stream studies. With a large number of streams, it is now possible to constrain the dark matter halos of galaxies in a cosmological volume and draw comparisons to theoretical expectations for the geometry of dark matter halos. This study combines Euclid imaging with visual detection and segmentation annotations to analyse strea… ▽ More Wide-field surveys like Euclid mark a new era of extragalactic stellar stream studies. With a large number of streams, it is now possible to constrain the dark matter halos of galaxies in a cosmological volume and draw comparisons to theoretical expectations for the geometry of dark matter halos. This study combines Euclid imaging with visual detection and segmentation annotations to analyse streams. We use projected stream morphologies to constrain the shape and centre-of-mass position (CoM) of each host galaxy's potential, jointly probing baryonic and dark matter distributions. These inferences complement weak lensing methods, with sensitivity to halo profile and geometry on sub-virial scales. The method enables both stacked, population-level constraints on halo flattening and CoM position, and constraints on these quantities for individual halos. We also present a novel method for transforming segmentation maps of stellar streams into smooth, curvature-preserving tracks optimised for fast and robust dynamical inference. This approach enables rapid modelling of stream morphology, supports a statistically rigorous combination of constraints across multiple streams within a single galaxy, and enables joint inference across galactic hosts. From our study of 13 galaxies with prominent tidal streams, we find agreement with spherical halos, albeit a mild preference for flattening with $q = 0.95^{+0.05}_{-0.10}$ at 68\% confidence. This is promising early agreement with $Λ$CDM predictions. With thousands more discovered streams expected across \Euclid's mission, our programme will enable precise measurements of halo shapes and CoM positions across large samples and redshifts, offering constraints on the geometry of dark matter halos. △ Less

Submitted 19 June, 2026; originally announced June 2026.

Comments: Euclid Key Paper. Submitted to A&A. Paper: 21 pages, 8 figures, 1 table. Total with Appendices: 40 pages, 21 figures, 2 tables

Euclid: Quick Data Release (Q1) -- LensMC shear measurement catalogue for cluster lensing science

Authors: G. Congedo, M. Sereno, H. Miyatake, S. Guerrini, M. Kilbinger, M. Radovich, H. Jansen, F. Kleinebreil, T. Schrabback, A. N. Taylor, B. Altieri, L. Amendola, S. Andreon, N. Auricchio, C. Baccigalupi, M. Baldi, S. Bardelli, P. Battaglia, A. Biviano, R. P. Blake, E. Branchini, M. Brescia, S. Camera, V. Capobianco, C. Carbone , et al. (118 additional authors not shown)

Abstract: We present a LensMC lensing analysis of Euclid Quick Release 1 images that were made available in March 2025. We measured shapes, positions, weights, and other morphological parameters of galaxies with a surface number density of $26\;\text{arcmin}^{-2}$ for $I_{\scriptscriptstyle\rm E}<24.5$, achieving 75 arcmin$^{-2}$ for $I_{\scriptscriptstyle\rm E}<27$, in 63 deg$^2$ of Euclid VIS images. This… ▽ More We present a LensMC lensing analysis of Euclid Quick Release 1 images that were made available in March 2025. We measured shapes, positions, weights, and other morphological parameters of galaxies with a surface number density of $26\;\text{arcmin}^{-2}$ for $I_{\scriptscriptstyle\rm E}<24.5$, achieving 75 arcmin$^{-2}$ for $I_{\scriptscriptstyle\rm E}<27$, in 63 deg$^2$ of Euclid VIS images. This is the first shear measurement catalogue produced with \lensmc in anticipation of the Euclid Data Release 1. To within the scope of this work and availability of survey volume, we found no spatial dependency in the additive biases. However, we developed an empirical bias correction based jointly on galaxy sizes and magnitudes, which was applied to the measurement of the lensing signal over the whole area. In order to validate the quality of our measurements, we calculated two-point statistics and cluster profile measurements, and cross-checked results with external cluster catalogues from WISE and the Dark Energy Survey. Additionally, by stacking shear profiles of random clusters we found that the low-redshift, large radii bins may be still contaminated by residual systematic effects. Thanks to Euclid image resolution and depth and overall good control of systematic errors, we are able to constrain the lensing profiles of clusters with masses of $10^{14}M_\odot$ out to $z\approx2$ over nearly 10 Gyr of evolution history. △ Less

Submitted 18 June, 2026; originally announced June 2026.

Comments: 11 pages, 10 figures, 1 table

Euclid Quick Data Release (Q1): The impact of AGN emission on SED-derived physical properties

Authors: Euclid Collaboration, B. Laloux, A. Bongiorno, M. Salvato, V. Allevato, M. Mezcua, W. Roster, T. Matamoro Zatarain, S. Paltani, R. Shirley, F. Tarsitano, C. Saulder, S. Fotopoulou, C. Andonie, J. Buchner, F. La Franca, V. Le Brun, F. Ricci, D. Scott, F. Shankar, M. Siudek, J. G. Sorce, L. Spinoglio, Y. Toba, A. Viitanen , et al. (287 additional authors not shown)

Abstract: The Euclid Quick Data Release (Q1) is a powerful dataset to study active galactic nuclei (AGN) and their host galaxies. Deriving their physical properties through multi-component spectral energy distribution (SED) fitting is a challenging task for AGN, but it is greatly aided by the Euclid near-infrared photometry. Here we present a new method to quantify the reliability of SED-derived parameters,… ▽ More The Euclid Quick Data Release (Q1) is a powerful dataset to study active galactic nuclei (AGN) and their host galaxies. Deriving their physical properties through multi-component spectral energy distribution (SED) fitting is a challenging task for AGN, but it is greatly aided by the Euclid near-infrared photometry. Here we present a new method to quantify the reliability of SED-derived parameters, such as AGN bolometric and monochromatic luminosities, host's stellar mass $M_\star$, star-formation rate (SFR) and specific star-formation rate (sSFR), by using mock SEDs of AGN built by combining observed SEDs of QSOs and galaxies. We apply this methodology to the ${\sim}1$ million Q1 AGN candidates, constructing a catalogue of AGN and host galaxy properties, alongside their respective reliability values. With a reliability threshold at 0.5, we find 88\% of sources with robust stellar masses and 76\% with reliable AGN luminosities. Moreover, through SED fitting we also measure the AGN fraction $f_{\rm AGN}$ of the total mid-infrared flux and we use its lower-limit to select AGN. A $f_{\rm AGN, \, low} > 0.075$ threshold yields 85\% completeness and purity. Comparable to colour-colour AGN selections, this method has the advantage of being less affected by redshift evolution and exploring fainter magnitudes. Additionally, by comparing the AGN and host galaxy parameters across different identification methods, we find that the probed range in stellar mass and AGN luminosity can be quite different. This highlights the importance of combining different approaches and accounting for their selection biases when studying AGN and their role in galaxy evolution. Finally, for the X-ray detected sample, we present the X-ray to mid-IR luminosity relation, and the correlation between stellar mass and bolometric luminosity as a function of redshift, in good agreement with previous results. △ Less

Submitted 5 June, 2026; originally announced June 2026.

Comments: accepted in A&A, 27 pages, 16 figures

\textit{Euclid} preparation. Baryon acoustic oscillations extraction techniques: comparison and optimisation

Authors: Euclid Collaboration, E. Sarpa, A. Veropalumbo, M. Bonici, M. Kärcher, M. Crocce, E. Sefusatti, E. Maragliano, E. Branchini, C. Oliveri, G. Gambardella, B. Camacho Quevedo, C. Moretti, P. Monaco, J. Bautista, M. Viel, W. J. Percival, S. Nadathur, A. Pezzotta, A. Eggemeier, A. G. Sánchez, J. Bel, C. Carbone, A. Crespi, S. Radinović , et al. (274 additional authors not shown)

Abstract: We present the first end-to-end validation of the Euclid baryon acoustic oscillation (BAO) analysis pipeline, encompassing density-field reconstruction, two-point correlation function measurement, and cosmological-parameter inference. Using eight Euclid-like mock catalogues from each of four Flagship I snapshots, designed to reproduce the expected statistical properties of the first Euclid data re… ▽ More We present the first end-to-end validation of the Euclid baryon acoustic oscillation (BAO) analysis pipeline, encompassing density-field reconstruction, two-point correlation function measurement, and cosmological-parameter inference. Using eight Euclid-like mock catalogues from each of four Flagship I snapshots, designed to reproduce the expected statistical properties of the first Euclid data release (DR1), we assess the two standard BAO reconstruction methods based on the Zel'dovich approximation, RecSym and RecIso, across $0.9 \leq z \leq 1.8$. The pipeline introduces several methodological advances: an emulator-based model evaluator (Bora.jl) combined with a Hamiltonian Monte Carlo sampler (NUTS), achieving more than a 500-fold speed-up relative to standard Markov chain Monte Carlo, and a semi-analytical covariance estimator (BeXiCov+WinCov) that enables robust error estimates from only eight mock realisations while remaining stable under fiducial-cosmology variations. These components ensure computational efficiency while reducing the risk of underestimating parameter uncertainties. Both reconstruction schemes yield unbiased BAO measurements across all redshifts and analysis choices, including smoothing scale and fiducial cosmology. In each snapshot, reconstruction enhances the figure of merit for $\{Ω_m, H_0 r_s\}$ by $\sim3$, equivalent to tripling the effective survey volume. Combining the four redshift bins, the improvement remains substantial, with BAO-only constraints reaching $\sim10\%$ precision on $Ω_m$ and $\sim3\%$ on $H_0 r_s$. Results from RecSym and RecIso are consistent within uncertainties, though we recommend RecSym during testing due to its lower sensitivity to covariance variations. These findings establish the accuracy, robustness, and scalability of the Euclid BAO pipeline for DR1, providing a solid foundation for future cosmological analyses. △ Less

Submitted 5 May, 2026; originally announced May 2026.

Euclid Quick Data Release (Q1). AstroVink: A vision transformer approach to find strong gravitational lens systems

Authors: Euclid Collaboration, S. H. Vincken, K. Rojas, M. Melchior, N. E. P. Lines, T. E. Collett, A. Verma, P. Holloway, G. Despali, S. Schuldt, R. B. Metcalf, R. Gavazzi, F. Courbin, J. A. Acevedo Barroso, B. Clément, T. Li, D. Sluse, J. Wilde, A. Melo, A. Sonnenfeld, C. Tortora, T. T. Thai, M. Millon, C. Spiniello, A. Manjón-García , et al. (280 additional authors not shown)

Abstract: We present AstroVink, a vision transformer classifier designed for automated identification of strong lens candidates in Euclid imaging. We build upon the DINOv2 encoder, fine tuned to distinguish between lens and non-lens galaxies. Our base model, trained on simulated strong lens systems and labelled non lenses, recovers 88 of the 110 lens candidates within the top 500 ranked candidates, correspo… ▽ More We present AstroVink, a vision transformer classifier designed for automated identification of strong lens candidates in Euclid imaging. We build upon the DINOv2 encoder, fine tuned to distinguish between lens and non-lens galaxies. Our base model, trained on simulated strong lens systems and labelled non lenses, recovers 88 of the 110 lens candidates within the top 500 ranked candidates, corresponding to an inspection efficiency of one lens per 5.7 inspected objects in our test set. After the Q1 data release, which yielded about 500 lens candidates, we retrained the model using high confidence lens candidates and new negatives, initially flagged as potential lenses by other classifiers but rejected during visual inspection. The retrained network further improves performance, achieving recovery of all 110 systems within the same ranking and reducing the inspection effort to one lens per 4.5 inspected objects, demonstrating that incorporating real examples significantly enhances model generalisation. An analysis of training subsets revealed that the inclusion of realistic negative examples played a key role in this improvement. Finally, we applied the retrained model to the Q1 original selection of 1.08M targets, followed by a new round of Space Warps citizen science inspection and expert vetting, where we identified a total of eight Grade A and 26 Grade B new lens candidates. These results demonstrate that transformer based architectures can recover strong lens candidates with high efficiency in real Euclid data, while substantially reducing the number of candidates requiring visual inspection. △ Less

Submitted 23 April, 2026; originally announced April 2026.

Comments: 21 pages, 6 figures, 1 table,submitted to A&A journal, dataset available at https://zenodo.org/records/17425610, software available at https://github.com/SaamieVincken/AstroVink

Euclid: Quick Data Release (Q1) -- Dual AGN in low-mass galaxies

Authors: M. Mezcua, B. Laloux, M. Scialpi, M. Siudek, A. Eróstegui, F. Ricci, T. Matamoro Zatarain, S. Visser, H. J. A. Rottgering, C. M. Gutierrez, A. Feltre, L. Bisigello, C. Saulder, L. Ulivi, J. H. Knapen, H. Domínguez Sánchez, G. Zamorani, K. Rubinur, J. Calhau, L. Spinoglio, F. Shankar, D. Stern, R. Pucha, A. Viitanen, B. Altieri , et al. (142 additional authors not shown)

Abstract: Dual active galactic nuclei (AGNs) are expected in hierarchical galaxy evolution models, in which low-mass galaxies merge to build more massive ones. While observational evidence for dual AGNs is growing in massive galaxies, no clear detection has yet been found in the low-mass regime. We used photometry and spectroscopy from the first \Euclid Quick Data Release, combined with a collection of mult… ▽ More Dual active galactic nuclei (AGNs) are expected in hierarchical galaxy evolution models, in which low-mass galaxies merge to build more massive ones. While observational evidence for dual AGNs is growing in massive galaxies, no clear detection has yet been found in the low-mass regime. We used photometry and spectroscopy from the first \Euclid Quick Data Release, combined with a collection of multi-wavelength data from the Dark Energy Spectroscopic Instrument (DESI), the LOw-Frequency ARray (LOFAR) high band antenna, and counterparts in X-ray and mid-infrared catalogues to identify dual AGNs at redshift $z \lesssim 1$. Focusing on low-mass galaxies with stellar masses below 10$^{10}$ M$_{\odot}$, we find nine dual AGN candidates with projected separations ranging from $\sim$20 to 51 kpc. We also find 49 dual AGN candidates in more massive galaxies. We derive a dual AGN fraction of 0.1\% for the low-mass galaxies and estimate that these systems likely trace a population of progenitor black hole pairs that may evolve into bound binaries and eventually coalesce, emitting gravitational waves in the LISA band. These results constitute the first sample of spectroscopically confirmed dual AGN candidates in low-mass galaxies and have important implications for models in which supermassive black holes grow from lower-mass black holes located in low-mass galaxies, as well as for predictions of gravitational waves from low-mass binary black holes. △ Less

Submitted 14 April, 2026; originally announced April 2026.

Comments: Accepted for publication in A&A

Euclid Quick Data Release (Q1). AgileLens: A scalable CNN-based pipeline for strong gravitational lens identification

Authors: Euclid Collaboration, X. Xu, R. Chen, T. Li, A. R. Cooray, S. Schuldt, J. A. Acevedo Barroso, D. Stern, D. Scott, M. Meneghetti, G. Despali, J. Chopra, Y. Cao, M. Cheng, J. Buda, J. Zhang, J. Furumizo, R. Valencia, Z. Jiang, C. Tortora, N. E. P. Lines, T. E. Collett, S. Fotopoulou, A. Galan, A. Manjón-García , et al. (286 additional authors not shown)

Abstract: We present an end-to-end, iterative pipeline for efficient identification of strong galaxy--galaxy lensing systems, applied to the Euclid Q1 imaging data. Starting from VIS catalogues, we reject point sources, apply a magnitude cut (I$_E$ $\leq$ 24) on deflectors, and run a pixel-level artefact/noise filter to build 96 $\times$ 96 pix cutouts; VIS+NISP colour composites are constructed with a VIS-… ▽ More We present an end-to-end, iterative pipeline for efficient identification of strong galaxy--galaxy lensing systems, applied to the Euclid Q1 imaging data. Starting from VIS catalogues, we reject point sources, apply a magnitude cut (I$_E$ $\leq$ 24) on deflectors, and run a pixel-level artefact/noise filter to build 96 $\times$ 96 pix cutouts; VIS+NISP colour composites are constructed with a VIS-anchored luminance scheme that preserves VIS morphology and NISP colour contrast. A VIS-only seed classifier supplies clear positives and typical impostors, from which we curate a morphology-balanced negative set and augment scarce positives. Among the six CNNs studied initially, a modified VGG16 (GlobalAveragePooling + 256/128 dense layers with the last nine layers trainable) performs best; the training set grows from 27 seed lenses (augmented to 1809) plus 2000 negatives to a colour dataset of 30,686 images. After three rounds of iterative fine-tuning, human grading of the top 4000 candidates ranked by the final model yields 441 Grade A/B candidate lensing systems, including 311 overlapping with the existing Q1 strong-lens catalogue, and 130 additional A/B candidates (9 As and 121 Bs) not previously reported. Independently, the model recovers 740 out of 905 (81.8%) candidate Q1 lenses within its top 20,000 predictions, considering off-centred samples. Candidates span I$_E$ $\simeq$ 17--24 AB mag (median 21.3 AB mag) and are redder in Y$_E$--H$_E$ than the parent population, consistent with massive early-type deflectors. Each training iteration required a week for a small team, and the approach easily scales to future Euclid releases; future work will calibrate the selection function via lens injection, extend recall through uncertainty-aware active learning, explore multi-scale or attention-based neural networks with fast post-hoc vetters that incorporate lens models into the classification. △ Less

Submitted 7 April, 2026; originally announced April 2026.

Comments: 30 pages, 16 figures

Euclid Quick Data Release (Q1). The Strong Lensing Discovery Engine F -- Bright and low-redshift strong lenses

Authors: Euclid Collaboration, L. R. Ecker, M. Fabricius, S. Seitz, R. Saglia, N. E. P. Lines, P. Holloway, T. Li, A. Verma, F. Balzer, Q. Jin, A. Manjón-García, S. H. Vincken, J. Wilde, J. A. Acevedo Barroso, J. W. Nightingale, K. Rojas, S. Schuldt, M. Walmsley, T. E. Collett, G. Despali, A. Sonnenfeld, C. Tortora, R. B. Metcalf, R. Bender , et al. (324 additional authors not shown)

Abstract: We present 72 additional galaxy-galaxy strong lenses that complement the sample discovered in the Euclid Quick Release 1 data (63.1 deg^2) of the Strong Lens Discovery Engine (SLDE) papers A-E. It is shown that previous pre-selection of potential lenses, which excluded objects from the Gaia catalogue, led to missing several bright and low-redshift strong lenses, adding more than 10% new strong len… ▽ More We present 72 additional galaxy-galaxy strong lenses that complement the sample discovered in the Euclid Quick Release 1 data (63.1 deg^2) of the Strong Lens Discovery Engine (SLDE) papers A-E. It is shown that previous pre-selection of potential lenses, which excluded objects from the Gaia catalogue, led to missing several bright and low-redshift strong lenses, adding more than 10% new strong lens candidates compared to the previous search. In total, the catalogue includes 38 "grade A" (confident) and 34 "grade B" (probable) candidates. These lenses are identified through a combination of two independent searches for bright nearby objects: one based on machine-learning models followed by expert visual inspection, and the other based solely on expert visual inspection, targeting objects not included in the initial machine-learning selection (a limitation identified only after extensive visual inspection). With these additional strong lens candidates, we augment the expected number of high-confidence candidates in the Euclid Wide Survey from previous forecasts to 120000. Detailed semi-automated lens modelling confirms at least 41 systems out of 72, a fraction consistent with that found in SLDE A (315 out of 488). These include: multiple edge-on disc lenses; sources with arcs near the lens centre; "red sources"; and an edge-on disk galaxy lensing a galaxy merger, producing two sets of lensed features, an Einstein ring and a doubly imaged component. The median redshift of these systems is $Δ$ z ~ 0.3 lower than that of the SLDE A sample. △ Less

Submitted 30 March, 2026; originally announced March 2026.

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

Authors: Euclid Collaboration, S. Joudaki, V. Pettorino, L. Blot, M. Bonici, S. Camera, G. Cañas-Herrera, V. F. Cardone, P. Carrilho, S. Casas, S. Davini, S. Di Domizio, S. Farrens, L. W. K. Goh, S. Gouyou Beauchamps, S. Ilić, F. Keil, A. M. C. Le Brun, M. Martinelli, C. Moretti, A. Pezzotta, Z. Sakr, A. G. Sánchez, D. Sciotti, K. Tanidis , et al. (323 additional authors not shown)

Abstract: 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 incl… ▽ More 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 will focus on the primary probes of Euclid and will 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. △ Less

Submitted 23 March, 2026; originally announced March 2026.

Comments: Second in a series of six papers presenting CLOE, the Euclid likelihood code; 43 pages, 11 figures, accepted for publication in A&A (with a table of contents added)

Euclid. Properties and performance of the NISP signal estimator

Authors: Euclid Collaboration, F. Cogato, B. Kubik, R. Barbier, S. Conseil, E. Medinaceli, Y. Copin, E. Franceschi, L. Valenziano, N. Aghanim, B. Altieri, S. Andreon, N. Auricchio, C. Baccigalupi, M. Baldi, A. Balestra, S. Bardelli, P. Battaglia, A. Biviano, E. Branchini, M. Brescia, J. Brinchmann, S. Camera, G. Cañas-Herrera, V. Capobianco , et al. (270 additional authors not shown)

Abstract: The Euclid spacecraft, located at the second Lagrangian point of the Sun-Earth system, hosts the Near-Infrared Spectrometer and Photometer (NISP) instrument. NISP is equipped with a mosaic of 16 HgCdTe-based detectors to acquire near-infrared photometric and spectroscopic data. To meet the spacecraft's constraints on computational resources and telemetry bandwidth, the near-infrared signal is proc… ▽ More The Euclid spacecraft, located at the second Lagrangian point of the Sun-Earth system, hosts the Near-Infrared Spectrometer and Photometer (NISP) instrument. NISP is equipped with a mosaic of 16 HgCdTe-based detectors to acquire near-infrared photometric and spectroscopic data. To meet the spacecraft's constraints on computational resources and telemetry bandwidth, the near-infrared signal is processed onboard via a dedicated hardware-software architecture designed to fulfil the stringent Euclid's data-quality requirements. A custom application software, running on the two NISP data processing units, implements the NISP signal estimator: an ad-hoc algorithm which delivers accurate flux measurements and simultaneously estimates the quality of signal estimation through the quality factor parameter. This paper investigates the properties of the NISP signal estimator by evaluating its performance during the early flight operations of the NISP instrument. First, we revisit the assumptions on which the inference of the near-infrared signal is based and investigate the origin of the main systematics of the signal estimator through Monte Carlo simulations. Then, we test the flight performance of the NISP signal estimator. Results indicate a systematic bias lower than 0.01 e/s for 99% of the NISP pixel array, well within the noise budget of the estimated signal. We also derive an analytical expression for the variance of the NISP signal estimator, demonstrating its validity, particularly when the covariance matrix is not pre-computed. Finally, we provide a robust statistical framework to interpret the QF parameter, analyse its dependence on the signal estimator bias, and show its sensitivity to cosmic ray hits on NISP detectors. Our findings corroborate previous results on the NISP signal estimator and suggest a leading-order correction based on the agreement between flight data and simulations. △ Less

Submitted 3 February, 2026; originally announced February 2026.

Euclid preparation. 3D reconstruction of the cosmic web with simulated Euclid Deep spectroscopic samples

Authors: Euclid Collaboration, K. Kraljic, C. Laigle, M. Balogh, P. Jablonka, U. Kuchner, N. Malavasi, F. Sarron, C. Pichon, G. De Lucia, M. Bethermin, F. Durret, M. Fumagalli, C. Gouin, M. Magliocchetti, J. G. Sorce, O. Cucciati, F. Fontanot, M. Hirschmann, Y. Kang, M. Spinelli, N. Aghanim, A. Amara, S. Andreon, N. Auricchio , et al. (299 additional authors not shown)

Abstract: The ongoing Euclid mission aims to measure spectroscopic redshifts for approximately two million galaxies using the H $α$ line emission detected in near-infrared slitless spectroscopic data from the Euclid Deep Fields (EDFs). These measurements will reach a flux limit of $5\times 10^{-17}\,{\rm erg}\,{\rm cm}^{-2}\,{\rm s}^{-1}$ in the redshift range $0.4<z<1.8$, opening the door to numerous inves… ▽ More The ongoing Euclid mission aims to measure spectroscopic redshifts for approximately two million galaxies using the H $α$ line emission detected in near-infrared slitless spectroscopic data from the Euclid Deep Fields (EDFs). These measurements will reach a flux limit of $5\times 10^{-17}\,{\rm erg}\,{\rm cm}^{-2}\,{\rm s}^{-1}$ in the redshift range $0.4<z<1.8$, opening the door to numerous investigations involving galaxy evolution, extending well beyond the mission's core objectives. The achieved H $α$ luminosity depth will lead to a sufficiently high sampling, enabling the reconstruction of the large-scale galaxy environment. We assess the quality of the reconstruction of the galaxy cosmic web environment with the expected spectroscopic dataset in EDFs. The analysis is carried out on the Flagship and GAEA galaxy mock catalogues. The quality of the reconstruction is first evaluated using geometrical and topological statistics measured on the cosmic web, namely the length of filaments, the area of walls, the volume of voids, and its connectivity and multiplicity. We then quantify how accurately gradients in galaxy properties with distance from filaments can be recovered. As expected, the small-scale redshift-space distortions, have a strong impact on filament lengths and connectivity, but can be mitigated by compressing galaxy groups before skeleton extraction. The cosmic web reconstruction is biased when relying solely on H $α$ emitters. This limitation can be mitigated by applying stellar mass weighting during the reconstruction. However, this approach introduces non-trivial biases that need to be accounted for when comparing to theoretical predictions. Redshift uncertainties pose the greatest challenge in recovering the expected dependence of galaxy properties, though the well-established stellar mass transverse gradients towards filaments can still be observed. △ Less

Submitted 15 January, 2026; originally announced January 2026.

Comments: 30 pages, 23 figures. Accepted for publication in A&A

Journal ref: A&A 708, A164 (2026)

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

Authors: Euclid Collaboration, R. Paviot, B. Joachimi, K. Hoffmann, S. Codis, I. Tutusaus, D. Navarro-Gironés, J. Blazek, F. Hervas-Peters, B. Altieri, S. Andreon, N. Auricchio, C. Baccigalupi, M. Baldi, S. Bardelli, A. Biviano, E. Branchini, M. Brescia, S. Camera, G. Cañas-Herrera, V. Capobianco, C. Carbone, V. F. Cardone, J. Carretero, S. Casas , et al. (271 additional authors not shown)

Abstract: 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 Li… ▽ More 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. △ Less

Submitted 12 January, 2026; originally announced January 2026.

Star formation quenching precedes morphological transformation in COSMOS-WEB's richest galaxy groups

Authors: Z. Ghaffari, G. Gozaliasl, A. Biviano, G. Toni, S. Taamoli, M. Maturi, L. Moscardini, A. Zacchei, F. Gentile, M. Haas, H. Akins, R. C. Arango-Toro, Y. Cheng, C. Casey, M. Franco, S. Harish, H. Hatamnia, O. Ilbert, J. Kartaltepe, A. H. Khostovan, A. M. Koekemoer, D. Liu, G. A. Mamon, H. J. McCracken, J. McKinney , et al. (4 additional authors not shown)

Abstract: We analyzed the 25 richest galaxy groups in COSMOS-Web at z = 0.18-3.65, identified via the AMICO algorithm. These groups contain 20-30 galaxies with high (>75%) membership probability. Our study reveals both passive-density and active-density relations: late-type galaxies (LTGs) prefer higher central overdensities than early-type galaxies (ETGs) across all groups, and many massive LTGs exhibit co… ▽ More We analyzed the 25 richest galaxy groups in COSMOS-Web at z = 0.18-3.65, identified via the AMICO algorithm. These groups contain 20-30 galaxies with high (>75%) membership probability. Our study reveals both passive-density and active-density relations: late-type galaxies (LTGs) prefer higher central overdensities than early-type galaxies (ETGs) across all groups, and many massive LTGs exhibit colors typical of quiescent galaxies. We identify red sequences (RS) in 5 groups, prominently established at z < 1, with early emergence in the RS locus up to z ~ 2.2. This suggests group environments represent a transitional phase where star formation quenching precedes morphological transformation, contrasting with the classical morphology-density relation in rich clusters. In the central regions (~33 arcsec / 100 kpc from centers), we identified 86 galaxies: 23 (~27%) ETGs and 63 (~73%) LTGs. High-mass galaxies (M_star > 10^10.5 M_sun) undergo rapid quenching over ~1 Gyr, becoming predominantly spheroidal ETGs. This indicates morphological transformation accelerates in massive systems during peak cosmic star formation. Intermediate-mass galaxies (10^9 < M_star/M_sun < 10^10.5) show mild quenching, while low-mass galaxies (M_star < 10^9 M_sun) remain largely star-forming; here, environmental processes suppress star formation without destroying disks, suggesting group quenching operates on longer timescales than mass quenching. Overall, mass-dependent quenching dominates the high-mass end, while environment shapes lower-mass systems. The HLAGN fraction for both groups and field increases with redshift, peaking at z ~ 2, with groups consistently showing higher fractions. We suggest AGN feedback partially drives rapid quenching in high-mass galaxies, while mergers may trigger AGN activity. △ Less

Submitted 9 January, 2026; originally announced January 2026.

Journal ref: A&A 708, A129 (2026)

Euclid preparation. Review of forecast constraints on dark energy and modified gravity

Authors: Euclid Collaboration, N. Frusciante, M. Martinelli, L. Lombriser, A. Silvestri, M. Archidiacono, M. Baldi, M. Ballardini, N. Bartolo, E. Bellini, G. Benevento, D. Bertacca, C. Bonvin, B. Bose, P. Brax, V. F. Cardone, S. Casas, M. Y. Elkhashab, P. G. Ferreira, F. Finelli, F. Hassani, S. Ilić, K. Koyama, M. Kunz, F. Lepori , et al. (304 additional authors not shown)

Abstract: The Euclid mission has been designed to provide, as one of its main deliverables, information on the nature of the gravitational interaction, which determines the expansion of the Universe and the formation of structures. Thus, Euclid has the potential to test deviations from general relativity that will allow us to shed light on long-lasting problems in the standard cosmological model, $Λ$CDM. Eu… ▽ More The Euclid mission has been designed to provide, as one of its main deliverables, information on the nature of the gravitational interaction, which determines the expansion of the Universe and the formation of structures. Thus, Euclid has the potential to test deviations from general relativity that will allow us to shed light on long-lasting problems in the standard cosmological model, $Λ$CDM. Euclid will mainly do this by using two complementary probes: weak gravitational lensing and galaxy clustering. In this paper we review pre-launch Euclid analyses for dark energy and modified gravity. These include forecast constraints with future Euclid data on cosmological parameters for different cosmological models, such as a time-varying dark energy component, phenomenological modifications of the perturbation sector and specific modified gravity models, with further extensions that include neutrino physics and the coupling to the electromagnetic sector through the fine-structure constant. We review the study of the impact of nonlinear clustering methods on beyond-$Λ$CDM constraints with Euclid. This is of fundamental importance to efficiently predict the large-scale clustering of matter and dark matter halos, given that we will have access to a wealth of information on scales beyond the linear regime. We inspect the extension of theoretical predictions for observable quantities in alternative cosmologies to $Λ$CDM at fully nonlinear scales by means of $N$-body simulations. We discuss the impact of relativistic corrections in extended cosmological models. Overall, this review highlights the significant potential of the Euclid mission to tightly constrain parameters of dark energy and modified gravity models, or perhaps to detect possible signatures of a $Λ$CDM failure. △ Less

Submitted 10 December, 2025; originally announced December 2025.

Comments: 28 pages, 3 figures

Euclid Quick Data Release (Q1): Euclid spectroscopy of quasars. 1. Identification and redshift determination of 3500 bright quasars

Authors: Euclid Collaboration, Y. Fu, R. Bouwens, K. I. Caputi, D. Vergani, M. Scialpi, B. Margalef-Bentabol, L. Wang, M. Bolzonella, M. Banerji, E. Bañados, A. Feltre, Y. Toba, J. Calhau, F. Tarsitano, P. A. C. Cunha, A. Humphrey, G. Vietri, F. Mannucci, S. Bisogni, F. Ricci, H. Landt, L. Spinoglio, T. Matamoro Zatarain, D. Stern , et al. (332 additional authors not shown)

Abstract: The slitless spectroscopy mode of the NISP onboard Euclid has enabled efficient spectroscopy of objects within a large FoV. We present a large and homogeneous sample of bright quasars identified from the Euclid Quick Data Release (Q1) by combining high-purity candidate selections from Gaia and WISE with the NISP spectra. Through visual inspection of the Euclid spectra of these quasar candidates, w… ▽ More The slitless spectroscopy mode of the NISP onboard Euclid has enabled efficient spectroscopy of objects within a large FoV. We present a large and homogeneous sample of bright quasars identified from the Euclid Quick Data Release (Q1) by combining high-purity candidate selections from Gaia and WISE with the NISP spectra. Through visual inspection of the Euclid spectra of these quasar candidates, we identify approximately 3500 quasars with reliable redshifts at $0<z\lesssim 4.8$. Of these, 2686 are new spectroscopic identifications relative to existing public compilations. We generate the first Euclid composite spectrum of quasars covering rest-frame NUV to NIR wavelengths without telluric lines, which will be pivotal to NIR quasar spectral analysis. We obtain an empirical spectroscopic depth of $J_{\rm E}\lesssim 21.5$ and $H_{\rm E}\lesssim 21.3$ at the sensitivity of the Wide Field Survey, beyond which the number of securely identified quasars declines sharply. We analyse VIS morphologies using Sersic and CAS metrics, and a deep-learning PSF fraction to track nuclear dominance. At low redshift ($z<0.5$), obvious host structures are common and a single Sersic model fits about half of the sources; at intermediate redshift ($0.5<z<2$), the nuclear component dominates, with 90% of the Sersic fits saturating at the upper index limit. In this intermediate redshift regime, $f_{\rm PSF}$ is available, and we use it as a more reliable compactness measure than the single-Sersic and CAS parameters to quantify nuclear versus host emission. We also explore the novel Euclid NIR colour space and discuss the role of these quasars in refining AGN selection techniques for future Euclid data releases. The spectroscopic bright quasar catalogue of this work, and the composite quasar spectrum, will be available at https://cdsarc.cds.unistra.fr/. (abridged) △ Less

Submitted 4 March, 2026; v1 submitted 9 December, 2025; originally announced December 2025.

Comments: 25 pages, 16 figures, 5 tables, accepted by A&A Euclid Quick Data Release (Q1) special issue. Data available upon request

Journal ref: A&A, 711, A285 (2026)

Euclid Quick Data Release (Q1). From simulations to sky: Advancing machine-learning lens detection with real Euclid data

Authors: Euclid Collaboration, N. E. P. Lines, T. E. Collett, P. Holloway, K. Rojas, S. Schuldt, R. B. Metcalf, T. Li, A. Verma, G. Despali, F. Courbin, R. Gavazzi, C. Tortora, B. Clément, N. Aghanim, B. Altieri, L. Amendola, S. Andreon, N. Auricchio, C. Baccigalupi, M. Baldi, A. Balestra, S. Bardelli, P. Battaglia, A. Biviano , et al. (279 additional authors not shown)

Abstract: In the era of large-scale surveys like Euclid, machine learning has become an essential tool for identifying rare yet scientifically valuable objects, such as strong gravitational lenses. However, supervised machine-learning approaches require large quantities of labelled examples to train on, and the limited number of known strong lenses has lead to a reliance on simulations for training. A well-… ▽ More In the era of large-scale surveys like Euclid, machine learning has become an essential tool for identifying rare yet scientifically valuable objects, such as strong gravitational lenses. However, supervised machine-learning approaches require large quantities of labelled examples to train on, and the limited number of known strong lenses has lead to a reliance on simulations for training. A well-known challenge is that machine-learning models trained on one data domain often underperform when applied to a different domain: in the context of lens finding, this means that strong performance on simulated lenses does not necessarily translate into equally good performance on real observations. In Euclid's Quick Data Release 1 (Q1), covering 63 deg2, 500 strong lens candidates were discovered through a synergy of machine learning, citizen science, and expert visual inspection. These discoveries now allow us to quantify this performance gap and investigate the impact of training on real data. We find that a network trained only on simulations recovers up to 92% of simulated lenses with 100% purity, but only achieves 50% completeness with 24% purity on real Euclid data. By augmenting training data with real Euclid lenses and non-lenses, completeness improves by 25-30% in terms of the expected yield of discoverable lenses in Euclid DR1 and the full Euclid Wide Survey. Roughly 20% of this improvement comes from the inclusion of real lenses in the training data, while 5-10% comes from exposure to a more diverse set of non-lenses and false-positives from Q1. We show that the most effective lens-finding strategy for real-world performance combines the diversity of simulations with the fidelity of real lenses. This hybrid approach establishes a clear methodology for maximising lens discoveries in future data releases from Euclid, and will likely also be applicable to other surveys such as LSST. △ Less

Submitted 5 December, 2025; originally announced December 2025.

Comments: 16 pages

Euclid Structural-Thermal-Optical Performance

Authors: Euclid Collaboration, A. Anselmi, R. Laureijs, G. D. Racca, G. Costa, L. Courcould Mifsud, J. -C. Cuillandre, M. Gottero, H. Hoekstra, K. Kuijken, V. Mareschi, L. Miller, S. Mottini, D. Stramaccioni, B. Altieri, A. Amara, S. Andreon, N. Auricchio, C. Baccigalupi, M. Baldi, A. Balestra, S. Bardelli, R. Bender, A. Biviano, E. Branchini , et al. (268 additional authors not shown)

Abstract: The Euclid system performance is defined in terms of image quality metrics tuned to the weak gravitational lensing (WL) cosmological probe. WL induces stringent requirements on the shape and stability of the VIS instrument system point spread function (PSF). The PSF is affected by error contributions from the telescope, the focal plane and image motion, and is controlled by a global error budget w… ▽ More The Euclid system performance is defined in terms of image quality metrics tuned to the weak gravitational lensing (WL) cosmological probe. WL induces stringent requirements on the shape and stability of the VIS instrument system point spread function (PSF). The PSF is affected by error contributions from the telescope, the focal plane and image motion, and is controlled by a global error budget with error allocations to each contributor. Aims. During spacecraft development, we verified through a structural-thermal-optical performance (STOP) analysis that the built and verified telescope with its spacecraft interface meets the in-orbit steady-state and transient image quality requirements. Methods. For the purposes of the STOP analysis, a detailed finite-element mathematical model was set up and a standard set of test cases, both steady-state and transient, was defined, comprising combinations of worst-case boundary conditions. Results. The STOP analysis addressed the interaction of all spacecraft components in transmitting temperature-induced loads that lead to optical train deformation. The results of the prelaunch analysis demonstrated that temperature-induced optical perturbations will be well below the allowable limits for all permitted observing conditions. During the first year in orbit, we used the STOP analysis predictions to help interpret the measured performance as a function of environmental variables. Unpredicted disturbances were discovered and unexpected sensitivities were revealed. In-orbit temperature variations are small (<300 mK) and so are their effects on the telescope structure, but they are detected in the time histories of the image quality metrics and are a non-negligible factor in the PSF stability budget demanded by the WL science. Taking everything into account, our analysis confirms the excellent overall performance of the telescope. △ Less

Submitted 30 November, 2025; originally announced December 2025.

Comments: 21 pages, 15 figures, submitted to A&A

Journal ref: A&A 709, A15 (2026)

Euclid preparation. XCII. Controlling angular systematics in the Euclid spectroscopic galaxy sample

Authors: Euclid Collaboration, P. Monaco, M. Y. Elkhashab, B. R. Granett, J. Salvalaggio, E. Sefusatti, C. Scarlata, B. Zabelle, M. Bethermin, S. Bruton, C. Carbone, S. de la Torre, S. Dusini, A. Eggemeier, L. Guzzo, G. Lavaux, S. Lee, K. Markovic, K. S. McCarthy, M. Moresco, F. Passalacqua, W. J. Percival, I. Risso, A. G. Sánchez, D. Scott , et al. (276 additional authors not shown)

Abstract: We present the strategy used to identify and mitigate potential sources of angular systematics in the \textit{Euclid} spectroscopic galaxy survey, and we quantify their impact on galaxy clustering measurements and cosmological parameter estimation. We first surveyed the \textit{Euclid} processing pipeline to identify all evident, potential sources of systematics, and classified them into two broad… ▽ More We present the strategy used to identify and mitigate potential sources of angular systematics in the \textit{Euclid} spectroscopic galaxy survey, and we quantify their impact on galaxy clustering measurements and cosmological parameter estimation. We first surveyed the \textit{Euclid} processing pipeline to identify all evident, potential sources of systematics, and classified them into two broad classes: angular systematics, which modulate the galaxy number density across the sky, and catastrophic redshift errors, which lead to interlopers in the galaxy sample. We then used simulated spectroscopic surveys to test our ability to mitigate angular systematics by constructing a random catalogue that represents the `visibility mask' of the survey; this is a dense set of intrinsically unclustered objects, subject to the same selection effects as the data catalogue. The construction of this random catalogue relies on a detection model, which gives the probability of reliably measuring the galaxy redshift as a function of the signal-to-noise ratio (S/N) of its emission lines. We demonstrate that, in the ideal case of a perfect knowledge of the visibility mask, the galaxy power spectrum in the presence of systematics is recovered, to within sub-per cent accuracy, by convolving a theory power spectrum with a window function obtained from the random catalogue itself. In the case of only approximate knowledge of the visibility mask, we test the stability of power spectrum measurements and cosmological parameter posteriors by using perturbed versions of the random catalogue. We find that significant effects are limited to very large scales, and parameter estimation remains robust; the most impacting effects are connected to the calibration of the detection model. △ Less

Submitted 27 March, 2026; v1 submitted 25 November, 2025; originally announced November 2025.

Comments: A&A in press; 36 pages, 18 figures, 4 appendices

Euclid Quick Data Release (Q1). Searching for giant gravitational arcs in galaxy clusters with mask region-based convolutional neural networks

Authors: Euclid Collaboration, L. Bazzanini, G. Angora, P. Bergamini, M. Meneghetti, P. Rosati, A. Acebron, C. Grillo, M. Lombardi, R. Ratta, M. Fogliardi, G. Di Rosa, D. Abriola, M. D'Addona, G. Granata, L. Leuzzi, A. Mercurio, S. Schuldt, E. Vanzella, C. Tortora, B. Altieri, S. Andreon, N. Auricchio, C. Baccigalupi, M. Baldi , et al. (284 additional authors not shown)

Abstract: Strong gravitational lensing (SL) by galaxy clusters is a powerful probe of their inner mass distribution and a key test bed for cosmological models. However, the detection of SL events in wide-field surveys such as Euclid requires robust, automated methods capable of handling the immense data volume generated. In this work, we present an advanced deep learning (DL) framework based on mask region-… ▽ More Strong gravitational lensing (SL) by galaxy clusters is a powerful probe of their inner mass distribution and a key test bed for cosmological models. However, the detection of SL events in wide-field surveys such as Euclid requires robust, automated methods capable of handling the immense data volume generated. In this work, we present an advanced deep learning (DL) framework based on mask region-based convolutional neural networks (Mask R-CNNs), designed to autonomously detect and segment bright, strongly-lensed arcs in Euclid's multi-band imaging of galaxy clusters. The model is trained on a realistic simulated data set of cluster-scale SL events, constructed by injecting mock background sources into Euclidised Hubble Space Telescope images of 10 massive lensing clusters, exploiting their high-precision mass models constructed with extensive spectroscopic data. The network is trained and validated on over 4500 simulated images, and tested on an independent set of 500 simulations, as well as real Euclid Quick Data Release (Q1) observations. The trained network achieves high performance in identifying gravitational arcs in the test set, with a precision and recall of 76% and 58%, respectively, processing 2'x2' images in a fraction of a second. When applied to a sample of visually confirmed Euclid Q1 cluster-scale lenses, our model recovers 66% of gravitational arcs above the area threshold used during training. While the model shows promising results, limitations include the production of some false positives and challenges in detecting smaller, fainter arcs. Our results demonstrate the potential of advanced DL computer vision techniques for efficient and scalable arc detection, enabling the automated analysis of SL systems in current and future wide-field surveys. The code, ARTEMIDE, is open source and will be available at github.com/LBasz/ARTEMIDE. △ Less

Submitted 3 March, 2026; v1 submitted 4 November, 2025; originally announced November 2025.

Comments: Accepted by A&A (aa57590-25) [12 pages, 6 figures]

Euclid Quick Data Release (Q1). Spectroscopic unveiling of highly ionised lines at z = 2.48-3.88

Authors: Euclid Collaboration, D. Vergani, S. Quai, F. Ricci, Y. Fu, S. Serjeant, M. Salvato, W. Roster, M. Mezcua, M. Siudek, A. Enia, G. Zamorani, L. Bisigello, A. Feltre, S. Fotopoulou, T. Matamoro Zatarain, L. Pozzetti, D. Scott, B. Laloux, J. G. Sorce, P. A. C. Cunha, A. Viitanen, C. Saulder, E. Rossetti, M. Moresco , et al. (294 additional authors not shown)

Abstract: This study explores a rare population of sources in a currently uncharted region of spectroscopic redshift space in the Euclid Quick Data Release (Q1), and is intended potentially to support upcoming spectroscopic studies. Our goal is to identify and investigate a population of sources characterised by highly ionised emission lines in their spectra, which are indicative of active galactic nucleus… ▽ More This study explores a rare population of sources in a currently uncharted region of spectroscopic redshift space in the Euclid Quick Data Release (Q1), and is intended potentially to support upcoming spectroscopic studies. Our goal is to identify and investigate a population of sources characterised by highly ionised emission lines in their spectra, which are indicative of active galactic nucleus activity, extreme shock phenomena, or Wolf--Rayet stars. A comprehensive visual inspection of spectra is conducted to ensure the reliability of the sample, focusing on the simultaneous detection of both NeV and OII emission-line measurements, a condition that restricts the Euclid spectroscopic redshift range to z=2.48--3.88. To characterise this population, we analysed the morpho-spectrophotometric properties of their host galaxies. This allowed for a direct comparison with control sources that exhibit similar OII properties and spectroscopic redshifts, but not NeV lines. We identify sources solely based on spectroscopic criteria in the redshift range beyond the Halpha regime. Encompassing 65 potential NeV candidates, the resulting sample delivers the first systematic probe of these NeV candidate emitters at high redshift. We found a good agreement, within 1$σ$, between the spectral measurements calculated using both direct integration and Gaussian fitting methodologies. The NeV candidates exhibit colours similar to bright QSOs, with only a few in the tail of very red quasars. We observed a higher stellar mass content, a lower continuum around the 4000A break, and a similar Sérsic index distribution compared to the control sample. This unique sample paves the way for a wide range of scientific investigations, which will be pursued in the forthcoming data releases. △ Less

Submitted 4 November, 2025; originally announced November 2025.

Euclid Quick Data Release (Q1). The average far-infrared properties of Euclid-selected star-forming galaxies

Authors: Euclid Collaboration, R. Hill, A. Abghari, D. Scott, M. Bethermin, S. C. Chapman, D. L. Clements, S. Eales, A. Enia, B. Jego, A. Parmar, P. Tanouri, L. Wang, S. Andreon, N. Auricchio, C. Baccigalupi, M. Baldi, A. Balestra, S. Bardelli, P. Battaglia, A. Biviano, E. Branchini, M. Brescia, S. Camera, G. Cañas-Herrera , et al. (280 additional authors not shown)

Abstract: The first Euclid Quick Data Release contains millions of galaxies with excellent optical and near-infrared (IR) coverage. To complement this dataset, we investigate the average far-IR properties of Euclid-selected main sequence (MS) galaxies using existing Herschel and SCUBA-2 data. We use 17.6deg$^2$ (2.4deg$^2$) of overlapping Herschel (SCUBA-2) data, containing 2.6 million (240000) MS galaxies.… ▽ More The first Euclid Quick Data Release contains millions of galaxies with excellent optical and near-infrared (IR) coverage. To complement this dataset, we investigate the average far-IR properties of Euclid-selected main sequence (MS) galaxies using existing Herschel and SCUBA-2 data. We use 17.6deg$^2$ (2.4deg$^2$) of overlapping Herschel (SCUBA-2) data, containing 2.6 million (240000) MS galaxies. We bin the Euclid catalogue by stellar mass and photometric redshift and perform a stacking analysis following SimStack, which takes into account galaxy clustering and bin-to-bin correlations. We detect stacked far-IR flux densities across a significant fraction of the bins. We fit modified blackbody spectral energy distributions in each bin and derive mean dust temperatures, dust masses, and star-formation rates (SFRs). We find similar mean SFRs compared to the Euclid catalogue, and we show that the average dust-to-stellar mass ratios decreased from z$\simeq$1 to the present day. Average dust temperatures are largely independent of stellar mass and are well-described by the function $T_2+(T_1-T_2){\rm e}^{-t/τ}$, where $t$ is the age of the Universe, $T_1=79.7\pm7.4$K, $T_2=23.2\pm0.1$K, and $τ=1.6\pm0.1$Gyr. We argue that since the dust temperatures are converging to a non-zero value below $z=1$, the dust is now primarily heated by the existing cooler and older stellar population, as opposed to hot young stars in star-forming regions at higher redshift. We show that since the dust temperatures are independent of stellar mass, the correlation between dust temperature and SFR depends on stellar mass. Lastly, we estimate the contribution of the Euclid catalogue to the cosmic IR background (CIB), finding that it accounts for >60% of the CIB at 250, 350, and 500$μ$m. Forthcoming Euclid data will extend these results to higher redshifts, lower stellar masses, and recover more of the CIB. △ Less

Submitted 10 July, 2026; v1 submitted 4 November, 2025; originally announced November 2025.

Comments: Submitted to A&A as part of the second Euclid Q1 paper splash. V2 fixed typo in title. V3 accepted by A&A. V4 fixed typo in Table B.1

Euclid: Quick Data Release (Q1) -- Secondary nuclei in early-type galaxies

Authors: M. Fabricius, R. Saglia, F. Balzer, L. R. Ecker, J. Thomas, R. Bender, J. Gracia-Carpio, M. Magliocchetti, O. Marggraf, A. Rawlings, J. G. Sorce, K. Voggel, L. Wang, A. van der Wel, B. Altieri, A. Amara, S. Andreon, N. Auricchio, C. Baccigalupi, M. Baldi, A. Balestra, S. Bardelli, A. Biviano, E. Branchini, M. Brescia , et al. (143 additional authors not shown)

Abstract: Massive early-type galaxies (ETGs) are believed to form primarily through mergers of less massive progenitors, leaving behind numerous traces of violent formation histories, such as stellar streams and shells. A particularly striking signature of these mergers is the formation of supermassive black hole (SMBH) binaries, which can create depleted stellar cores through interactions with stars on rad… ▽ More Massive early-type galaxies (ETGs) are believed to form primarily through mergers of less massive progenitors, leaving behind numerous traces of violent formation histories, such as stellar streams and shells. A particularly striking signature of these mergers is the formation of supermassive black hole (SMBH) binaries, which can create depleted stellar cores through interactions with stars on radial orbits - a process known as core scouring. The secondary SMBH in such systems may still carry a dense stellar envelope and thereby remain observable for some time as a secondary nucleus, while it is sinking towards the shared gravitational potential of the merged galaxy. We leverage Euclid's Q1 Early Release data to systematically search for secondary nuclei in ETGs. We present a preliminary sample of 666 candidate systems distributed over 504 hosts (some of which contain multiple secondary nuclei). The vast majority of these fall at separations of 3 kpc to 15 kpc, indicative of normal mergers. 44 fall at projected separations of less than 2 kpc. We argue those candidates at very close angular separations are unlikely to be a consequence of chance alignments. We show that their stellar masses are mostly too large for them to be globular clusters and that a significant subset are unresolved even at Euclid's spatial resolution, rendering them too small to be dwarf galaxies. These may represent the highest-density nuclei of a previously merged galaxy, currently sinking into the centre of the new, common gravitational potential and thus likely to host a secondary SMBH. We then demonstrate that convolutional neural networks offer a viable avenue to detect multiple nuclei in the thirty-times larger sky coverage of the future Euclid DR1. Finally, we argue that our method could detect the remnants of a recoil event from two merged SMBHs. △ Less

Submitted 4 November, 2025; originally announced November 2025.

Comments: 17 pages, 22 figures

Euclid Quick Data Release (Q1). Quenching precedes bulge formation in dense environments but follows it in the field

Authors: Euclid Collaboration, F. Gentile, E. Daddi, D. Elbaz, A. Enia, B. Magnelli, J-B. Billand, P. Corcho-Caballero, C. Cleland, G. De Lucia, C. D'Eugenio, M. Fossati, M. Franco, C. Lobo, Y. Lyu, M. Magliocchetti, G. A. Mamon, L. Quilley, J. G. Sorce, M. Tarrasse, M. Bolzonella, F. Durret, L. Gabarra, S. Guo, L. Pozzetti , et al. (299 additional authors not shown)

Abstract: (Abridged) The bimodality between star-forming discs and quiescent spheroids requires the existence of two main processes: the galaxy quenching and the morphological transformation. In this paper, we aim to understand the link between these processes and their relation with the stellar mass of galaxies and their local environment. Taking advantage of the first data released by the Euclid Collabora… ▽ More (Abridged) The bimodality between star-forming discs and quiescent spheroids requires the existence of two main processes: the galaxy quenching and the morphological transformation. In this paper, we aim to understand the link between these processes and their relation with the stellar mass of galaxies and their local environment. Taking advantage of the first data released by the Euclid Collaboration, covering more than 60 deg2 with space-based imaging and photometry, we analyse a mass-complete sample of nearly one million galaxies in the range 0.25<z<1 with $M_\ast>10^{9.5} M_\odot$. We divide the sample into four sub-populations of galaxies, based on their star-formation activity and morphology. We then analyse the physical properties of these populations and their relative abundances in the stellar mass vs. local density plane. Together with confirming the passivity-density relation and the morphology-density relation, we find that quiescent discy galaxies are more abundant in the low-mass regime of high-density environment. At the same time, star-forming bulge-dominated galaxies are more common in field regions, preferentially at high masses. Building on these results and interpreting them through comparison with simulations, we propose a scenario where the evolution of galaxies in the field significantly differs from that in higher-density environments. The morphological transformation in the majority of field galaxies takes place before the onset of quenching and is mainly driven by secular processes taking place within the main sequence, leading to the formation of star-forming bulge-dominated galaxies as intermediate-stage galaxies. Conversely, quenching of star formation precedes morphological transformation for most galaxies in higher-density environments. This causes the formation of quiescent disc-dominated galaxies before their transition into bulge-dominated ones. △ Less

Submitted 19 January, 2026; v1 submitted 4 November, 2025; originally announced November 2025.

Comments: Accepted for publication by A&A in the Special Issue `Euclid Quick Data Release (Q1)', 16 pages, 7 figures, 1 appendix

Euclid preparation. Cosmology Likelihood for Observables in Euclid (CLOE). 6: Impact of systematic uncertainties on the cosmological analysis

Authors: Euclid Collaboration, L. Blot, K. Tanidis, G. Cañas-Herrera, P. Carrilho, M. Bonici, S. Camera, V. F. Cardone, S. Casas, S. Davini, S. Di Domizio, S. Farrens, L. W. K. Goh, S. Gouyou Beauchamps, S. Ilić, S. Joudaki, F. Keil, A. M. C. Le Brun, M. Martinelli, C. Moretti, V. Pettorino, A. Pezzotta, Z. Sakr, A. G. Sánchez, D. Sciotti , et al. (287 additional authors not shown)

Abstract: Extracting cosmological information from the Euclid galaxy survey will require modelling numerous systematic effects during the inference process. This implies varying a large number of nuisance parameters, which have to be marginalised over before reporting the constraints on the cosmological parameters. This is a delicate process, especially with such a large parameter space, which could result… ▽ More Extracting cosmological information from the Euclid galaxy survey will require modelling numerous systematic effects during the inference process. This implies varying a large number of nuisance parameters, which have to be marginalised over before reporting the constraints on the cosmological parameters. This is a delicate process, especially with such a large parameter space, which could result in biased cosmological results. In this work, we study the impact of different choices for modelling systematic effects and prior distribution of nuisance parameters for the final Euclid Data Release, focusing on the 3$\times$2pt analysis for photometric probes and the galaxy power spectrum multipoles for the spectroscopic probes. We explore the effect of intrinsic alignments, linear galaxy bias, magnification bias, multiplicative cosmic shear bias and shifts in the redshift distribution for the photometric probes, as well as the purity of the spectroscopic sample. We find that intrinsic alignment modelling has the most severe impact with a bias up to $6\,σ$ on the Hubble constant $H_0$ if neglected, followed by mis-modelling of the redshift evolution of galaxy bias, yielding up to $1.5\,σ$ on the parameter $S_8\equivσ_8\sqrt{Ω_{\rm m} /0.3}$. Choosing a too optimistic prior for multiplicative bias can also result in biases of the order of $0.7\,σ$ on $S_8$. We also find that the precision on the estimate of the purity of the spectroscopic sample will be an important driver for the constraining power of the galaxy clustering full-shape analysis. These results will help prioritise efforts to improve the modelling and calibration of systematic effects in Euclid. △ Less

Submitted 11 October, 2025; originally announced October 2025.

Comments: 22 pages, 16 figures. Submitted to A&A

Cosmology Likelihood for Observables in \Euclid (CLOE). 1. Theoretical recipe

Authors: Euclid Collaboration, V. F. Cardone, S. Joudaki, L. Blot, M. Bonici, S. Camera, G. Cañas-Herrera, P. Carrilho, S. Casas, S. Davini, S. Di Domizio, S. Farrens, L. W. K. Goh, S. Gouyou Beauchamps, S. Ilić, F. Keil, A. M. C. Le Brun, M. Martinelli, C. Moretti, V. Pettorino, A. Pezzotta, A. G. Sánchez, Z. Sakr, D. Sciotti, K. Tanidis , et al. (301 additional authors not shown)

Abstract: As the statistical precision of cosmological measurements increases, the accuracy of the theoretical description of these measurements needs to increase correspondingly in order to infer the underlying cosmology that governs the Universe. To this end, we have created the Cosmology Likelihood for Observables in Euclid (CLOE), which is a novel cosmological parameter inference pipeline developed with… ▽ More As the statistical precision of cosmological measurements increases, the accuracy of the theoretical description of these measurements needs to increase correspondingly in order to infer the underlying cosmology that governs the Universe. To this end, we have created the Cosmology Likelihood for Observables in Euclid (CLOE), which is a novel cosmological parameter inference pipeline developed within the Euclid Consortium to translate measurements and covariances into cosmological parameter constraints. In this first in a series of six papers, we describe the theoretical recipe of this code for the Euclid primary probes. These probes are composed of the photometric 3x2pt observables of cosmic shear, galaxy-galaxy lensing, and galaxy clustering, along with spectroscopic galaxy clustering. We provide this description in both Fourier and configuration space for standard and extended summary statistics, including the wide range of systematic uncertainties that affect them. This includes systematic uncertainties such as intrinsic galaxy alignments, baryonic feedback, photometric and spectroscopic redshift uncertainties, shear calibration uncertainties, sample impurities, photometric and spectroscopic galaxy biases, as well as magnification bias. The theoretical descriptions are further able to accommodate both Gaussian and non-Gaussian likelihoods and extended cosmologies with non-zero curvature, massive neutrinos, evolving dark energy, and simple forms of modified gravity. These theoretical descriptions that underpin CLOE will form a crucial component in revealing the true nature of the Universe with next-generation cosmological surveys such as Euclid. △ Less

Submitted 10 October, 2025; originally announced October 2025.

Comments: First in a series of six papers presenting CLOE, the Euclid likelihood code; 27 pages, 10 figures, A&A submitted

Euclid preparation. LXXXV. Toward a DR1 application of higher-order weak lensing statistics

Authors: Euclid Collaboration, S. Vinciguerra, F. Bouchè, N. Martinet, L. Castiblanco, C. Uhlemann, S. Pires, J. Harnois-Déraps, C. Giocoli, M. Baldi, V. F. Cardone, A. Vadalà, N. Dagoneau, L. Linke, E. Sellentin, P. L. Taylor, J. C. Broxterman, S. Heydenreich, V. Tinnaneri Sreekanth, N. Porqueres, L. Porth, M. Gatti, D. Grandón, A. Barthelemy, F. Bernardeau , et al. (262 additional authors not shown)

Abstract: This is the second paper in the HOWLS (higher-order weak lensing statistics) series exploring the usage of non-Gaussian statistics for cosmology inference within Euclid. With respect to our first paper, we develop a full tomographic analysis based on realistic photometric redshifts that allows us to derive Fisher forecasts in the ($σ_8$, $w_0$) plane for a Euclid-like data release 1 (DR1) setup. W… ▽ More This is the second paper in the HOWLS (higher-order weak lensing statistics) series exploring the usage of non-Gaussian statistics for cosmology inference within Euclid. With respect to our first paper, we develop a full tomographic analysis based on realistic photometric redshifts that allows us to derive Fisher forecasts in the ($σ_8$, $w_0$) plane for a Euclid-like data release 1 (DR1) setup. We find that the five higher-order statistics (HOS) that satisfy the Gaussian likelihood assumption of the Fisher formalism (one-point probability distribution function, $\ell$1-norm, peak counts, Minkowski functionals, and Betti numbers) each outperform the shear two-point correlation functions by a factor of $2.5$ on the $w_0$ forecasts, with only marginal improvement when used in combination with two-point estimators, suggesting that every HOS is able to retrieve both the non-Gaussian and Gaussian information of the matter density field. The similar performance of the different estimators is explained by a homogeneous use of multi-scale and tomographic information, optimized to lower computational costs. These results hold for the three mass mapping techniques of the Euclid pipeline, aperture mass, Kaiser--Squires, and Kaiser--Squires plus, and they are unaffected by the application of realistic star masks. Finally, we explored the use of HOS with the Bernardeau--Nishimichi--Taruya (BNT) nulling scheme approach, finding promising results toward applying physical scale cuts to HOS. △ Less

Submitted 5 January, 2026; v1 submitted 6 October, 2025; originally announced October 2025.

Comments: Accepted by A&A on 15/12/2025

Journal ref: A&A 707, A235 (2026)

Euclid preparation. Establishing the quality of the 2D reconstruction of the filaments of the cosmic web with DisPerSE using Euclid photometric redshifts

Authors: Euclid Collaboration, N. Malavasi, F. Sarron, U. Kuchner, C. Laigle, K. Kraljic, P. Jablonka, M. Balogh, S. Bardelli, M. Bolzonella, J. Brinchmann, G. De Lucia, F. Fontanot, C. Gouin, M. Hirschmann, Y. Kang, M. Magliocchetti, T. Moutard, J. G. Sorce, M. Spinelli, L. Wang, L. Xie, A. M. C. Le Brun, E. Tsaprazi, O. Cucciati , et al. (291 additional authors not shown)

Abstract: Cosmic filaments are prominent structures of the matter distribution of the Universe. Modern detection algorithms are an efficient way to identify filaments in large-scale observational surveys of galaxies. Many of these methods were originally designed to work with simulations and/or well-sampled spectroscopic surveys. When spectroscopic redshifts are not available, the filaments of the cosmic we… ▽ More Cosmic filaments are prominent structures of the matter distribution of the Universe. Modern detection algorithms are an efficient way to identify filaments in large-scale observational surveys of galaxies. Many of these methods were originally designed to work with simulations and/or well-sampled spectroscopic surveys. When spectroscopic redshifts are not available, the filaments of the cosmic web can be detected in projection using photometric redshifts in slices along the Line of Sight, which enable the exploration of larger cosmic volumes. However, this comes at the expense of a lower redshift precision. It is therefore crucial to assess the differences between filaments extracted from exact redshifts and from photometric redshifts for a specific survey. We apply this analysis to capture the uncertainties and biases of filament extractions introduced by using the photometric sample of the Euclid Wide Survey. The question that we address in this work is how can we compare two filament samples derived with redshifts of different precisions in the Euclid Wide Survey context. We apply the cosmic web detection algorithm DisPerSE, in the redshift range $0.1 \leq z \leq 0.5$, to the GAlaxy Evolution and Assembly (GAEA) simulated galaxy sample which reproduces several characteristics of the Euclid Wide Survey. We develop a method to compare skeletons derived from photometric redshifts to those derived from true galaxy positions. This method expands the commonly used measure of distance between filaments to include geometrical (angles between filaments) and astrophysical considerations (galaxy mass gradients and connectivity-mass relations). We assess whether this approach strengthens our ability to correctly identify filaments in very large surveys such as the Euclid Wide Survey. [abridged] △ Less

Submitted 21 August, 2025; originally announced August 2025.

Comments: 27 pages, 14 Figures

Euclid preparation: Expected constraints on initial conditions

Authors: Euclid Collaboration, F. Finelli, Y. Akrami, A. Andrews, M. Ballardini, S. Casas, D. Karagiannis, Z. Sakr, J. Valiviita, G. Alestas, N. Bartolo, J. R. Bermejo-Climent, S. Nesseris, D. Paoletti, D. Sapone, I. Tutusaus, A. Achúcarro, G. Cañas-Herrera, J. Jasche, G. Lavaux, N. Aghanim, B. Altieri, A. Amara, L. Amendola, S. Andreon , et al. (285 additional authors not shown)

Abstract: The Euclid mission of the European Space Agency will deliver galaxy and cosmic shear surveys, which will be used to constrain initial conditions and statistics of primordial fluctuations. We present highlights for the Euclid scientific capability to test initial conditions beyond LCDM with the main probes, i.e. 3D galaxy clustering from the spectroscopic survey, the tomographic approach to 3x2pt s… ▽ More The Euclid mission of the European Space Agency will deliver galaxy and cosmic shear surveys, which will be used to constrain initial conditions and statistics of primordial fluctuations. We present highlights for the Euclid scientific capability to test initial conditions beyond LCDM with the main probes, i.e. 3D galaxy clustering from the spectroscopic survey, the tomographic approach to 3x2pt statistics from photometric galaxy survey, and their combination. We provide Fisher forecasts from the combination of Euclid spectroscopic and photometric surveys for spatial curvature, running of the spectral index of the power spectrum of curvature perturbations, isocurvature perturbations, and primordial features. For the parameters of these models we also provide the combination of Euclid forecasts (pessimistic and optimistic) with current and future measurements of the cosmic microwave background (CMB) anisotropies., i.e. Planck, the Simons Observatory (SO), and CMB-S4. We provide Fisher forecasts for how the power spectrum and bispectrum from the Euclid spectroscopic survey will constrain the local, equilateral, and orthogonal shapes of primordial non-Gaussianity. We also review how Bayesian field-level inference of primordial non-Gaussianity can constrain local primordial non-Gaussianity. We show how Euclid, with its unique combination of the main probes, will provide the tightest constraints on low redshift to date. By targeting a markedly different range in redshift and scale, Euclid's expected uncertainties are complementary to those obtained by CMB primary anisotropy, returning the tightest combined constraints on the physics of the early Universe. △ Less

Submitted 21 July, 2025; originally announced July 2025.

Comments: Abstract abridged, 25 pages, 6 tables, 11 figures

Euclid preparation. Overview of Euclid infrared detector performance from ground tests

Authors: Euclid Collaboration, B. Kubik, R. Barbier, J. Clemens, S. Ferriol, A. Secroun, G. Smadja, W. Gillard, N. Fourmanoit, A. Ealet, S. Conseil, J. Zoubian, R. Kohley, J. -C. Salvignol, L. Conversi, T. Maciaszek, H. Cho, W. Holmes, M. Seiffert, A. Waczynski, S. Wachter, K. Jahnke, F. Grupp, C. Bonoli, L. Corcione , et al. (319 additional authors not shown)

Abstract: The paper describes the objectives, design and findings of the pre-launch ground characterisation campaigns of the Euclid infrared detectors. The pixel properties, including baseline, bad pixels, quantum efficiency, inter pixel capacitance, quantum efficiency, dark current, readout noise, conversion gain, response nonlinearity, and image persistence were measured and characterised for each pixel.… ▽ More The paper describes the objectives, design and findings of the pre-launch ground characterisation campaigns of the Euclid infrared detectors. The pixel properties, including baseline, bad pixels, quantum efficiency, inter pixel capacitance, quantum efficiency, dark current, readout noise, conversion gain, response nonlinearity, and image persistence were measured and characterised for each pixel. We describe in detail the test flow definition that allows us to derive the pixel properties and we present the data acquisition and data quality check software implemented for this purpose. We also outline the measurement protocols of all the pixel properties presented and we provide a comprehensive overview of the performance of the Euclid infrared detectors as derived after tuning the operating parameters of the detectors. The main conclusion of this work is that the performance of the infrared detectors Euclid meets the requirements. Pixels classified as non-functioning accounted for less than 0.2% of all science pixels. IPC coupling is minimal and crosstalk between adjacent pixels is less than 1% between adjacent pixels. 95% of the pixels show a QE greater than 80% across the entire spectral range of the Euclid mission. The conversion gain is approximately 0.52 ADU/e-, with a variation less than 1% between channels of the same detector. The reset noise is approximately equal to 23 ADU after reference pixels correction. The readout noise of a single frame is approximately 13 $e^-$ while the signal estimator noise is measured at 7 $e^-$ in photometric mode and 9 $e^-$ in spectroscopic acquisition mode. The deviation from linear response at signal levels up to 80 k$e^-$ is less than 5% for 95% of the pixels. Median persistence amplitudes are less than 0.3% of the signal, though persistence exhibits significant spatial variation and differences between detectors. △ Less

Submitted 15 July, 2025; originally announced July 2025.

Comments: 22 pages, 20 figures, 4 pages of annexes. Submitted to A&A

Journal ref: A&A 707, A230 (2026)

Euclid VI. NISP-P optical ghosts

Authors: Euclid Collaboration, K. Paterson, M. Schirmer, K. Okumura, B. Venemans, K. Jahnke, N. Aghanim, B. Altieri, A. Amara, S. Andreon, C. Baccigalupi, M. Baldi, A. Balestra, S. Bardelli, P. Battaglia, A. Biviano, A. Bonchi, E. Branchini, M. Brescia, J. Brinchmann, S. Camera, G. Cañas-Herrera, V. Capobianco, J. Carretero, S. Casas , et al. (287 additional authors not shown)

Abstract: The Near-Infrared Spectrometer and Photometer (NISP) onboard Euclid includes several optical elements in its path, which introduce artefacts into the data from non-nominal light paths. To ensure uncontaminated source photometry, these artefacts must be accurately accounted for. This paper focuses on two specific optical features in NISP's photometric data (NISP-P): ghosts caused by the telescope's… ▽ More The Near-Infrared Spectrometer and Photometer (NISP) onboard Euclid includes several optical elements in its path, which introduce artefacts into the data from non-nominal light paths. To ensure uncontaminated source photometry, these artefacts must be accurately accounted for. This paper focuses on two specific optical features in NISP's photometric data (NISP-P): ghosts caused by the telescope's dichroic beamsplitter, and the bandpass filters within the NISP fore-optics. Both ghost types exhibit a characteristic morphology and are offset from the originating stars. The offsets are well modelled using 2D polynomials, with only stars brighter than approximately 10 magnitudes in each filter producing significant ghost contributions. The masking radii for these ghosts depend on both the source-star brightness and the filter wavelength, ranging from 20 to 40 pixels. We present the final relations and models used in the near-infrared (NIR) data pipeline to mask these ghosts for Euclid's Quick Data Release (Q1). △ Less

Submitted 15 July, 2025; originally announced July 2025.

Comments: 13 pages, 10 figures, submitted

Journal ref: A&A 707, A226 (2026)

Euclid preparation: The NISP spectroscopy channel, on ground performance and calibration

Authors: Euclid Collaboration, W. Gillard, T. Maciaszek, E. Prieto, F. Grupp, A. Costille, K. Jahnke, J. Clemens, S. Dusini, M. Carle, C. Sirignano, E. Medinaceli, S. Ligori, E. Franceschi, M. Trifoglio, W. Bon, R. Barbier, S. Ferriol, A. Secroun, N. Auricchio, P. Battaglia, C. Bonoli, L. Corcione, F. Hormuth, D. Le Mignant , et al. (334 additional authors not shown)

Abstract: ESA's Euclid cosmology mission relies on the very sensitive and accurately calibrated spectroscopy channel of the Near-Infrared Spectrometer and Photometer (NISP). With three operational grisms in two wavelength intervals, NISP provides diffraction-limited slitless spectroscopy over a field of $0.57$ deg$^2$. A blue grism $\text{BG}_\text{E}$ covers the wavelength range $926$--$1366$\,nm at a spec… ▽ More ESA's Euclid cosmology mission relies on the very sensitive and accurately calibrated spectroscopy channel of the Near-Infrared Spectrometer and Photometer (NISP). With three operational grisms in two wavelength intervals, NISP provides diffraction-limited slitless spectroscopy over a field of $0.57$ deg$^2$. A blue grism $\text{BG}_\text{E}$ covers the wavelength range $926$--$1366$\,nm at a spectral resolution $R=440$--$900$ for a $0.5''$ diameter source with a dispersion of $1.24$ nm px$^{-1}$. Two red grisms $\text{RG}_\text{E}$ span $1206$ to $1892$\,nm at $R=550$--$740$ and a dispersion of $1.37$ nm px$^{-1}$. We describe the construction of the grisms as well as the ground testing of the flight model of the NISP instrument where these properties were established. △ Less

Submitted 18 September, 2025; v1 submitted 9 June, 2025; originally announced June 2025.

Comments: 18 pages 15 figures with additional 8 pages of annexes. Accepted to A&A

Journal ref: A&A 707, A227 (2026)

Euclid preparation: TBD. Cosmic Dawn Survey: evolution of the galaxy stellar mass function across 0.2<z<6.5 measured over 10 square degrees

Authors: Euclid Collaboration, L. Zalesky, J. R. Weaver, C. J. R. McPartland, G. Murphree, I. Valdes, C. K. Jespersen, S. Taamoli, N. Chartab, N. Allen, S. W. J. Barrow, D. B. Sanders, S. Toft, B. Mobasher, I. Szapudi, B. Altieri, A. Amara, S. Andreon, N. Auricchio, C. Baccigalupi, M. Baldi, S. Bardelli, P. Battaglia, A. Biviano, D. Bonino , et al. (282 additional authors not shown)

Abstract: The Cosmic Dawn Survey Pre-launch (PL) catalogues cover an effective 10.13 deg$^{2}$ area with uniform deep Spitzer/IRAC data ($m\sim25$ mag, 5$σ$), the largest area covered to these depths in the infrared. These data are used to gain new insight into the growth of stellar mass across cosmic history by characterising the evolution of the galaxy stellar mass function (GSMF) through… ▽ More The Cosmic Dawn Survey Pre-launch (PL) catalogues cover an effective 10.13 deg$^{2}$ area with uniform deep Spitzer/IRAC data ($m\sim25$ mag, 5$σ$), the largest area covered to these depths in the infrared. These data are used to gain new insight into the growth of stellar mass across cosmic history by characterising the evolution of the galaxy stellar mass function (GSMF) through $0.2 < z \leq 6.5$. The total volume (0.62 Gpc$^{3}$) represents a tenfold increase compared to previous works that have explored $z > 3$ and significantly reduces cosmic variance, yielding strong constraints on the abundance of massive galaxies. Results are generally consistent with the literature but now provide firm estimates of number density where only upper limits were previously available. Contrasting the GSMF with the dark matter halo mass function suggests that massive galaxies ($M \gtrsim10^{11}$ M$_{\odot}$) at $z > 3.5$ required integrated star-formation efficiencies of $M/(M_{\rm h}f_{\rm b}) \gtrsim$ 0.25--0.5, in excess of the commonly-held view of ``universal peak efficiency" from studies on the stellar-to-halo mass relation (SHMR). Such increased efficiencies imply an evolving peak in the SHMR at $z > 3.5$ which can be maintained if feedback mechanisms from active galactic nuclei and stellar processes are ineffective at early times. In addition, a significant fraction of the most massive quiescent galaxies are observed to be in place already by $z\sim 2.5$--3. The apparent lack in change of their number density by $z\sim 0.2$ is consistent with relatively little mass growth from mergers. Utilising the unique volume, evidence for an environmental dependence of the galaxy stellar mass function is found all the way through $z\sim 3.5$ for the first time, though a more careful characterisation of the density field is ultimately required for confirmation. △ Less

Submitted 24 April, 2025; originally announced April 2025.

Comments: - Submitted to A&A - Catalogues available here: https://dawn.calet.org/pl/

Journal ref: A&A 708, A104 (2026)

Euclid Quick Data Release (Q1) Ultracool dwarfs in the Euclid Deep Field North

Authors: A. Mohandasan, R. L. Smart, C. Reylé, V. Le Brun, A. Pérez-Garrido, E. Bañados, B. Goldman, S. L. Casewell, M. R. Zapatero Osorio, T. Dupuy, M. Rejkuba, E. L. Martín, C. Dominguez-Tagle, M. {Ž}erjal, N. Huélamo, N. Lodieu, P. Cruz, R. Rebolo, M. W. Phillips, J. -Y. Zhang, N. Aghanim, B. Altieri, A. Amara, S. Andreon, N. Auricchio , et al. (154 additional authors not shown)

Abstract: Ultracool dwarfs (UCDs) encompass the lowest mass stars and brown dwarfs, defining the stellar substellar boundary. They have significant potential for advancing the understanding of substellar physics; however, these objects are challenging to detect due to their low luminosity. The wide coverage and deep sensitivity of the Euclid survey will increase the number of confirmed and well characterise… ▽ More Ultracool dwarfs (UCDs) encompass the lowest mass stars and brown dwarfs, defining the stellar substellar boundary. They have significant potential for advancing the understanding of substellar physics; however, these objects are challenging to detect due to their low luminosity. The wide coverage and deep sensitivity of the Euclid survey will increase the number of confirmed and well characterised UCDs by several orders of magnitude. In this study, we take advantage of the Euclid Quick Data Release (Q1) and in particular we look in detail at the known and new UCDs in the Euclid Deep Field North (22.9 deg2 down to JE = 24.5 mag), to understand the advantages of using the slitless Euclid spectroscopy. We compile a comparison sample of known UCDs and use their spectra to demonstrate the capability of Euclid to derive spectral types using a template matching method. This method is then applied to the spectra of the newly identified candidates. We confirm that 33 of these candidates are new UCDs, with spectral types ranging from M7 to T1 and JE = 17 to 21 mag. We look at their locus in colour colour diagrams and compare them with the expected colours of QSOs. A machine readable catalogue is provided for further study, containing both the comparison sample and the newly identified UCDs, along with their spectral classifications where the Q1 spectra quality allows for confident determination △ Less

Submitted 28 March, 2025; originally announced March 2025.

Comments: 14 pages, 9 figures, 2 tables

Euclid: Quick Data Release (Q1) -- A photometric search for ultracool dwarfs in the Euclid Deep Fields

Authors: M. Žerjal, C. Dominguez-Tagle, N. Vitas, N. Sedighi, E. L. Martín, M. R. Zapatero Osorio, J. Olivares, S. Muñoz Torres, S. Tsilia, J. -Y. Zhang, D. Barrado, V. J. S. Béjar, H. Bouy, A. Burgasser, P. Cruz, N. Lodieu, P. Mas Buitrago, N. Phan-Bao, E. Solano, R. Tata, B. Goldman, A. Mohandasan, C. Reylé, R. L. Smart, N. Aghanim , et al. (156 additional authors not shown)

Abstract: We present a catalogue of 5306 new ultracool dwarf (UCD) candidates in the three Euclid Deep Fields in the Q1 data release. They range from late M to late T dwarfs, and include 1200 L and T dwarfs. A total of 546 objects have been spectroscopically confirmed, including 329 L dwarfs and 26 T dwarfs. We also provide empirical Euclid colours as a function of spectral type. Our UCD selection criteria… ▽ More We present a catalogue of 5306 new ultracool dwarf (UCD) candidates in the three Euclid Deep Fields in the Q1 data release. They range from late M to late T dwarfs, and include 1200 L and T dwarfs. A total of 546 objects have been spectroscopically confirmed, including 329 L dwarfs and 26 T dwarfs. We also provide empirical Euclid colours as a function of spectral type. Our UCD selection criteria are based only on colour ($I_\mathrm{E}-Y_\mathrm{E}>2.5$). The combined requirement for optical detection and stringent signal-to-noise ratio threshold ensure a high purity of the sample, but at the expense of completeness, especially for T dwarfs. The detections range from magnitudes 19 and 24 in the near-infrared bands, and extend down to 26 in the optical band. We discuss Euclid's capability to identify UCD candidates based on its photometric passbands. The average surface density of detected UCDs on the sky is approximately 100 objects per $\mathrm{deg}^2$, including 20 L and T dwarfs per $\mathrm{deg}^2$. This leads to an expectation of at least 1.4 million ultracool dwarfs in the final data release of the Euclid Wide Survey, including at least $300\,000$ L dwarfs, and more than 2600 T dwarfs, using the strict selection criteria from this work. △ Less

Submitted 9 December, 2025; v1 submitted 28 March, 2025; originally announced March 2025.

Comments: Accepted in A&A. 19 pages, 15 figures, 5 tables

Euclid Quick Data Release (Q1). The first Euclid view of Planck galaxy protocluster candidates at cosmic noon

Authors: Euclid Collaboration, T. Dusserre, H. Dole, F. Sarron, G. Castignani, N. Ramos-Chernenko, N. Aghanim, A. Garic, I. -E. Mellouki, N. Dagoneau, O. Chapuis, B. L. Frye, M. Polletta, H. Dannerbauer, M. Langer, L. Maurin, E. Soubrie, A. Biviano, S. Mei, N. Mai, B. Altieri, A. Amara, S. Andreon, N. Auricchio, C. Baccigalupi , et al. (317 additional authors not shown)

Abstract: [ABRIGED ABSTRACT] A large catalogue of candidate galaxy protoclusters with high star-formation rates was produced by the Planck collaboration. We search, in the first data release (Q1) of the Euclid survey, for the visible and infrared counterparts of the Planck galaxy protocluster candidates expected to be above $z > 1.5$. Eight of them are in Euclid Q1. Our goal is to investigate the optical na… ▽ More [ABRIGED ABSTRACT] A large catalogue of candidate galaxy protoclusters with high star-formation rates was produced by the Planck collaboration. We search, in the first data release (Q1) of the Euclid survey, for the visible and infrared counterparts of the Planck galaxy protocluster candidates expected to be above $z > 1.5$. Eight of them are in Euclid Q1. Our goal is to investigate the optical nature of these overdensities previously detected in the submillimetre wavelength range. We search for overdensities using the DETECTIFz algorithm, an overdensity finder based on Delaunay tessellation that uses photometric redshift probability distributions through Monte Carlo simulations. Focusing our search on the eight high-star forming Planck protocluster candidates, we find that two of them have one Euclid counterpart, and six have between two and four Euclid counterparts, which amounts to a total of 20 Euclid counterparts. These Euclid counterparts lie at photometric redshifts $1.4<z_{\rm ph} < 2.7$ and 12 of them also have partial Herschel coverage. All detections have also been confirmed by at least one other independent protocluster detection algorithm. We study the colours, derived stellar masses and star-formation rates (SFRs) of the detected member galaxies of those protocluster candidate counterparts. We also estimate the total stellar masses, SFRs, and the halo mass lower limits for all Euclid protocluster candidates. We find that in the dark matter halo mass ($M_{\rm h}$) / redshift plane, these Planck and Euclid overdense regions lie in the region $12.6 <\log_{10} (M_{\rm h}/M_\odot)< 13.4$, $1.4<z< 2.7$. This means that the halos of our objects are expected to have experienced a transition between cold flows in hot media to accretion of hot material. △ Less

Submitted 31 March, 2026; v1 submitted 27 March, 2025; originally announced March 2025.

Comments: 13+7 pages, 11 figures. Submitted to A&A. Improved and updated data analysis

Euclid Quick Data Release (Q1). First detections from the galaxy cluster workflow

Authors: Euclid Collaboration, S. Bhargava, C. Benoist, A. H. Gonzalez, M. Maturi, J. -B. Melin, S. A. Stanford, E. Munari, M. Vannier, C. Murray, S. Maurogordato, A. Biviano, J. Macias-Perez, J. G. Bartlett, F. Pacaud, A. Widmer, M. Meneghetti, B. Sartoris, M. Aguena, G. Alguero, S. Andreon, S. Bardelli, L. Baumont, M. Bolzonella, R. Cabanac , et al. (329 additional authors not shown)

Abstract: The first survey data release by the Euclid mission covers approximately $63\,\mathrm{deg^2}$ in the Euclid Deep Fields to the same depth as the Euclid Wide Survey. This paper showcases, for the first time, the performance of cluster finders on Euclid data and presents examples of validated clusters in the Quick Release 1 (Q1) imaging data. We identify clusters using two algorithms (AMICO and PZWa… ▽ More The first survey data release by the Euclid mission covers approximately $63\,\mathrm{deg^2}$ in the Euclid Deep Fields to the same depth as the Euclid Wide Survey. This paper showcases, for the first time, the performance of cluster finders on Euclid data and presents examples of validated clusters in the Quick Release 1 (Q1) imaging data. We identify clusters using two algorithms (AMICO and PZWav) implemented in the Euclid cluster-detection pipeline. We explore the internal consistency of detections from the two codes, and cross-match detections with known clusters from other surveys using external multi-wavelength and spectroscopic data sets. This enables assessment of the Euclid photometric redshift accuracy and also of systematics such as mis-centring between the optical cluster centre and centres based on X-ray and/or Sunyaev--Zeldovich observations. We report 426 joint PZWav and AMICO-detected clusters with high signal-to-noise ratios over the full Q1 area in the redshift range $0.2 \leq z \leq 1.5$. The chosen redshift and signal-to-noise thresholds are motivated by the photometric quality of the early Euclid data. We provide richness estimates for each of the Euclid-detected clusters and show its correlation with various external cluster mass proxies. Out of the full sample, 77 systems are potentially new to the literature. Overall, the Q1 cluster catalogue demonstrates a successful validation of the workflow ahead of the Euclid Data Release 1, based on the consistency of internal and external properties of Euclid-detected clusters. △ Less

Submitted 3 September, 2025; v1 submitted 24 March, 2025; originally announced March 2025.

Comments: Paper accepted as part of the A&A Special Issue 'Euclid Quick Data Release (Q1),' 24 pages, 17 figures

Euclid: Quick Data Release (Q1) -- A census of dwarf galaxies across a range of distances and environments

Authors: F. R. Marleau, R. Habas, D. Carollo, C. Tortora, P. -A. Duc, E. Sola, T. Saifollahi, M. Fügenschuh, M. Walmsley, R. Zöller, A. Ferré-Mateu, M. Cantiello, M. Urbano, E. Saremi, R. Ragusa, R. Laureijs, M. Hilker, O. Müller, M. Poulain, R. F. Peletier, S. J. Sprenger, O. Marchal, N. Aghanim, B. Altieri, A. Amara , et al. (182 additional authors not shown)

Abstract: The Euclid Q1 fields were selected for calibration purposes in cosmology and are therefore relatively devoid of nearby galaxies. However, this is precisely what makes them interesting fields in which to search for dwarf galaxies in local density environments. We take advantage of the unprecedented depth, spatial resolution, and field of view of the Euclid Quick Release (Q1) to build a census of dw… ▽ More The Euclid Q1 fields were selected for calibration purposes in cosmology and are therefore relatively devoid of nearby galaxies. However, this is precisely what makes them interesting fields in which to search for dwarf galaxies in local density environments. We take advantage of the unprecedented depth, spatial resolution, and field of view of the Euclid Quick Release (Q1) to build a census of dwarf galaxies in these regions. We have identified dwarfs in a representative sample of 25 contiguous tiles in the Euclid Deep Field North (EDF-N), covering an area of 14.25 sq. deg. The dwarf candidates were identified using a semi-automatic detection method, based on properties measured by the Euclid pipeline and listed in the MER catalogue. A selection cut in surface brightness and magnitude was used to produce an initial dwarf candidate catalogue, followed by a cut in morphology and colour. This catalogue was visually classified to produce a final sample of dwarf candidates, including their morphology, number of nuclei, globular cluster (GC) richness, and presence of a blue compact centre. We identified 2674 dwarf candidates, corresponding to 188 dwarfs per sq. deg. The visual classification of the dwarfs reveals a slightly uneven morphological mix of 58% ellipticals and 42% irregulars, with very few potentially GC-rich (1.0%) and nucleated (4.0%) candidates but a noticeable fraction (6.9%) of dwarfs with blue compact centres. The distance distribution of 388 (15%) of the dwarfs with spectroscopic redshifts peaks at about 400 Mpc. Their stellar mass distribution confirms that our selection effectively identifies dwarfs while minimising contamination. The most prominent dwarf overdensities are dominated by dEs, while dIs are more evenly distributed. This work highlights Euclid's remarkable ability to detect and characterise dwarf galaxies across diverse masses, distances, and environments. △ Less

Submitted 19 March, 2025; originally announced March 2025.

Comments: Paper submitted as part of the A&A Special Issue `Euclid Quick Data Release (Q1)', 27 pages, 19 figures, 5 tables

Euclid: Quick Data Release (Q1) -- Photometric studies of known transients

Authors: C. Duffy, E. Cappellaro, M. T. Botticella, I. M. Hook, F. Poidevin, T. J. Moriya, A. A. Chrimes, V. Petrecca, K. Paterson, A. Goobar, L. Galbany, R. Kotak, C. Gall, C. M. Gutierrez, C. Tao, L. Izzo, N. Aghanim, B. Altieri, A. Amara, S. Andreon, N. Auricchio, C. Baccigalupi, M. Baldi, A. Balestra, S. Bardelli , et al. (152 additional authors not shown)

Abstract: We report on serendipitous Euclid observations of previously known transients, using the Euclid Q1 data release. By cross-matching with the Transient Name Server (TNS) we identify 164 transients that coincide with the data release. Although the Euclid Q1 release only includes single-epoch data, we are able to make Euclid photometric measurements at the location of 161 of these transients. Euclid o… ▽ More We report on serendipitous Euclid observations of previously known transients, using the Euclid Q1 data release. By cross-matching with the Transient Name Server (TNS) we identify 164 transients that coincide with the data release. Although the Euclid Q1 release only includes single-epoch data, we are able to make Euclid photometric measurements at the location of 161 of these transients. Euclid obtained deep photometric measurements or upper limits of these transients in the $I_E$, $Y_E$, $J_E$, and $H_E$ bands at various phases of the transient light-curves, including before, during, and after the observations of ground-based transient surveys. Approximately 70\% of known transients reported in the six months before the Euclid observation date and with discovery magnitude brighter than 24 were detected in Euclid $\IE$ images. Our observations include one of the earliest near-infrared detections of a Type~Ia supernova (SN 2024pvw) 15 days prior to its peak brightness, and the late-phase (435.9 days post peak) observations of the enigmatic core-collapse SN 2023aew. Euclid deep photometry provides valuable information on the nature of these transients such as their progenitor systems and power sources, with late time observations being a uniquely powerful contribution. In addition, Euclid is able to detect the host galaxies of some transients that were previously classed as hostless. The Q1 data demonstrate the power of the Euclid data even with only single-epoch observations available, as will be the case for much larger areas of sky in the Euclid Wide Survey. △ Less

Submitted 19 March, 2025; originally announced March 2025.

Comments: Paper submitted as part of the A&A Special Issue `Euclid Quick Data Release (Q1), 27 pages, 10 figures

Euclid Quick Data Release (Q1). Galaxy shapes and alignments in the cosmic web

Authors: Euclid Collaboration, C. Laigle, C. Gouin, F. Sarron, L. Quilley, C. Pichon, K. Kraljic, F. Durret, N. E. Chisari, U. Kuchner, N. Malavasi, M. Magliocchetti, H. J. McCracken, J. G. Sorce, Y. Kang, C. J. R. McPartland, S. Toft, N. Aghanim, B. Altieri, A. Amara, S. Andreon, N. Auricchio, H. Aussel, C. Baccigalupi, M. Baldi , et al. (319 additional authors not shown)

Abstract: Galaxy morphologies and shape orientations are expected to correlate with their large-scale environment, since they grow by accreting matter from the cosmic web and are subject to interactions with other galaxies. Cosmic filaments are extracted in projection from the Euclid Quick Data Release 1 (covering 63.1 $\mathrm{deg}^2$) at $0.5<z<0.9$ in tomographic slices of 170 comoving… ▽ More Galaxy morphologies and shape orientations are expected to correlate with their large-scale environment, since they grow by accreting matter from the cosmic web and are subject to interactions with other galaxies. Cosmic filaments are extracted in projection from the Euclid Quick Data Release 1 (covering 63.1 $\mathrm{deg}^2$) at $0.5<z<0.9$ in tomographic slices of 170 comoving $h^{-1}\mathrm{Mpc}$ using photometric redshifts. Galaxy morphologies are accurately retrieved thanks to the excellent resolution of VIS data. The distribution of massive galaxies ($M_* > 10^{10} M_\odot$) in the projected cosmic web is analysed as a function of morphology measured from VIS data. Specifically, the 2D alignment of galaxy shapes with large-scale filaments is quantified as a function of Sérsic indices and masses. We find the known trend that more massive galaxies are closer to filament spines. At fixed stellar masses, morphologies correlate both with densities and distances to large-scale filaments. In addition, the large volume of this data set allows us to detect a signal indicating that there is a preferential alignment of the major axis of massive early-type galaxies along projected cosmic filaments. Overall, these results demonstrate our capabilities to carry out detailed studies of galaxy environments with Euclid, which will be extended to higher redshift and lower stellar masses with the future Euclid Deep Survey. △ Less

Submitted 19 March, 2025; originally announced March 2025.

Comments: Paper submitted as part of the A&A Special Issue `Euclid Quick Data Release (Q1)', 22 pages, 19 figures

Euclid Quick Data Release (Q1). The role of cosmic connectivity in shaping galaxy clusters

Authors: Euclid Collaboration, C. Gouin, C. Laigle, F. Sarron, T. Bonnaire, J. G. Sorce, N. Aghanim, M. Magliocchetti, L. Quilley, P. Boldrini, F. Durret, C. Pichon, U. Kuchner, N. Malavasi, K. Kraljic, R. Gavazzi, Y. Kang, S. A. Stanford, P. Awad, B. Altieri, A. Amara, S. Andreon, N. Auricchio, H. Aussel, C. Baccigalupi , et al. (315 additional authors not shown)

Abstract: The matter distribution around galaxy clusters is distributed over several filaments, reflecting their positions as nodes in the large-scale cosmic web. The number of filaments connected to a cluster, namely its connectivity, is expected to affect the physical properties of clusters. Using the first Euclid galaxy catalogue from the Euclid Quick Release 1 (Q1), we investigate the connectivity of ga… ▽ More The matter distribution around galaxy clusters is distributed over several filaments, reflecting their positions as nodes in the large-scale cosmic web. The number of filaments connected to a cluster, namely its connectivity, is expected to affect the physical properties of clusters. Using the first Euclid galaxy catalogue from the Euclid Quick Release 1 (Q1), we investigate the connectivity of galaxy clusters and how it correlates with their physical and galaxy member properties. Around 220 clusters located within the three fields of Q1 (covering $\sim 63 \ \text{deg}^2$), are analysed in the redshift range $0.2 < z < 0.7$. Due to the photometric redshift uncertainty, we reconstruct the cosmic web skeleton, and measure cluster connectivity, in 2-D projected slices with a thickness of 170 comoving $h^{-1}.\text{Mpc}$ and centred on each cluster redshift, by using two different filament finder algorithms on the most massive galaxies ($M_*\ > 10^{10.3} \ M_\odot$). In agreement with previous measurements, we recover the mass-connectivity relation independently of the filament detection algorithm, showing that the most massive clusters are, on average, connected to a larger number of cosmic filaments, consistent with hierarchical structure formation models. Furthermore, we explore possible correlations between connectivities and two cluster properties: the fraction of early-type galaxies and the Sérsic index of galaxy members. Our result suggests that the clusters populated by early-type galaxies exhibit higher connectivity compared to clusters dominated by late-type galaxies. These preliminary investigations highlight our ability to quantify the impact of the cosmic web connectivity on cluster properties with Euclid. △ Less

Submitted 29 May, 2026; v1 submitted 19 March, 2025; originally announced March 2025.

Comments: Paper accepted as part of the A&A Special Issue 'Euclid Quick Data Release (Q1)', 13 pages, 9 figures

Euclid Quick Data Release (Q1). Combined Euclid and Spitzer galaxy density catalogues at $z>$ 1.3 and detection of significant Euclid passive galaxy overdensities in Spitzer overdense regions

Authors: Euclid Collaboration, N. Mai, S. Mei, C. Cleland, R. Chary, J. G. Bartlett, G. Castignani, H. Dannerbauer, G. De Lucia, F. Fontanot, D. Scott, S. Andreon, S. Bhargava, H. Dole, T. DUSSERRE, S. A. Stanford, V. P. Tran, J. R. Weaver, P. -A. Duc, I. Risso, N. Aghanim, B. Altieri, A. Amara, N. Auricchio, H. Aussel , et al. (286 additional authors not shown)

Abstract: Euclid will detect tens of thousands of clusters and protoclusters at $z$>1.3. With a total coverage of 63.1deg$^2$, the Euclid Quick Data Release 1 (Q1) is large enough to detect tens of clusters and hundreds of protoclusters at these early epochs. The Q1 photometric redshift catalogue enables us to detect clusters out to $z$ < 1.5; however, infrared imaging from Spitzer extends this limit to hig… ▽ More Euclid will detect tens of thousands of clusters and protoclusters at $z$>1.3. With a total coverage of 63.1deg$^2$, the Euclid Quick Data Release 1 (Q1) is large enough to detect tens of clusters and hundreds of protoclusters at these early epochs. The Q1 photometric redshift catalogue enables us to detect clusters out to $z$ < 1.5; however, infrared imaging from Spitzer extends this limit to higher redshifts by using high local projected densities of Spitzer-selected galaxies as signposts for cluster and protocluster candidates. We use Spitzer imaging of the Euclid Deep Fields (EDFs) to derive densities for a sample of Spitzer-selected galaxies at redshifts $z$ > 1.3, building Spitzer IRAC1 and IRAC2 photometric catalogues that are 95% complete at a magnitude limit of IRAC2=22.2, 22.6, and 22.8 for the EDF-S, EDF-F, and EDF-N, respectively. We apply two complementary methods to calculate galaxy densities: (1) aperture and surface density; and (2) the Nth-nearest-neighbour method. When considering a sample selected at a magnitude limit of IRAC2 < 22.2, at which all three EDFs are 95% complete, our surface density distributions are consistent among the three EDFs and with the SpUDS blank field survey. We also considered a deeper sample (IRAC2 < 22.8), finding that 2% and 3% of the surface densities in the North and Fornax fields are 3$σ$ higher than the average field distribution and similar to densities found in the CARLA cluster survey. Our surface densities are also consistent with predictions from the GAEA semi-analytical model. Using combined Euclid and ground-based i-band photometry we show that our highest Spitzer-selected galaxy overdense regions, found at $z$~1.5, also host high densities of passive galaxies. This means that we measure densities consistent with those found in clusters and protoclusters at $z$>1.3. △ Less

Submitted 20 March, 2025; v1 submitted 19 March, 2025; originally announced March 2025.

Comments: Paper submitted as part of the A&A Special Issue `Euclid Quick Data Release (Q1)', 18 pages, 7 figures

Euclid Quick Data Release (Q1). The first catalogue of strong-lensing galaxy clusters

Authors: Euclid Collaboration, P. Bergamini, M. Meneghetti, A. Acebron, B. Clément, M. Bolzonella, C. Grillo, P. Rosati, D. Abriola, J. A. Acevedo Barroso, G. Angora, L. Bazzanini, R. Cabanac, B. C. Nagam, A. R. Cooray, G. Despali, G. Di Rosa, J. M. Diego, M. Fogliardi, A. Galan, R. Gavazzi, G. Granata, N. B. Hogg, K. Jahnke, L. Leuzzi , et al. (353 additional authors not shown)

Abstract: We present the first catalogue of strong lensing galaxy clusters identified in the Euclid Quick Release 1 observations (covering $63.1\,\mathrm{deg^2}$). This catalogue is the result of the visual inspection of 1260 cluster fields. Each galaxy cluster was ranked with a probability, $\mathcal{P}_{\mathrm{lens}}$, based on the number and plausibility of the identified strong lensing features. Specif… ▽ More We present the first catalogue of strong lensing galaxy clusters identified in the Euclid Quick Release 1 observations (covering $63.1\,\mathrm{deg^2}$). This catalogue is the result of the visual inspection of 1260 cluster fields. Each galaxy cluster was ranked with a probability, $\mathcal{P}_{\mathrm{lens}}$, based on the number and plausibility of the identified strong lensing features. Specifically, we identified 83 gravitational lenses with $\mathcal{P}_{\mathrm{lens}}>0.5$, of which 14 have $\mathcal{P}_{\mathrm{lens}}=1$, and clearly exhibiting secure strong lensing features, such as giant tangential and radial arcs, and multiple images. Considering the measured number density of lensing galaxy clusters, approximately $0.3\,\mathrm{deg}^{-2}$ for $\mathcal{P}_{\mathrm{lens}}>0.9$, we predict that \Euclid\ will likely see more than 4500 strong lensing clusters over the course of the mission. Notably, only three of the identified cluster-scale lenses had been previously observed from space. Thus, \Euclid has provided the first high-resolution imaging for the remaining $80$ galaxy cluster lenses, including those with the highest probability. The identified strong lensing features will be used for training deep-learning models for identifying gravitational arcs and multiple images automatically in \Euclid observations. This study confirms the huge potential of \Euclid for finding new strong lensing clusters, enabling exciting new discoveries on the nature of dark matter and dark energy and the study of the high-redshift Universe. △ Less

Submitted 19 March, 2025; originally announced March 2025.

Comments: Paper submitted as part of the A&A Special Issue `Euclid Quick Data Release (Q1)', 16 pages, 10 figures

Euclid Quick Data Release (Q1). The Strong Lensing Discovery Engine E -- Ensemble classification of strong gravitational lenses: lessons for Data Release 1

Authors: Euclid Collaboration, P. Holloway, A. Verma, M. Walmsley, P. J. Marshall, A. More, T. E. Collett, N. E. P. Lines, L. Leuzzi, A. Manjón-García, S. H. Vincken, J. Wilde, R. Pearce-Casey, I. T. Andika, J. A. Acevedo Barroso, T. Li, A. Melo, R. B. Metcalf, K. Rojas, B. Clément, H. Degaudenzi, F. Courbin, G. Despali, R. Gavazzi, S. Schuldt , et al. (321 additional authors not shown)

Abstract: The Euclid Wide Survey (EWS) is expected to identify of order $100\,000$ galaxy-galaxy strong lenses across $14\,000$deg$^2$. The Euclid Quick Data Release (Q1) of $63.1$deg$^2$ Euclid images provides an excellent opportunity to test our lens-finding ability, and to verify the anticipated lens frequency in the EWS. Following the Q1 data release, eight machine learning networks from five teams were… ▽ More The Euclid Wide Survey (EWS) is expected to identify of order $100\,000$ galaxy-galaxy strong lenses across $14\,000$deg$^2$. The Euclid Quick Data Release (Q1) of $63.1$deg$^2$ Euclid images provides an excellent opportunity to test our lens-finding ability, and to verify the anticipated lens frequency in the EWS. Following the Q1 data release, eight machine learning networks from five teams were applied to approximately one million images. This was followed by a citizen science inspection of a subset of around $100\,000$ images, of which $65\%$ received high network scores, with the remainder randomly selected. The top scoring outputs were inspected by experts to establish confident (grade A), likely (grade B), possible (grade C), and unlikely lenses. In this paper we combine the citizen science and machine learning classifiers into an ensemble, demonstrating that a combined approach can produce a purer and more complete sample than the original individual classifiers. Using the expert-graded subset as ground truth, we find that this ensemble can provide a purity of $52\pm2\%$ (grade A/B lenses) with $50\%$ completeness (for context, due to the rarity of lenses a random classifier would have a purity of $0.05\%$). We discuss future lessons for the first major Euclid data release (DR1), where the big-data challenges will become more significant and will require analysing more than $\sim300$ million galaxies, and thus time investment of both experts and citizens must be carefully managed. △ Less

Submitted 19 March, 2025; originally announced March 2025.

Comments: Paper submitted as part of the A&A Special Issue `Euclid Quick Data Release (Q1)', 15 pages, 8 figures

Euclid Quick Data Release (Q1). The Strong Lensing Discovery Engine D -- Double-source-plane lens candidates

Authors: Euclid Collaboration, T. Li, T. E. Collett, M. Walmsley, N. E. P. Lines, K. Rojas, J. W. Nightingale, W. J. R. Enzi, L. A. Moustakas, C. Krawczyk, R. Gavazzi, G. Despali, P. Holloway, S. Schuldt, F. Courbin, R. B. Metcalf, D. J. Ballard, A. Verma, B. Clément, H. Degaudenzi, A. Melo, J. A. Acevedo Barroso, L. Leuzzi, A. Manjón-García, R. Pearce-Casey , et al. (313 additional authors not shown)

Abstract: Strong gravitational lensing systems with multiple source planes are powerful tools for probing the density profiles and dark matter substructure of the galaxies. The ratio of Einstein radii is related to the dark energy equation of state through the cosmological scaling factor $β$. However, galaxy-scale double-source-plane lenses (DSPLs) are extremely rare. In this paper, we report the discovery… ▽ More Strong gravitational lensing systems with multiple source planes are powerful tools for probing the density profiles and dark matter substructure of the galaxies. The ratio of Einstein radii is related to the dark energy equation of state through the cosmological scaling factor $β$. However, galaxy-scale double-source-plane lenses (DSPLs) are extremely rare. In this paper, we report the discovery of four new galaxy-scale double-source-plane lens candidates in the Euclid Quick Release 1 (Q1) data. These systems were initially identified through a combination of machine learning lens-finding models and subsequent visual inspection from citizens and experts. We apply the widely-used {\tt LensPop} lens forecasting model to predict that the full \Euclid survey will discover 1700 DSPLs, which scales to $6 \pm 3$ DSPLs in 63 deg$^2$, the area of Q1. The number of discoveries in this work is broadly consistent with this forecast. We present lens models for each DSPL and infer their $β$ values. Our initial Q1 sample demonstrates the promise of \Euclid to discover such rare objects. △ Less

Submitted 19 March, 2025; originally announced March 2025.

Comments: Paper submitted as part of the A&A Special Issue `Euclid Quick Data Release (Q1), 16 pages, 11 figures

Euclid Quick Data Release (Q1). The Strong Lensing Discovery Engine C: Finding lenses with machine learning

Authors: Euclid Collaboration, N. E. P. Lines, T. E. Collett, M. Walmsley, K. Rojas, T. Li, L. Leuzzi, A. Manjón-García, S. H. Vincken, J. Wilde, P. Holloway, A. Verma, R. B. Metcalf, I. T. Andika, A. Melo, M. Melchior, H. Domínguez Sánchez, A. Díaz-Sánchez, J. A. Acevedo Barroso, B. Clément, C. Krawczyk, R. Pearce-Casey, S. Serjeant, F. Courbin, G. Despali , et al. (328 additional authors not shown)

Abstract: Strong gravitational lensing has the potential to provide a powerful probe of astrophysics and cosmology, but fewer than 1000 strong lenses have been confirmed so far. With a 0.16'' resolution covering a third of the sky, the Euclid telescope will revolutionise the identification of strong lenses, with 170 000 lenses forecasted to be discovered amongst the 1.5 billion galaxies it will observe. We… ▽ More Strong gravitational lensing has the potential to provide a powerful probe of astrophysics and cosmology, but fewer than 1000 strong lenses have been confirmed so far. With a 0.16'' resolution covering a third of the sky, the Euclid telescope will revolutionise the identification of strong lenses, with 170 000 lenses forecasted to be discovered amongst the 1.5 billion galaxies it will observe. We present an analysis of the performance of five machine-learning models at finding strong gravitational lenses in the quick release of Euclid data (Q1) covering 63 deg2. The models have been validated by citizen scientists and expert visual inspection. We focus on the best-performing network: a fine-tuned version of the Zoobot pretrained model originally trained to classify galaxy morphologies in heterogeneous astronomical imaging surveys. Of the one million Q1 objects that Zoobot was tasked to find strong lenses within, the top 1000 ranked objects contain 122 grade A lenses (almost-certain lenses) and 41 grade B lenses (probable lenses). A deeper search with the five networks combined with visual inspection yielded 250 (247) grade A (B) lenses, of which 224 (182) are ranked in the top 20 000 by Zoobot. When extrapolated to the full Euclid survey, the highest ranked one million images will contain 75 000 grade A or B strong gravitational lenses. △ Less

Submitted 26 June, 2025; v1 submitted 19 March, 2025; originally announced March 2025.

Comments: Paper accepted for the A&A Special Issue `Euclid Quick Data Release (Q1)', 24 pages

Euclid Quick Data Release (Q1) The Strong Lensing Discovery Engine B -- Early strong lens candidates from visual inspection of high velocity dispersion galaxies

Authors: Euclid Collaboration, K. Rojas, T. E. Collett, J. A. Acevedo Barroso, J. W. Nightingale, D. Stern, L. A. Moustakas, S. Schuldt, G. Despali, A. Melo, M. Walmsley, D. J. Ballard, W. J. R. Enzi, T. Li, A. Sainz de Murieta, I. T. Andika, B. Clément, F. Courbin, L. R. Ecker, R. Gavazzi, N. Jackson, A. Kovács, P. Matavulj, M. Meneghetti, S. Serjeant , et al. (314 additional authors not shown)

Abstract: We present a search for strong gravitational lenses in Euclid imaging with high stellar velocity dispersion ($σ_ν> 180$ km/s) reported by SDSS and DESI. We performed expert visual inspection and classification of $11\,660$ \Euclid images. We discovered 38 grade A and 40 grade B candidate lenses, consistent with an expected sample of $\sim$32. Palomar spectroscopy confirmed 5 lens systems, while DE… ▽ More We present a search for strong gravitational lenses in Euclid imaging with high stellar velocity dispersion ($σ_ν> 180$ km/s) reported by SDSS and DESI. We performed expert visual inspection and classification of $11\,660$ \Euclid images. We discovered 38 grade A and 40 grade B candidate lenses, consistent with an expected sample of $\sim$32. Palomar spectroscopy confirmed 5 lens systems, while DESI spectra confirmed one, provided ambiguous results for another, and help to discard one. The \Euclid automated lens modeler modelled 53 candidates, confirming 38 as lenses, failing to model 9, and ruling out 6 grade B candidates. For the remaining 25 candidates we could not gather additional information. More importantly, our expert-classified non-lenses provide an excellent training set for machine learning lens classifiers. We create high-fidelity simulations of \Euclid lenses by painting realistic lensed sources behind the expert tagged (non-lens) luminous red galaxies. This training set is the foundation stone for the \Euclid galaxy-galaxy strong lensing discovery engine. △ Less

Submitted 19 March, 2025; originally announced March 2025.

Comments: Paper submitted as part of the A&A Special Issue `Euclid Quick Data Release (Q1)', 18 pages, 9 figures

Euclid Quick Data Release (Q1): The Strong Lensing Discovery Engine A -- System overview and lens catalogue

Authors: Euclid Collaboration, M. Walmsley, P. Holloway, N. E. P. Lines, K. Rojas, T. E. Collett, A. Verma, T. Li, J. W. Nightingale, G. Despali, S. Schuldt, R. Gavazzi, A. Melo, R. B. Metcalf, I. T. Andika, L. Leuzzi, A. Manjón-García, R. Pearce-Casey, S. H. Vincken, J. Wilde, V. Busillo, C. Tortora, J. A. Acevedo Barroso, H. Dole, L. R. Ecker , et al. (350 additional authors not shown)

Abstract: We present a catalogue of 497 galaxy-galaxy strong lenses in the Euclid Quick Release 1 data (63 deg$^2$). In the initial 0.45\% of Euclid's surveys, we double the total number of known lens candidates with space-based imaging. Our catalogue includes 250 grade A candidates, the vast majority of which (243) were previously unpublished. Euclid's resolution reveals rare lens configurations of scienti… ▽ More We present a catalogue of 497 galaxy-galaxy strong lenses in the Euclid Quick Release 1 data (63 deg$^2$). In the initial 0.45\% of Euclid's surveys, we double the total number of known lens candidates with space-based imaging. Our catalogue includes 250 grade A candidates, the vast majority of which (243) were previously unpublished. Euclid's resolution reveals rare lens configurations of scientific value including double-source-plane lenses, edge-on lenses, complete Einstein rings, and quadruply-imaged lenses. We resolve lenses with small Einstein radii ($θ_{\rm E} < 1''$) in large numbers for the first time. These lenses are found through an initial sweep by deep learning models, followed by Space Warps citizen scientist inspection, expert vetting, and system-by-system modelling. Our search approach scales straightforwardly to Euclid Data Release 1 and, without changes, would yield approximately 7000 high-confidence (grade A or B) lens candidates by late 2026. Further extrapolating to the complete Euclid Wide Survey implies a likely yield of over 100000 high-confidence candidates, transforming strong lensing science. △ Less

Submitted 19 March, 2025; originally announced March 2025.

Comments: Data: https://doi.org/10.5281/zenodo.15003116. Paper submitted as part of the A&A Special Issue `Euclid Quick Data Release (Q1)'. 20 pages, 11 figures, plus appendices