Luca Costantin

We present an analysis on the kinematics and physical properties of the ionised gas in the lensed galaxy RXCJ2248-ID at z = 6.105 based on high-resolution JWST NIRSpec/IFU data. This source is a nitrogen-rich high- z galaxy showing a compact morphological structure and high star formation and stellar surface densities. The magnification factor of the observed image of the source (ID3, μ = 5.3) along with the rich dataset available, including NIRSpec and ALMA spectroscopy, makes this target an ideal benchmark to study the physical properties of nitrogen-rich galaxies. In this work, we use the large number of optical lines detected at a high signal-to-noise (S/N) in our NIRSpec high-resolution ( R = 2700) integral field spectra to analyse potential differences in kinematics, dust distribution, and electron temperature and density. In addition, we combine our data with available ALMA observations to derive the optical-to-far-IR [O III ] λ 5008/[O III ] 88 μm ratio in order to estimate the electron density of the O ⁺⁺ regions. Our analysis revealed a high electron temperature ( T _e ∼ 30 000 K) in the ionised gas independent of the ionisation level. We measured a wide range in the electron densities of the ionised interstelllar medium (ISM) from our [O III ] λ 5008/[O III ] 88 μm and [Ar IV ] λ 4713/[Ar IV ] λ 4742 ratios (log( n _e [cm ⁻³ ]) ∼ 2.7−3.8) and previous values (i.e. 4.8 < log( n _e [cm ⁻³ ]) < 5.5) based on high-ionisation rest-UV emission lines. The ionised gas appears to be clumpy with a low filling factor ranging from about 10% to 0.2% for the low- and high-density clouds. In addition, we observed a very complex ISM kinematic structure traced by the [O III ] λ 5008 and H α lines. Concretely, we derived the presence of two distinct broad and very broad components (FWHM ∼ 210−250 and ∼1000−1500 km s ⁻¹ ) in addition to the systemic component (FWHM ∼ 60−70 km s ⁻¹ ). These broad components are heavily extinct ( A _V ∼ 1.5 and 2.5, respectively), based on their Balmer decrements, while the gas associated with the narrow component is consistent with no extinction. The maximal velocities of these outflows (∼115−500 km s ⁻¹ ) are such that a fraction of the total outflowing gas (0.16−2.1 × 10 ⁷ M _⊙ ) could escape into the intergalactic medium. The rest of the gas will fall back to the central regions, remaining available for additional star formation episodes. The presence of dusty outflows and a clumpy (i.e. low filling factor) ISM give support to the attenuation-free scenario proposed to explain the high- z UV-bright compact galaxies such as RXCJ2248-ID. On the other hand, the high densities in the ISM together with the high sta formation rate surface brightness and the amount of returning outflowing mass give support to the feedback-free starburst scenario.

Context . Collisional ring galaxies are a rare class of interacting systems, comprising only ∼0.01% of galaxies in the local Universe. Their formation is typically attributed to a head-on collision of a massive galaxy with a compact satellite (intruder), triggering density waves propagating outwards that produce the characteristic ring morphology. Here, we report the discovery and detailed analysis of GS18660, the most distant ring galaxy known to date, at z = 3.076, based on JWST data obtained within of the GA-NIFS programme. Aims . This work aims to characterise the physical and dynamical properties of GS18660 and shed light on the formation of its ring. Specifically, we analysed the ionised gas properties, stellar populations, and gas kinematics of the system, and used the observed geometry to constrain the timescale of the collision. Methods . Our analysis is based on NIRSpec integral field spectrograph (IFS) data, including low-resolution ( R ∼ 100) spectroscopy covering 0.6–5.3 μ m (∼0.2–1.3 μ m rest-frame), and high-resolution ( R ∼ 2700) spectroscopy covering 1.66–3.17 μ m (0.4–0.8 μ m rest-frame). Multi-wavelength techniques (e.g. emission line diagnostics, full spectral fitting, and gas dynamics) were applied to derive nebular gas conditions and stellar population properties. Results . Gas kinematic analysis reveals that GS18660 exhibits a rotating disc component with an additional radial expansion velocity of ∼200 km s ⁻¹ , consistent with a propagating collisional wave. Nebular line diagnostics indicate an intense star formation (star formation rate SFR ∼100 M _⊙ yr ⁻¹ ) along the ring and in the nucleus. Stellar population analysis shows that the most recent star formation episode, occurring within the last ≈50 Myr, predominantly took place in the ring. We also identify a close companion, the intruder galaxy responsible for the collision, moving away with a relative velocity of ∼425 km s ⁻¹ . Conclusions . The evidence favours a collisional origin for the ring in GS18660, though the presence of a recently formed bar (and hence a resonance ring) cannot be completely excluded. This discovery provides a unique window to the physics of galaxy interactions and the rapid assembly of stellar structures in the early Universe.

The formation of stellar bars is an important milestone in the secular evolution of spiral galaxies, which typically indicates the presence of a massive rotationally supported disk. Determining when these structures first appeared in the early Universe is crucial to constraining the timeline of Galactic disk assembly. Here, we report the discovery of COSMOS-74706, a barred spiral galaxy at z _spec = 3.159. Imaging of COSMOS-74706 with JWST/NIRCam indicates a disk-like morphology and spiral structure with an elongated central feature aligned between the spiral arms, most conspicuously visible in the F200W, F277W, and F356W filters. Three independent methods all support the presence of a bar: visual inspection of residuals from Sérsic profile fitting shows a linear structure, isophotal ellipse-fitting displays characteristic profiles of ellipticity and position angle consistent with a bar signature, and Fourier decomposition of the galaxy produces a central bisymmetric mode above a threshold strength calibrated to z = 1−3 barred spirals. Leveraging archival Keck/MOSDEF spectroscopy overlapping with a blue clump on the edge of the galaxy, a robust redshift is inferred, with photometric constraints indicating that this structure lies at the same redshift as the main spiral. This spectroscopic evidence placing an unlensed barred spiral at z > 3 supports the idea that galaxies with rotationally supported disks and disk-halo properties that are conducive to bar formation were already in place within 2 Gyr after the Big Bang.

The classical picture for the formation of stellar bars—key dynamical drivers of the evolution of galaxies—is through secular evolution of instability in gas-poor, stellar-dominated disks. The detection with the James Webb Space Telescope of stellar bars and spiral arms in galaxies at early cosmic times has thus challenged ΛCDM-based expectations, which recent studies reconcile by suggesting that these galaxies are baryon-dominated and have already consumed most of their gas. Yet, a paradox arises, as early galaxies are expected to be increasingly rich in gas, which is generally considered to prevent or slow down stellar bar formation. Here, we show the detection of a stellar bar in GN20, a gas-rich star-forming disk galaxy at a redshift of z = 4.055, only 1.5 Gyr after the Big Bang. Simultaneous observations of the stars, gas, and dust reveal that GN20 is indeed baryon-dominated (over dark matter; 72% ± 34%), but the baryonic mass is largely in the form of gas (74% ± 25%). This discovery demonstrates that gas-rich disks do support rapid stellar bar formation in the early Universe, motivating a new theoretical perspective on bar formation in gas-rich systems, and providing a potential new mechanism for very early galaxy assembly and quenching.

We present a study of 27 very UV-bright (−22.0 ≲ M _UV ≲ −24.4) star-forming galaxies at z ∼ 6 identified in the Subaru High- z Exploration of Low-Luminosity Quasars (SHELLQs) survey. Stacking their rest-frame UV spectra revealed a prominent N V λ 1240 P Cygni feature, consistent with very young (∼6 Myr) stellar populations dominated by massive and hot stars. Ultraviolet-bright galaxies in the reionization epoch are thus powerful and efficient ionizing sources, with an average ionizing photon production efficiency of log( ξ _ion /Hz erg ⁻¹ ) = 25.54 ^+0.09 _−0.12 . For one representative source, J0217–0208 at z = 6.204 ( M _UV = −23.4), we analyzed available JWST/NIRCam and NIRSpec observations. Its spectral energy distribution indicates a young (∼5 Myr) starburst with a stellar mass of 10 ⁹ M _⊙ and a high specific star formation rate (∼100 Gyr ⁻¹ ). Together with its very compact NIRCam-measured size ( r _eff ≃ 260 pc), this corresponds to stellar mass and star formation rate surface densities approximately one hundred times higher than those of typical galaxies at comparable redshifts. NIRSpec spectroscopy further revealed strong nebular emission, for which we derived a high electron density ( n _e ≃ 10 ³ cm ⁻³ ), a metallicity of 12 + log(O/H) = 8.20 ± 0.11 (from the direct method), and a super-solar N/O ratio (log(N/O) ≃ −0.30). Furthermore, J0217–0208 shows broad components in several rest-optical emission lines, indicating powerful ionized outflows. From the Balmer decrement, these outflows appear to be heavily obscured ( E ( B − V ) _out ≃ 0.6), in contrast to the nearly dust-free stellar continuum ( E ( B − V ) _★ = 0.01 ± 0.01) obtained from its steep UV slope, β _UV ≃ −2.6. Combined with ALMA detections of a massive ( M _dust ≃ 2 × 10 ⁸ M _⊙ ), extended (∼1.4 kpc), and cold ( T _dust ≃ 25 K) dust reservoir, these findings point to dusty feedback-driven outflows carrying and pushing dust well beyond the stellar core and likely boosting the observed UV luminosity. Taken together, our results suggest that UV-bright galaxies at high redshift represent short-lived but extreme phases of rapid stellar mass growth, efficient ionizing photon production, and strong feedback. The extreme properties of J0217–0208, such as a supersolar N/O, a steep UV slope, a compact size, and very high surface densities, closely mirror those of the brightest galaxies at z > 10, suggesting a shared evolutionary pathway.

We report the detection of a clumpy, blue-dominated Lyman- α (Ly α ) emission at z = 3.066 located in the heart of a cosmic web filament in the MUSE eXtremely Deep Field (MXDF). It is spatially associated with the formation of two compact star-forming regions revealed by deep JWST/NIRCam imaging. Gas accretion in these regions is indicated by the blue-dominated Ly α profiles, spectral signatures that are rarely observed. Radiative transfer simulations of the Ly α profile using a clumpy multiphase model suggest a radial inflow of gas clumps with a velocity of ∼100 km/s. Embedded in this Ly α structure, the associated main galaxy dominates the stellar mass budget, while the two compact ultra-low-mass systems (log( M _★ / M _⊙ ) = 6.3 − 6.9) have formed the bulk of their stellar mass in less than 7 Myr. These two components also have high specific star-formation rates and elevated ionisation parameters, consistent with recent bursty star formation. This system provides compelling observational evidence of how gas accretion, most likely from the cosmic web, can induce starbursts in ultra-low-mass galaxies.

We present a detailed analysis of a massive barred galaxy at z _spec = 3.1591 using deep multiband imaging from the Hubble Space Telescope (HST) and JWST. For the first time, we resolve its morphology and stellar structures thanks to the JWST/NIRCam near-infrared (NIR) and mid-infrared (MIR) photometry. The galaxy possesses two distinct components with significantly different colors. Through careful image decomposition and masking, we isolate and characterize the flux contribution from each component. The galaxy exhibits a clear spiral morphology, and in a separate companion paper, we present evidence suggesting the presence of a stellar bar. Based on spatially resolved spectral energy distribution modeling with Prospector , we derive the star formation histories and other physical properties of the bar and the surrounding regions. The total stellar mass of the galaxy is constrained as log ( M * / M ⊙ ) = 10.63 ± 0.13 . We find that the bar region contains around 30% of the total stellar mass, but only accounts for around 8% of the recent star formation rate. The region containing the potential bar shows a significantly older mass-weighted stellar age, supporting the inside-out scenario for galaxy formation, and providing tentative evidence for bar quenching in the early stage. The quick onset of a stellar bar at this redshift requires a low dark matter fraction, suggesting the baryon-dominated nature of high- z massive galaxies, and offering rare insight into galaxy evolution at around 2 billion yr after the Big Bang.

Aims. We present an investigation of the evolutionary pathways of green valley (GV) galaxies, defined as galaxies with a distance to the star-forming main sequence in the range −1.3 < Δ _SFMS < −0.5, drawn from the SDSS-IV/MaNGA survey. Our goal is to examine the connection between the dynamical galaxy structure, specifically, its stellar angular momentum, and its key physical properties, including gas-phase metallicity, star formation history (SFH), and chemical enrichment history (ChEH). By exploring these correlations, we aim to constrain the physical processes that govern the evolution of green valley galaxies. Methods. We divided our sample into fast- and slow-rotating galaxies using a criterion motivated by the bimodal distribution of the stellar spin and compared their integrated stellar and gas-phase metallicities. Additionally, using a simple but comprehensive galaxy chemical evolution model, optimised to fit the gas-phase metallicity estimates and the integrated stellar spectra for each galaxy, we reconstructed the past star formation and chemical enrichment histories of the galaxies. The derived SFHs and ChEHs, along with parameters governing the gas-infall timescale and outflow strengths, offer valuable insights into the physical processes driving the evolution and quenching of our GV sample galaxies. Results. We found that fast-rotating galaxies exhibit systematically higher metallicities in the gas and stellar phases compared to slow-rotating galaxies. However, in the gas phase, the difference is significant only at the low-mass end, while the stellar metallicity offset is observed consistently across the full stellar mass range. Our modelling framework yields simple but physically motivated explanations for these trends. At low stellar masses, the model predicts similar gas-infall and star formation timescales for fast- and slow-rotating galaxies, but the stronger outflows inferred for the slower population substantially reduce their chemical content in the gas and stars. At high masses, slow-rotating galaxies show shorter gas-infall and star formation timescales; the combination of a reduced pristine gas inflow and more efficient gas removal produces gas-phase metallicities that are comparable to those of fast-rotating galaxies, but systematically lower stellar metallicities. Conclusions. The systematic differences in stellar and gas-phase metallicity, as well as in model-inferred gas-accretion and outflow parameters, highlight the contrasting properties of fast- and slow-rotating galaxies in our GV sample. Interpreting these differences within our chemical evolution framework and combining evidence from theoretical studies suggests distinct past evolutions. Slow-rotating galaxies likely experienced more merger events, usually associated with strong gas removal processes, leading to their systematically lower metallicities. At low masses, stronger supernova-driven outflows reduce their chemical content while leaving star formation timescales similar to those of fast-rotating galaxies. At high masses, merger-triggered feedback from active galactic nuclei may rapidly deplete and suppress gas infall, producing the shorter star formation timescales seen in slow-rotating galaxies. Alternative environmental and assembly-driven scenarios are also discussed.

We present measurements of morphological parameters from fitting 53,885 galaxies detected to a magnitude limit of F356W < 28.5 in the CEERS NIRCam imaging with galfit in six broadband filters: F115W, F150W, F200W, F277W, F356W, and F444W. We provide a public catalog of Sérsic indices, effective semimajor axes, axis ratios, integrated magnitudes, and position angles for these galaxies in each of the filters. Uncertainties in the measured parameters are estimated from simulated galaxies that have similar noise and background properties as the observed galaxies. We compare our measurements with those in the CANDELS/Extended Groth Strip field measured with Hubble Space Telescope/WFC3 and find that the sizes agree to within 0.09 dex and the Sérsic indices agree to within 0.13 dex. We further present the evolution in the size–mass relation, and find that the evolution to z ∼ 9 is consistent with previous results derived at lower redshift. Finally, we look at the color gradients of galaxies at 1 < z < 5 and find that for late-type galaxies ( n < 2.5), there is a strong dependence on mass, but no apparent evolution with redshift, indicating that the stellar populations and dust attenuation in more massive galaxies vary substantially with radius and contribute to significant morphological k -corrections. For early-type galaxies ( n > 2.5), the color gradients are nearly flat with no dependence on mass, indicating that the stellar populations are more uniform throughout. The structural measurements presented are accurate to 20% or better for most galaxies with F356W < 27.0 mag and will enable further studies of galaxy morphology to z ∼ 10.

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.