Filippo Maria Zerbi

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 (Teledyne’s H2RG) 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 on board via a dedicated hardware-software architecture designed to fulfil Euclid ’s stringent data quality requirements. A custom application software, running on the two NISP data-processing units, implements the NISP signal estimator: an ad hoc algorithm that delivers accurate flux measurements and simultaneously estimates the quality of signal estimation through the quality factor (QF) parameter. This paper further 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 explain how we tested the flight performance of the NISP signal estimator. Results indicate a systematic bias lower than 0.01 e ⁻ s ⁻¹ for 99% of the NISP pixel array, which is well within the noise budget of the estimated signal. We also derived 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.

Stage-IV galaxy surveys will provide the opportunity to test cosmological models and the underlying theory of gravity with unparal-leled precision. In this context, it is crucial for the Euclid mission to leverage its spectroscopic and photometric probes to systematically investigate and incorporate non-standard cosmological models, including modified gravity, alternative dark energy scenarios, massive neutrinos, and primordial non-Gaussianity. For this, we produce and release publicly simulated galaxy catalogues from a broad suite of non-standard cosmological simulations, which we processed through a model-independent analytical pipeline, making use of ROCKSTAR for halo identification, and a modified version of the SciPic library for the galaxy-halo connection using the halo occupation distribution framework. One of the goals of this paper is also to test the validity of the template-fitting method for galaxy clustering analyses, even in cases where the template significantly differ from data. For this purpose, we investigate the multipoles of the two-point correlation function in redshift space and VDG _∞ , a highly performant, state-of-the-art model for galaxy clustering. Across a wide range of models, the linear growth rate multiplied by the matter density within spheres of radius of 12 Mpc, fσ ₁₂ , exhibits a notable robustness to the choice of cosmological template. Compared to previous works, our study extends this result to numerous scenarios with markedly distinct gravitational or dark energy dynamics. We find that the most of the scatter in cosmological parameter inference already appears when using the cosmological model of the simulations as templates. Using a ‘wrong’ template can also introduce an additional scatter, although with smaller amplitude. Often, we find deviations much larger than error bars, meaning that the Gaussian approximation for the covariance might need to be further studied. Looking ahead, future cosmological investigations must broaden their scope to include a diverse array of non-standard theoretical frameworks extending beyond ΛCDM and rudimentary dynamic dark energy models.

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 extracted from each of the four snapshots of the Flagship 1 (FS1) simulation, which is designed to replicate the statistical properties of the first Euclid data release (DR1), we assessed the performance of the two standard BAO reconstruction methods based on the Zeldovich approximation, R EC S YM and R EC I SO , in four redshift snapshots (0.9 ≤ z ≤ 1.8). The pipeline introduces several methodological advances: an emulator-based model evaluator ( Bora.jl ) combined with a Hamiltonian Monte Carlo sampler ( NUTS ), achieving a speed-up of more than 500 times relative to standard Monte Carlo Markov chains, and a semi-analytical covariance estimator ( BeXiCov+WinCov ) that enables robust error estimates with only eight mock realisations and remains stable under variations in the fiducial cosmology. Together, these components ensure computational efficiency while significantly reducing the risk of underestimating the parameter uncertainties. The two reconstruction schemes yield unbiased BAO measurements for all redshifts and user-defined choices, including the smoothing scale and fiducial cosmology. In each snapshot, reconstruction enhances the figure of merit for {Ω _m , H ₀ r _s } by a factor of ∼3, which is equivalent to tripling the effective survey volume. When we combined the four redshift bins, the improvement remained substantial, with BAO-only constraints reaching ∼10% precision on Ω _m and ∼3% on H ₀ r _s . The results from R EC S YM and R EC I SO are consistent within the uncertainties, but we recommend R EC S YM during testing because its sensitivity to covariance variations is lower. These findings establish the accuracy, robustness, and scalability of the Euclid BAO pipeline for DR1 and provide a solid foundation for future cosmological analyses.

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

We present the discovery of two disky titans in the first data release of the Euclid satellite. These sources are massive ( M _* > 10 ¹¹ M _⊙ ) star-forming (SFR ∼ 20 M _⊙ yr ⁻¹ ) disks located in strong over-dense environments at intermediate redshifts ( z ∼ 0.75). They represent a small fraction of the massive galaxies in over-dense regions (just four candidates in over 20 deg ² analysed in this study). Their existence is puzzling, given the abundance of passive, bulge-dominated sources commonly found at the centre of groups and clusters at low redshifts. First, our analysis shows that these objects are located in massive groups ( M _h ∼ 10 ^13.8 M _⊙ ), where rapid accretion of cold gas should be prevented by the formation of a static hot halo. Despite this, a millimetre follow-up with NOEMA shows significant cold gas reservoirs ( M H ₂ ∼ 10 ^10.3 M _⊙ ) within these sources. Second, our morphological analysis indicates a massive, passive bulge in these galaxies. This bulge is expected to stabilise the disk against fragmentation, thereby suppressing further star formation. However, these sources lie on the Schmidt–Kennicutt relation or even slightly above. Building on these observations, we propose a scenario where these disky titans are the product of a merger-induced rejuvenation episode, in which the most massive galaxy in a group accretes cold gas from another member and briefly restarts star formation. Such a scenario is supported by a comparison with the TNG300 simulation and easily explains the persistence of star formation activity in massive galaxies in over-dense environments as temporary stages of more complex evolution. More generally, our study showcases the ability of Euclid to find rare objects, thanks to the unprecedented statistic offered by its surveys and the scientific potential residing in the synergy between Euclid and other facilities observing at longer wavelengths.

The large-scale structure (LSS) of the Universe is an important probe for deviations from the canonical cosmological constant Λ and cold dark matter (ΛCDM) model. A statistically significant detection of any deviations would signify the presence of a new physics or the breakdown of any number of the underlying assumptions of the standard cosmological model or possible systematic errors in the data. In this paper, we quantify the ability of the LSS data products of the spectroscopic survey of the Euclid mission, together with other contemporary surveys, to improve the constraints on deviations from ΛCDM in the redshift range 0 < z < 1.75. We consider both currently available growth rate data and simulated data with specifications from Euclid and external surveys, based on ΛCDM and a modified gravity (MoG) model with an evolving Newton’s constant (denoted μCDM), and carry out a binning method and a machine learning reconstruction, based on genetic algorithms (GAs), of several LSS null tests. Using the forecast Euclid growth data from the spectroscopic survey in the range 0.95 < z < 1.75, we find that in combination with external data products (covering the range 0 < z < 0.95), Euclid will be able to improve the precision of current null tests of the LSS on average by a factor of eight when using a binning method and a factor of six when using the GAs. Our work highlights the need for synergies between Euclid and other surveys, but also the usefulness of statistical analyses, such as GAs, in order to disentangle any degeneracies in the cosmological parameters. Both are necessary to provide tight constraints over an extended redshift range and to probe for deviations from the ΛCDM model.

Context . Dwarf galaxies in dense cluster environments are susceptible to tidal interactions that can alter their morphology and kinematics. Boxy isophotes are well studied in massive galaxies but remain poorly understood in dwarfs. Aims . We aim to identify and characterise boxy/peanutty dwarf galaxies in the Perseus cluster and determine the origin of their isophotal shapes. Methods . Using Euclid Early Release Observations of the Perseus cluster, we present a cumulative light fraction method for robustly measuring the isophotal shape parameter c ₄ , particularly suited to low surface brightness regimes. From the ~1100 catalogued dwarfs, we select a clean sample of ~190 early-type systems sufficiently resolved and free of contamination for reliable c ₄ measurement. Observed trends are interpreted through comparison with mock Euclid observations of gravitational N -body simulations of tidally transformed dwarfs. Results . From this clean sample, we identify 13 dwarfs with significantly boxy isophotes ( c ₄ < −0.0175), confirmed through visual inspection. These galaxies lack visible thin disks, lie on the cluster red sequence, and show no preferential spatial concentration within Perseus (Kolmogorov–Smirnov p = 0.74 versus the full dE sample). We find a significant anticorrelation between c ₄ and effective radius: larger galaxies exhibit more boxy (more negative c ₄ ) isophotes. We caution that the linear (Pearson) coefficient is sensitive to the two most boxy galaxies; the rank-based (Spearman) coefficient remains significant ( r _s = −0.59, p = 0.035). The parameter c ₄ shows no correlation with Sérsic index or concentration. An analogous size–shape anticorrelation is recovered in the simulations, where inner regions are dominated by box orbits associated with a triaxial peanut structure and outer regions by short-axis tube orbits. Conclusions . Boxy/peanutty dwarf galaxies in Perseus are tidally transformed remnants of moderately rotating progenitors, with boxy isophotes tracing inner box-orbit-dominated peanut structures. The size–shape correlation arises from viewing geometry: face-on orientations reveal the rectangular profile of the elongated triaxial structure (large and boxy), while edge-on views yield rounder, compact morphologies. Our sample thus represents an orientation-selected subset of tidally transformed, peanutty dwarfs in the cluster.

The Euclid satellite will deliver a catalogue of optically selected galaxy clusters spanning from around 2000 deg ² in Data Release (DR) 1 to around 14 000 deg ² in DR3. In this work, we assess the validity of cluster clustering (CC) models for template-fitting, which complements the full-shape methodology by providing cosmological information from the anisotropy of the redshift-space two-point correlation function (2PCF). Both methods will be used to analyse the cluster 2PCF multipoles with Euclid . We examined the multipoles of the two-point redshift-space clustering of galaxy clusters simulated with the semi-analytic PINOCCHIO code using third-order Lagrangian perturbation theory, assuming a Euclid DR1-like footprint of 500 deg ² in the northern hemisphere and 1400 deg ² in the southern hemisphere. We estimated the first three even multipoles of the 2PCF and associated covariance matrix from 1000 DR1-like synthetic catalogues. We studied the impact of modelling the relevant non-linearities, halo bias, and photometric redshift uncertainties on the 2PCF. We applied three clustering models to the mock catalogues at 0 < z < 2 and with a virial mass of M _vir > 10 ¹⁴ h ⁻¹ M _⊙ under realistic and optimistic photometric redshift uncertainty scenarios. We formulated a set of permissive and conservative criteria that ought to be fulfilled by the multipole cut-off scales and validated them against 100 mock catalogues via an inference of the growth rate multiplied by the matter power spectrum normalisation parameter, fσ ₈ . We tested the dispersion, Scoccimarro, and Taruya–Nishimichi–Saito models. We find that the simplest of the three (i.e. the dispersion model) yields unbiased inferences on fσ ₈ from CC down to 10 h ⁻¹ Mpc in a DR1-like setting. All clustering models provide very similar goodness-of-fit metrics in the presence of DR1-like cluster redshift uncertainties.

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 the VIS catalogues, we rejected point sources, applied a magnitude cut ( I _E ≤ 24) on the deflectors, and ran a pixel-level artefact-and-noise filter to build 96 × 96 pixel cutouts. The VIS+NISP colour composites were 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, which we used to curate a morphology-balanced negative set and augment scarce positives. Among the six compact convolutional neural networks (CNNs) that had been studied initially, the modified VGG16 (GlobalAveragePooling + 256/128 dense layers with the last nine layers trainable) exhibited the best performance. In our run, the training set grew from 27 seed lenses (augmented 67× to 1809) plus 2000 negatives to a colour dataset of 30 686 images. After three rounds of iterative fine-tuning, a human grading of the top 4000 candidates ranked by the final model yielded 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 recovered 740 out of 905 (81.8%) candidate Q1 lenses within its top 20 000 predictions, considering off-centred samples. Candidates span I _E ≃ 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 about a week for a small team and the approach can easily be scaled to future wide-area Euclid releases. Subsequent works will focus on calibrating the selection function via lens injection, extending recall through uncertainty-aware active learning, and exploring multi-scale or attention-based neural networks with fast post hoc vetters that incorporate lens models into the classification.

Observatories of class up to 1m are commonly provided with COTS cameras for imaging and photometry. This is especially true for didactic, outreach, and private telescopes, although they are also successfully used for scientific purposes. Spectroscopy is generally relegated to larger, institutional facilities and their corresponding professional instruments. However, semiprofessional solutions exist. How to integrate them in small observatories? We present the first light of Cerberus, a three-headed instrument for the 80cm OARPAF telescope in Northern Italy: a new Nashmyth interface flange allows connecting not only the existing SBIG CCD camera, but also a LHIRES III long slit and a FLECHAS ´echelle spectrograph. Moreover, the photometry mode is upgraded with a tip-tilt lens to correct for tracking and vibrations up to 10Hz rate. The three instruments are alternatively selected through a linear stage provided with mirrors, and are remotely operated by a custom web-based graphical interface.