Michele Fumagalli

Context. Current and future large spectroscopic surveys are significantly enhancing the volume and resolution of quasar spectra that are observed, motivating the creation of efficient and precise automated techniques to detect absorption features. Aims. This study focuses on the detection of metal absorption features using a novel U-Net model on WEAVE-like mock spectra in the quasar rest-frame wavelength interval 1230 Å ≤ λ _RF ≤ 3095 Å. We test the network performance for absorption detection both on ideal data and after simulating the continuum fitting step as applied on real data. The ability to recover the correct line centers is also studied. Methods. The performance of these architectures is evaluated by the completeness, purity, and F1 score reached in bins of signal-to-noise (S/N _line ) for the absorption lines and with the absolute fractional flux error for the continuum. Results. The U-Net model reaches scores of ≈90% for all metrics (completeness, purity, and F1 score) at S/N _line ≈ 4. All false positive detections with S/N _line ≥ 5 fall in the tails of the broad Ly α absorbers distribution of damped Ly α systems. The combination of continuum fitting and line detections has negligible effects on the detection performance at S/N _line ≥ 4. Conclusions. Our proposed U-Net architecture offers a competitive tool for the analysis of absorption lines in current and upcoming large spectroscopic surveys and is well suited to the identification of any absorption line feature.

We present the first detailed forecasts for the detectability of patchy hydrogen reionization in the one-dimensional Lyα forest power spectrum to be measured by the WEAVE-QSO survey. Using the Sherwood-relics reionization simulations and a WEAVE-QSO survey configuration, we generate mock spectra in four redshift bins, z = 4.0, 4.2, 4.4, and 4.6, in which relic ionization and temperature fluctuations from patchy hydrogen reionization enhance the Lyα forest power spectrum on large scales (i.e., at wavenumber $k\sim 10^{-3}\rm \, s\, km^{-1}$). Our Lyα forest pipeline forecasts the power spectrum covariance by considering sample size, spectral resolution, noise subtraction, continuum placement, metal contamination, and damping wings from high-column density absorbers. Applying our covariance forecast within a Bayesian parameter inference framework, we find that the signature of patchy hydrogen reionization should be detectable at a significance of ≃ 4.5σ. The forthcoming WEAVE-QSO 1D power spectrum measurements should therefore be able to directly detect and characterize the large-scale relic imprint of patchy hydrogen reionization in the Lyα forest power spectrum at z ≥ 4.

We present a complete and homogeneous analysis of the Ly α emission properties of the cosmic web at 3 ≲ z ≲ 5 as a function of the overdensity of Ly α emitters (LAEs) in the MUSE Ultra Deep Field (MUDF) and the MUSE Extremely Deep Field (MXDF). We identified 41 overdensities, probing environments that are δ ≈ 2 − 10 times denser than the field. We searched for extended emission down to surface brightness (SB) levels of 3 − 5 × 10 ⁻²⁰ erg s ⁻¹ cm ⁻² arcsec ⁻² , revealing filamentary structures beyond the scale of the circumgalactic medium, confirming that LAEs act as signposts of the cosmic web. A trend with overdensity emerges: non-detections are mainly at δ < 2, a ≈1 dex scatter in SB appears for 2 ≲ δ ≲ 3.5, and detections reach a maximum intrinsic SB of ≈2 × 10 ⁻¹⁹ erg s ⁻¹ cm ⁻² arcsec ⁻² for δ ≳ 3.5. Emitting regions occupy a fraction of projected area f _s ≈ 0.20 − 0.25 inside filaments, yielding a cosmological incidence for Ly α emission in the cosmic web ℓ (Ly α ) of ≈1.22, which is similar to the one of partially neutral Lyman limit systems. This analysis indicates that the emitting gas is partially ionized at moderate densities ( n _H ≈ 10 ⁻³ − 10 ⁻¹ cm ⁻³ ), likely tracing the denser spines of intergalactic filaments and embedded substructures. Finally, we forecast how oriented stacking in larger samples of overdensities from shallower observations could yield an expanded view of the cosmic web before the next-generation wide-field spectroscopic instruments become operational.

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.

We present the KCWI Analysis of Gas around Galaxies (KAGG) survey of Ly α emission around damped Ly α absorbers (DLAs) at z ≈ 3–5 with the Keck Cosmic Web Imager (KCWI) integral-field spectrograph. The KAGG survey is characterized by the sample of 36 DLAs spanning a wide metallicity range, the wide search area for associated Ly α emission (impact parameter b ≲ 120 kpc), and the high emission sensitivity, reaching Ly α luminosities ≤10 ⁴² erg s ⁻¹ . We identify seven DLA Ly α emitters that are likely DLA galaxy candidates due to the good match between the Ly α emission and DLA redshifts. Based on a composite sample of 11 DLA Ly α emitters from KAGG and MUSE Analysis of Gas around Galaxies (MAGG), we find that the impact parameters of DLA Ly α emitters decrease with DLA metallicity, from b > 50 kpc at [M/H] ≈ −2 to b < 50 kpc at [M/H] ≈ −1. Including Ly α emitters at larger velocity offsets and at larger impact parameters from the DLAs, we construct samples of 35 “group” Ly α emitters and 17 “large-scale” Ly α emitters. We find that their impact parameters and incidence rates also depend on DLA metallicity. Based on simulations of our survey and comparisons with models of galaxy formation, we conclude that higher DLA metallicities are associated with higher halo masses. The low Ly α emission detection rate at low impact parameters suggests that most DLA galaxies either have low stellar masses ( M ≲ 10 ⁸ M _⊙ ) or have their Ly α emission suppressed due to H i resonant scattering and/or dust absorption.

The evolution of galaxies is profoundly influenced by the environment they reside in. Cosmic voids, the most underdense regions of the Universe, serve as pristine laboratories for studying galaxy evolution in the relative absence or weaker influence of the complex physical processes that dominate denser environments. We investigate the properties and merger histories of galaxies as a function of environment using the GAlaxy Evolution and Assembly (GAEA) mock-observation light cone, which replicates the Euclid Deep Survey as foreseen at the epoch of the first data release. The galaxy sample spans the redshift range 0.4<z<1.8, corresponding to the interval over which is accessible to Euclid slitless spectroscopy. We classified galaxies based on their void-centric distance and local density contrast. We compared their stellar masses, specific star formation rates, bulge-to-total stellar mass ratios, and halo masses across different environments, while controlling for the dependence on stellar mass of the other intrinsic properties. We further analysed the merger histories of these galaxies to study their assembly evolution. We find that galaxies located closer to void centres (d_ ̊m cc ̊m v ) are generally less massive, more actively star-forming, and more disc-dominated than galaxies in denser regions, with their distributions statistically distinct at more than 4 confidence according to Kolmogorov--Smirnov tests. Merger histories indicate that void galaxies do not experience fewer mergers but that these occur later, leading to stellar mass assembly delayed by approximately 0.5--1 Gyr relative to galaxies in high-density regions. This supports a scenario in which the environment regulates the timing and nature of mergers rather than their overall frequency, producing a slower evolutionary path in low-density regions. We conclude by discussing the extent to which these trends are shaped by environmental parametrisation methods and observational selection effects. Our analysis provides a framework for interpreting forthcoming Euclid data and demonstrates that Euclid will enable the identification of cosmic voids and characterisations of their galaxy populations with an unprecedented statistical power and redshift coverage, providing a quantitative forecast of Euclid's capability to probe environmental effects on galaxy evolution.

Damped Ly α absorbers (DLAs) are the highest H i column density ( N _{H I} ) absorption line systems detected in the spectra of background quasars. DLAs dominate the neutral gas content of the Universe (Ω _HI ) and are used to measure the metallicity evolution of H i gas. In this work, we introduce a sample of five recently detected DLAs at z > 4.7, found in mid- to high-resolution spectroscopy from VLT/X-shooter and Keck/HIRES. These DLAs were not preselected based on metallicity, enabling an unbiased study of the metallicity of H I gas at z ∼ 5. We also search for DLAs unbiased in metallicity at 0 < z < 5.5 from the literature, we apply a combined correction for dust depletion and α -enhancement (assuming no depletion of S, Si, and Zn) to Fe abundances, and we measure the cosmic metallicity evolution of α -elements using a linear fit with a slope of −0.22 ± 0.05 dex per unit redshift. For the highest redshift bin, we find an N _{H I} weighted average of 〈 Z 〉 = −2.00. This value is 4.4 σ deviant from the trend recovered at z < 4.7 and a K-S test comparison gives a 2.4 σ difference. We conclude that the metallicity of H I gas sharply decreases at z ∼ 5, in agreement with previous tentative evidence. This sharp decrease may be connected with the onset of the enrichment of galaxies’ circumgalactic media or with the end of cosmic reionization; though, we cannot exclude that it is driven by small sample statistics.

We investigate the connection between strong, blended Ly α absorption systems (SBLAs) and ≈1000 Ly α -emitting galaxies (LAEs) at z ≳ 3 in 28 quasar fields from the MUSE Analysis of Gas around Galaxies (MAGG) survey. Selecting SBLAs as spectral regions with transmitted flux −0.05 < F < 0.25 over ≈ 138 km s ⁻¹ bins, we find a strong correlation with LAEs within a projected distance of R ≤ 300 kpc and line-of-sight velocity separation of |Δ v | ≤ 300 km s ⁻¹ . The association rate increases significantly with decreasing flux, a trend that persists also at smaller separations ( R < 100 kpc). A two-dimensional cross-correlation analysis confirms significant clustering of LAEs around SBLAs, while no such clustering is seen for spectral regions with F > 0.25. The correlation appears to also depend on the width of the spectral window used to identify SBLAs, with a larger window yielding a stronger signal. Our analysis confirms that SBLAs serve as probes of the circumgalactic medium (CGM) at the interface between the Ly α forest and the optically thick Lyman limit systems. The significant dependence of the LAE-SBLA cross-correlation on the spectral binning used to select these absorbers motivates future tests of the current SBLA framework as a tracer of halos.

We present the first-year results of the MUSE Large Program “Cosmic Duets”, whose goal is to obtain adaptive-optics assisted MUSE observations with a typical angular resolution of 0.1″ − 0.2″ in order to provide integral-field spectroscopy of sub-arcsec separation dual and lensed active galactic nucleus (AGN) candidates. These observations reveal previously unexplored properties of dual and lensed systems that are key to understanding galaxy evolution, supermassive black hole mergers, and strong-lensing modeling. Targets were efficiently selected using the Gaia multipeak (GMP) technique, which identifies pairs of point-like sources with angular separations below 0.8″ in the Gaia catalog. MUSE spatially resolved spectroscopy provides accurate redshifts, ionization diagnostics, and identification of absorption systems along the line of sight. We report results for 30 GMP-selected targets at z = 0.5 − 3.5. All systems show at least two spatially resolved components. Nineteen objects are confirmed as AGN multiplets, including six dual AGN, ten doubly lensed quasars, and three quadruply lensed systems, while the remaining 11 correspond to chance alignments with foreground stars. Among all the spectroscopically confirmed dual AGN in the literature, 27 pairs have projected separations below 7 kpc in this redshift regime, and our sample accounts for 22% of the total. We studied dual and lensed AGN distributions as a function of redshift, magnitude, and projected separation while accounting for selection effects, and we find that bright systems ( J ≲ 16.5) are dominated by lensed quasars, whereas the relative fraction of dual AGN increases at fainter magnitudes. This first-year sample demonstrates the high efficiency of GMP preselection combined with MUSE spectroscopy for identifying sub-arcsec dual and lensed AGN. The full program, targeting ∼150 systems, will enable statistical studies of dual AGN incidence as a function of redshift, separation, and luminosity and allow for precise characterization of the mass composition and distribution in strong lensing galaxies.