Alessandro Pizzella

Context. Ultra-diffuse galaxies (UDGs) lie at the extreme end of the size–luminosity distribution of low-surface-brightness (LSB) galaxies. Their detection and characterization require deep imaging and reliable source detection techniques that can handle low signal-to-noise ratios and severe source blending. Aims. We aim at improving the detection and characterization of LSB galaxies and UDG candidates in different environments. To this end, we developed a new automated detection PYTHON based tool, named ATTILA. Methods. We used deep g - and r -band imaging from the VST Early-type GAlaxy Survey (VEGAS), covering the central region of Hydra I and three new additional fields. Sources were identified by combining tiling processing, source detection, and iterative deblending. Structural parameters were derived through surface brightness profile analysis and Sérsic modeling. Cluster membership was determined using the early-type galaxies color-magnitude relation. Results. We identify 24 new UDGs, doubling the known population in the Hydra I cluster to 48, consistent with expectations from halo mass scaling relations, and 92 additional LSB galaxies. In real data, ATTILA recovers more than 80% of previously known LSB galaxies and significantly improves the automated detection rate relative to standard methods. Conclusions. By improving the recovery of faint and diffuse sources while mitigating blending and contamination effects, ATTILA enables a more complete census of the LSB galaxy population, including UDGs. This is crucial for properly sampling the low-mass end of the galaxy population and for investigating the apparent tension between the observational downsizing scenario and the hierarchical assembly predicted by the Λ cold dark matter (ΛCDM) paradigm. In this context, ATTILA is particularly well suited for current and forthcoming large datasets, as efficiently covers wide areas with the depth required to unveil the LSB Universe.

We present the ground-based imaging campaign and light curves of Markarian 817 as part of the multiwavelength monitoring program AGN STORM 2. Observations were carried out over 1.4 yr in the uBgVriz filters, with a median cadence of 0.4 day in the g band. Reverberation lags are measured using three methods (interpolated cross-correlation function (ICCF), Just Another Vehicle for Estimating Lags In Nuclei, and PyROA ) with the Swift UVW2 band (1928 Å) as the reference light curve. The ICCF centroid lags range from 3.0 ± 0.8 days for the u band up to 7.9 ± 1.5 days for z , and are consistent with a τ ∝ λ ^4/3 dependence, the relation expected for lamppost reprocessing by a Shakura–Sunyaev disk. Lags measured with the other methods are systematically shorter, and deviate from a λ ^4/3 power-law spectrum at long wavelengths. The lags exceed thin-disk reprocessing predictions by factors of ∼3–6, similar to the “disk size discrepancy” seen in other Seyfert galaxies. We divide the campaign into three epochs with different levels of mean luminosity and X-ray obscuring column density and find that the lags vary by as much as a factor of 2 between epochs. The intrinsic spectral energy distribution is bluer and brighter during the first third of the campaign, and the longest continuum reverberation lags are obtained during that period. These results suggest that changes in ionizing luminosity can produce large variations in continuum lags on short timescales by altering the diffuse continuum luminosity emitted by the broad-line region (BLR) and/or obscuring outflow, although changes in obscuration between the central engine and BLR may also contribute to the lag variations.

Aims. The existence of massive disc galaxies with little or no bulge challenges conventional Λ cold dark matter model, which typically favours dynamically hot central structures due to early collapse and mergers. The study of these bulgeless disc galaxies is the aim of the Bulgeless Evolution And the Rise of Discs (BEARD) survey, as they offer a unique opportunity to investigate the link between galaxy morphology and the properties of their satellite systems. Methods. Using the high-resolution cosmological hydrodynamical simulation TNG50-1, we studied the satellite populations of 135 bulgeless galaxies. We compared their satellite properties to those of a bulge-dominated control sample with matched stellar masses. Our analysis focuses on satellite abundance, luminosity functions, spatial distribution, orbital alignment, and infall histories. Results. We find that satellite abundance is largely independent of host galaxy morphology. However, satellites around bulgeless galaxies exhibit luminosity functions with a steeper faint-end slope, are more centrally concentrated, and show stronger orbital alignment with the host disc plane. The orbital alignment originates from coherent post-infall dynamical evolution that depends on host galaxy morphology. The infall of more massive satellites can additionally perturb this process, contributing to a weakening or temporary stalling of the secular alignment. Conclusions. Due to the co-evolution of the host galaxy and the satellite system, the morphology of the central galaxy leaves a clear imprint on its satellite system. Bulgeless galaxies tend to have dynamically colder, more aligned, and more centrally concentrated satellite populations. These trends reflect a more quiet merger history and support the use of satellite properties as tracers of host galaxy formation pathways.

We study the formation and evolution of bulgeless galaxies within the Milky Way-Andromeda analogue sample of the TNG50 simulation. Through kinematic decomposition with M ORDOR , we identified bulgeless galaxies with a bulge-to-disc mass ratio of B / D ≤ 0.08, in line with the Bulgeless Evolution And the Rise of Discs (BEARD) survey and Milky Way constraints. We compared the identified bulgeless galaxies to those that are bulge-dominated, which have a bulge-to-disc mass ratio of B / D > 1. We find that 74% of bulgeless galaxies experience at least one major merger (stellar mass ratio 1:4) throughout their lifespan. Bulgeless galaxies form later ( z ₅₀ ∼ 0.7) compared to bulge-dominated counterparts ( z ₅₀ ∼ 1.2). Bulgeless galaxies have lower-mass haloes and higher specific stellar angular momentum, which is compatible with Milky Way observations. However, specific star formation rates and hydrogen gas fractions are slightly higher than Milky Way observations. Our analysis of the redshift evolution of stellar components reveals that bulgeless galaxies have gradual disc growth with high thin disc-to-total mass ratios ( D / T > 0.5) since z ∼ 1 and minimal bulge growth ( B / T < 0.1) since z ∼ 1.5. In contrast, bulge-dominated galaxies have earlier disc formation, which is disrupted, resulting in higher morphology evolution. Bulgeless galaxies are more likely to undergo gas-rich, coplanar, and corotating mergers, promoting disc survival, compared to bulge-dominated galaxies that encounter a broader spectrum of mergers. We also observed differences in galaxy structure between bulgeless and bulge-dominated galaxies without major mergers, suggesting the relevance of early gas accretion and alignment. Bulgeless galaxies have younger stellar populations and more extended star formation histories than bulge-dominated galaxies, which rapidly quench and have older stellar populations. These findings elucidate the distinct merger-driven and secular pathways that give rise to Milky Way galaxies.

In the standard Λ cold dark matter (ΛCDM) cosmology, galaxies grow through smooth accretion and hierarchical mergers. While this framework explains many large-scale structures, the existence of massive disc galaxies without prominent bulges-pure discs-remains a challenge. In this work, we investigate the physical origin of the scatter in the stellar mass–size relation of massive spiral galaxies, with a particular focus on bulgeless systems. Studying these systems is also key to understanding the evolutionary history of our own Galaxy, the Milky Way, which is known to host a low-mass bulge. We performed a structural analysis of 22 nearby bulgeless galaxies from the Bulgeless Evolution And the Rise of Discs (BEARD) survey. To minimise the scatter in the stellar mass–size relation, we adopted a proxy for the physically motivated definition for the galaxy size, based on the radius R ₁ , where the stellar mass surface density reaches Σ _* = 1 M _⊙ pc ⁻² . For this purpose, we used deep g - and r -band imaging obtained with the 2.5 m Isaac Newton Telescope-Wide Field Camera. We derived surface brightness, colour, and stellar mass density radial profiles, which allowed us to obtain precise measurements of R ₁ . Point spread function (PSF) effects were corrected through star subtraction and wavelet deconvolution. BEARD bulgeless galaxies follow the tight stellar mass– R ₁ relation defined in previous studies with a similar scatter (∼0.1 dex). We also constructed the same relation using galaxies from the IllustrisTNG50 simulation. We find a morphological segregation contributing to the scatter of the relation, with bulgeless (BEARD-like analogues) and bulge-dominated galaxies defining the upper and lower envelope, respectively. We find that this morphological trend shown by the simulations is strongly correlated with the specific central stellar mass density, Σ ^spec _1,kpc , defined as the stellar mass surface density enclosed within the central kiloparsec, normalised using the total galaxy mass. The observed discrepancy between observations and simulations can be attributed to the broader Σ ^spec _1,kpc distribution covered by our observed BEARD bulgeless galaxies. A deeper analysis of the physical driver of this morphological segregation reveals that the scatter in the mass–size relation is also related to the spatial configuration of merger events, rather than their frequency, with bulgeless systems tending to inhabit halos with a slightly higher spin.

Context. The new generation of optical spectrographs, including WEAVE, 4MOST, DESI, and WST, offers huge multiplexing capabilities and excellent spectral resolution. This is an unprecedented opportunity to statistically unveil the details of the star formation histories (SFHs) of galaxies. However, these observations are not easily comparable with the predictions of cosmological simulations. Aims. Our goal is to build a reference framework for comparing spectroscopic observations with cosmological simulations and to test the currently available tools for deriving the stellar population properties of mock galaxies as well as their SFHs. We focus on the observational strategy of the Stellar Population at intermediate redshift Survey (StePS) carried out with the WEAVE instrument. Methods. We created mock datasets of ∼750 galaxies at redshifts of z = 0.3, 0.5, and 0.7 from the TNG50 cosmological simulation. We performed radiative transfer calculations using SKIRT and analyzed the spectra with the pPXF algorithm, treating them as if they were real observations. Results. This work presents the methodology used to generate the mock datasets, providing an initial exploration of stellar population properties (i.e. mass-weighted ages and metallicities) and SFHs of a test sample of three galaxies at z = 0.7 and their descendants at z = 0.5 and 0.3. We show that there is very good agreement between mock WEAVE-like spectra compared to the intrinsic values in TNG50 (average difference of 0.2 ± 0.3 Gyr). We also report that an overall agreement is seen when retrieving the SFHs of galaxies, especially if they form the bulk of their stars on short timescales and at early epochs. While we did identify a tendency to overestimate the weight of old stellar populations in galaxies with complex SFHs, we were able to properly recover the timescales on which galaxies build up 90% of their mass, with almost no difference in the measured and intrinsic cumulative SFHs over the last 4 Gyr. Conclusions. We have released the datasets concurrently with this publication of this paper, which consist of multi-wavelength imaging and spectroscopic data of ∼750 galaxies at redshift z = 0.3, 0.5, and 0.7. This work provides a fundamental bench-test for forthcoming WEAVE observations, providing the community with realistic mock spectra of galaxies that can be used to test currently available tools for deriving first-order stellar populations parameters (i.e. ages and metallicities) as well as more complex diagnostics, such as mass and SFHs.

This paper examines the reliability of the Tremaine-Weinberg (TW) method in measuring the pattern speed of barred galaxies at high redshifts. Measuring pattern speeds at high redshift may help to shed light on the time evolution of interactions between galactic bars and dark matter halos. The TW method has been extensively employed for nearby galaxies, and its accuracy in determining bar pattern speeds has been validated through numerical simulations. For nearby galaxies, the method yields acceptable results when the inclination angle of the galaxy and the position angle of the bar fall within appropriate ranges. However, the application of the TW method to high-redshift galaxies remains unexplored in both observations and simulations. For this study we generated mock observations of barred galaxies from the TNG50 cosmological simulation. These simulated observations were tailored to mimic the integral field unit (IFU) spectroscopy data that the Near-Infrared Spectrograph (NIRSpec) on the James Webb Space Telescope (JWST) would capture at a redshift of ɀ ≃ 1.2. By applying the TW method to these mock observations and comparing the results with the known pattern speeds, we demonstrate that the TW method performs adequately for barred galaxies whose bars are sufficiently long to be detected by JWST at high redshifts. This work opens a new avenue for applying the TW method to investigate the properties of high-redshift barred galaxies.

Context. In the near-infrared (NIR) region, a large number of absorption features may be used to constrain the properties of unresolved stellar populations of galaxies. Aims. We present the results of a pilot project assessing the feasibility of measuring the distribution and radial trends of NIR line-strength indices on spatially resolved spectra. We used the ESO/KMOS multi-object spectrograph to observe the massive, nearby elliptical galaxy IC 4296. Methods. We analysed the distribution of the NIR line-strength indices that are most sensitive to the age and metallicity of the galaxy stellar populations both on 2D maps and on stacked spectra of five radial bins. This allowed us to build the radial profiles of the line-strength indices out to 0.80 R_e and compare them to those measured in the optical range. Results. For the particular case of IC 4296, we find that the metal indices have negative radial trends. The results are in agreement with literature radial profiles obtained from the analysis of the optical Lick/IDS indices and favour in situ galaxy formation over multiple merging events. Conclusions. The obtained results demonstrate the usefulness of the NIR wavelength domain to unveil the processes of galaxy formation and evolution.