Rafael Rebolo

Aims. We revisit the carbon abundance in the primitive iron-poor star J0815+4729 ([Fe/H] = −5.49). J0815+4729 is so enhanced in carbon and rich in oxygen that, in addition to the CH and C ₂ molecular lines, weaker neutral atomic transitions are measurable in its spectrum. This provides a unique opportunity to explore the line formation of C I and O I transitions in such an extremely iron-deficient star. Methods. Based on high-resolution spectroscopy and newly constructed atomic models for carbon and oxygen, we present the first investigation into deriving the carbon and oxygen abundances of this hyper-iron-poor star from neutral atomic transitions under nonlocal thermodynamic equilibrium (NLTE) conditions. Results. The carbon abundance we derived from C I lines is A(C) _NLTE = 8.08 ± 0.07, which is consistent with that inferred from C ₂ lines (A(C) _LTE = 7.96 ± 0.08), but higher by about 0.6 dex than the value from CH lines (A(C) _LTE = 7.47 ± 0.17). Since the CH lines are the most commonly used carbon abundance indicator for extremely metal-poor stars, this result casts doubt on the quantitative interpretation of CH absorption in the most primitive stars. We confirm that J0815+4729 is one of the most carbon-enhanced stars known to date, with [Fe/H] ≤ −4.5. We also determined A(O) _NLTE as 7.25 ± 0.11, with NLTE corrections of about −0.11 dex. The carbon-to-oxygen ratio of the star is [C/O] = 1.06 ± 0.11.

Context. Ground-based spectrographs operating in the near-infrared (NIR) regime are hampered by various absorption and emission features of Earth’s atmosphere. While considerable attention has been paid to mitigating telluric absorption, correcting telluric emission features remains non-trivial and can significantly affect the observation of faint targets. Aims. We aim to develop and implement automated algorithms for sky background emission correction in the context of high-resolution spectroscopy. These empirical-based algorithms have been officially integrated into both NIRPS data reduction pipelines: NIRPS DRS and APERO DRS. The NIRPS DRS algorithm is additionally functional at the telescope site, enabling quick, real-time telluric-corrected data processing. Designed for flexibility, these techniques can be incorporated into the reduction workflow of any high-resolution spectrograph to improve the radial velocity (RV) performance. Methods. In our approach, a reference sky spectrum is first created by deep-stacking NIRPS sky calibration frames on a common wavelength grid and calculating the weighted median flux per pixel, separately for both the high-accuracy and high-efficiency instrument modes. This process is repeated for all spectral orders and for both the object and sky-calibration fibres: fibre A and fibre B, respectively. In this reference sky spectrum, the sky emission lines can be identified and used to construct a static library. During the data reduction process, the emission features in the library are individually scaled in terms of flux using two distinct techniques, each specific to the two DRS pipelines. Finally, they are locally subtracted from the science observations to minimise their noise contribution to the final spectrum Results. We find that the correction algorithms significantly improve the RV measurements obtained using both the cross-correlation function and line-by-line techniques, enabling NIRPS to achieve submetre-per-second precision in the NIR. The techniques have been successfully validated and demonstrated on both bright (Proxima Centauri) and faint (TOI-406, TOI-3494, TOI-4552) targets, as well as in both high-efficiency and high-accuracy instrument modes. Conclusions. We present the first attempts at empirically subtracting emission line features from high-resolution spectra. These algorithms can be applied independently of the instrumentation and implemented into the data reduction workflow of any high-resolution instrument with minimal modifications required.

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.

We model intrinsic alignment (IA) in Euclid ’s Flagship simulation to investigate its impact on Euclid ’s weak lensing signal. Our IA implementation in the Flagship simulation takes the photometric properties of galaxies into account, along with their dark matter host halos. The simulation parameters are calibrated using combined constraints from observations and cosmological hydrodynamical simulations. We compare simulations against theory predictions, determining the parameters of two of the most widely used IA models: the nonlinear alignment (NLA) and the tidal alignment and tidal torquing (TATT) models. We measured the amplitude of the simulated IA signal as a function of galaxy magnitude and colour in the redshift range 0.1 < z < 2.1, close that of Euclid ’s main galaxy sample. We find that both NLA and TATT are able to accurately describe the IA signal in the simulation down to scales of 6–7 h ⁻¹ Mpc. We measured the alignment amplitudes for red galaxies that are comparable to those of the observations, using samples that had not been used in the initial calibration procedure. For blue galaxies, our constraints are consistent with zero alignments in our first redshift bin 0.1 < z < 0.3, but we detected a non-negligible signal at higher redshift; however, this signal is consistent with the upper limits set by observational constraints. Additionally, several hydrodynamical simulations have predicted alignments for spiral galaxies, in agreement with our findings. Finally, the evolution of alignment with redshift is realistic and comparable to what has been determined based on 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 improved agreement can be obtained when the 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.

Ultracool dwarfs (UCDs) encompass the lowest-mass stars and brown dwarfs at the stellar–substellar boundary. These intrinsically faint objects are challenging to detect. However, 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. We carried out a systematic study using Euclid Quick Data Release (Q1) low-resolution near-infrared (NIR) spectra to identify and characterise UCDs. A sample of previously known UCDs in Q1 was compiled to validate the analysis, which was then applied to newly identified UCD candidates in Euclid Deep Field North (EDF-N; 22.9 deg ² , down to J _E ≈ 24.5 mag). We confirm 33 new UCDs in EDF-N, spanning spectral types (SpTs) M7–T1 ( J _E = 17–21 mag) and distances of 50–600 pc. Six of them were also independently identified in other Q1 studies. We followed up on two of these objects with ground-based optical and NIR observations using the Gran Telescopio Canarias, providing an independent validation of the Euclid spectral classification and the ability of Q1 spectra to constrain the stellar parameters of UCDs. We determined the SpTs from Euclid Q1 spectra of 33 newly identified UCDs and 167 previously known UCDs by fitting spectral standards from the SpeX Prism Library Analysis Toolkit. The known UCD sample includes 103 objects in common with the Dark Energy Survey (M7–T0, J _E = 17–22 mag, and d = 50–600 pc), 6 objects from literature compilations of UCD sources (M8–T7, J _E = 18–20 mag, and d = 30–300 pc), and 58 in common with Gaia (M7–L1, J _E = 17–24 mag, and d = 70–300 pc). We derived independent stellar parameter estimates through atmospheric model fitting; the resulting effective temperatures are in good agreement with Euclid SpTs. Combined with complementary ground- and space-based data, these results demonstrate the potential for determining proper motions and identifying companions. Additionally, we investigated Gaia UCDs in Q1 by combining their mean XP spectra with Euclid NIR spectra, extending the wavelength coverage and enabling more comprehensive determinations of stellar parameters, including radii. To support broader classification efforts, we analysed the UCDs in colour–colour space and assessed their separation from quasar loci. We present a publicly available, machine-readable catalogue that includes Euclid observations of known and newly identified sources, with spectral classifications and stellar parameters. This work demonstrates Euclid ’s capability to identify and characterise the faintest stellar populations, establishing a foundation for large-scale UCD studies with forthcoming survey data.

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

The intrinsic alignments of galaxies are a potentially major contaminant among cosmological analyses of weak gravitational lensing. In this work, we constructed a semi-analytic model of galaxy ellipticities and alignments in the Euclid Flagship simulation to predict this contamination in Euclid ’s weak lensing observations. Galaxy shapes and orientations were determined by the corresponding properties of the host haloes in the underlying N -body simulation, as well as the relative positions of galaxies within their halo. Alignment strengths were moderated via stochastic misalignments, separately for central and satellite galaxies, and were dependent on the galaxy’s redshift, luminosity, and rest-frame colour. The resulting model was calibrated against galaxy ellipticity statistics from the COSMOS Survey, selected alignment measurements based on Sloan Digital Sky Survey samples, and galaxy orientations extracted from the Horizon-AGN hydrodynamic simulation at redshift z = 1. The best-fit model has a total of 12 alignment parameters and generally reproduces the calibration data sets well within the 1 σ statistical uncertainties of the observations and the Flagship simulation, with notable exceptions for the most luminous subsamples on small physical scales. The statistical power of the calibration data and the volume of the single Flagship realisation are still too small to provide informative prior ranges for intrinsic alignment amplitudes in relevant galaxy samples. Given these uncertainties, we predict that Euclid end-of-mission tomographic weak gravitational lensing two-point statistics will end up modified due to intrinsic alignments, by up to 10%.

Context . Condensates are ubiquitous to all Solar System planets with significant ( > 10 mbar) atmospheres. The same is true for most exoplanets with characterised atmospheres, with even ultra-hot exoplanets being able to form clouds on their cooler nightsides. One high-interest condensate is titanium, a highly refractory element that as an oxide (TiO) is a potent optical absorber long believed to be a driver of thermal inversions in highly irradiated exoplanet atmospheres. Observations have shown that some ultra-hot Jupiters have strong thermal inversions despite being significantly Ti-depleted, likely due to cold trapping, raising doubts about whether TiO is the main cause of their inverted temperature structures. Aims . Our aim was to retrieve the titanium-to-iron ratio of the dayside atmosphere of the ultra-hot Jupiter WASP-189b to determine whether the full titanium budget is accounted for in the gas phase. Methods . We analysed thermal emission observations of WASP-189b taken with the HARPS and NIRPS spectrographs using different atmospheric retrieval prescriptions to infer the planet’s atmospheric thermal structure and composition. Results . We observed Fe and Ti signals in cross-correlation and measured a sub-solar Ti/Fe ratio for WASP-189b using both free and chemical equilibrium retrieval approaches. We find the Ti/Fe of the planetary atmosphere to be between 28% and 81% (1- σ bounds) that of the stellar value. Conclusions . The slight underabundance of Ti with respect to Fe on the dayside atmosphere of WASP-189b suggests that some titanium is missing from the gas phase, potentially due to a partial nightside cold trap. In the context of the ultra-hot Jupiter population, the onset of titanium in exoplanetary atmospheres appears to occur progressively, coinciding not with when thermal inversions begin but rather when the vapourisation threshold of titanium is reached on the nightside.

Aims . We search for evidence of star–planet magnetic interactions in the nearby Proxima Centauri planetary system using high-quality, high-spectral-resolution optical observations spanning three years. Methods . We investigate the variability of photospheric and chromospheric features traced by oxides, H I , Na I , Fe I , He I , and Ca II using a differential spectroscopic method and narrowband light curves derived from the spectra. Results . We measure a photospheric stellar rotation period of 84.9 ± 0.6 d and a half-rotation period of 44.3 ± 0.2 d, consistent with previous studies. The stellar photospheric absorption lines show no variability correlated with the orbital periods of Proxima b or d, supporting their planetary nature. Using Fe I absorption and emission lines, we find that Proxima Centauri was flaring for 24.8 ± 4.7% of the observing time, with significant statistical evidence (≥99.8%) of flare events likely phase-locked to the inner Mars-mass planet Proxima d. Modeling the star–planet interaction via the helicity-driven reconnection mechanism with the Poynting flux formalism, we estimate a likely polar magnetic field of ∼16 G for Proxima d (assuming a Mars-sized radius), with a plausible range of 3–280 G accounting for radial and dipolar stellar magnetic field configurations, planetary radii comparable to Mars and Earth, and the observed range of stellar flare intensities. This represents the first such estimate for a terrestrial exoplanet. Evidence of a potential star–planet interaction with the outer, Earth-mass Proxima b arises not from phase-locked flare clustering, but from modulation of flare intensities. Applying a prewhitening analysis to the full time series of combined chromospheric H α , Na I D1 and D2, and Ca II H and K lines reveals peaks, in order, at half the stellar rotation period, Proxima b’s orbital period, the full stellar rotation, and Proxima d’s orbital period. All evidence suggests that both planets show magnetic interaction with their host star. Focusing on flaring epochs only, the periodogram of these chromospheric lines shows a peak consistent with the synodic period between half the stellar rotation and the mutual synodic period of Proxima b and d, implying prograde stellar rotation and planetary orbits. We also identify spectroscopic features at around 7794.0 and 7808.5 Å that behave differently from the rest of the spectrum.

We report the discovery of 31 new high- z quasars in the redshift range 6.6 < z < 7.8. These quasars were selected from approximately 3000 deg ² of sky covered during the first 1.5 years of the Euclid Wide Survey, representing the initial results of the Euclid high- z quasar search. Our candidate selection employed multiple machine-learning and probabilistic techniques applied to the EuclidI _E , Y _E , J _E , and H _E images, supplemented by ancillary z -band data when available. Spectroscopic follow-up observations were carried out with Keck, Magellan, and the Large Binocular Telescope (LBT). Among the new discoveries, there are 12 quasars at z ≥ 7, more than doubling the number of previously known quasars at z ≥ 7. The newly discovered quasars exhibit 21.2 < J _E < 23.2 (−25.5 < M ₁₄₅₀ < −23.6), extending quasar studies to the faint end of the quasar luminosity function (QLF) at z ≳ 7. The quasar with the highest- z , EUCL J172902.75+641018.1 at z ≈ 7.77, sets the new redshift record for the most distant quasar ever reported. These discoveries demonstrate Euclid ’s transformative role in high- z quasar discovery and set the stage for future follow-up studies of the early galaxies hosting quasars, supermassive black hole growth, and the intergalactic medium in the epoch of reionisation.