Miroslava Dessauges-Zavadsky

We present a spatially resolved analysis of stellar populations in the brightest z > 6 galaxy known to date (AB mag 23), the strongly lensed MACS0308−zD1 (dubbed the “Cosmic Spear”) at z _spec = 6.2. New JWST NIRCam imaging and high-resolution NIRSpec integral field unit (IFU) spectroscopy span the rest-frame ultraviolet to optical. The NIRCam imaging reveals bright star-forming clumps and a tail consisting of three distinct, extremely compact star clusters that are multiply imaged by gravitational lensing. The star clusters have delensed effective radii of R _eff ≲ 8 pc, stellar masses of M _* ∼ 10 ⁶ –10 ⁷ M _⊙ , and high stellar mass surface densities of Σ _* ≳ 2 × 10 ⁴ M _⊙ pc ⁻² . While their stellar populations are very young (∼6–11 Myr), their dynamical ages exceed unity, consistent with the clusters being gravitationally bound systems. Placing the star clusters in the size versus stellar mass density plane, we find they occupy a region similar to other high-redshift star clusters within galaxies observed recently with JWST, being significantly more massive and denser than local star clusters. Spatially resolved analysis of the brightest clump reveals a compact, intensely star-forming core. The ionizing photon production efficiency ( ξ _ion ) is slightly suppressed in this central region, potentially indicating a locally elevated Lyman continuum escape fraction facilitated by feedback-driven channels.

We report M1149-BSG-z5, a barred spiral galaxy at z = 5.102, identified in the parallel field of MACS J1149+2223 with JWST and the Hubble Space Telescope. M1149-BSG-z5 is the highest-redshift barred galaxy candidate to date. Both isophote ellipse fitting and structural modeling support a stellar bar of length ≈ 4.5 kpc, and extended spiral arms peaking at r ≈ 5.5 kpc. M1149-BSG-z5 is a massive star-forming main-sequence galaxy, with a stellar mass of 10 ^10.45 M _⊙ and a star formation rate of 144 M _⊙ yr ⁻¹ . A concentrated bulge is embedded in an extended disk with a global Sérsic index n = 2.37. With an effective radius of R _e = 2.61 kpc measured from a stacked F277W + F356W + F444W image, M1149-BSG-z5 is larger than typical galaxies at z ∼ 5 and comparable to barred galaxies at 2 < z < 4. M1149-BSG-z5 also hosts a broad-line active galactic nucleus, with a relatively low black hole-to-stellar mass ratio of M _BH / M _* ∼ 10 ⁻³ . Its metal-enriched emission-line properties indicate that it is already chemically evolved. Taken together, these properties imply that M1149-BSG-z5 assembled early and is already structurally evolved. We also find that M1149-BSG-z5 resides in an overdense region with a nearby companion galaxy, suggesting interaction-driven bar formation. Its concentrated light distribution, early assembly, and location on the star-forming main sequence also suggest baryon-dominated, gas-rich conditions, where gravitational instability may further accelerate bar formation.

We present the discovery of a strongly lensed galaxy at z ∼ 11–12, dubbed the “Misty Moons,” identified in the JWST Treasury Survey, Vast Exploration for Nascent, Unexplored Sources. The Misty Moons is gravitationally lensed by the galaxy cluster MACS J0257.1-2325 at z = 0.505, and has five multiple images suggested by two independent lens models. Two of the five images, ID1 and ID2 ( μ ∼ 20–30), are very bright (F200W∼26 AB mag) and exhibit blue SEDs with prominent Ly α breaks. In the source plane, the Misty Moons is a sub- L ^* galaxy ( M _UV ∼ −18.0 mag) resolved into multiple stellar clumps, each of which has an effective radius of r _eff ∼ 10–70 pc and a stellar mass of ∼10 ⁷ M _⊙ . These clumps dominate the stellar mass budget of the Misty Moons (≳80%), similar to other high- z clumps, which suggests a highly clustered mode of star formation in the early Universe, unlike seen in local dwarf galaxies. We convolve the source-plane image with the JWST/NIRCam point-spread function to produce a mock NIRCam image of the Misty Moons without lensing magnification and find that the intrinsic galaxy has a radial surface-brightness profile comparable to those of z ≳ 10 faint galaxies, such as JADES-GS-z13-0 and JADES-GS-z14-1, indicating that the Misty Moons represents a typical z ≳ 10 faint galaxy. The Misty Moons, a lensed galaxy with resolved internal structures, provides an ideal laboratory for exploring the early stages of galaxy formation at z ≳ 10.

We present the first sample of 218 high-redshift ( z > 0.5) star clusters, detected with JWST/NIRCam in 78 magnified galaxies from different galaxy cluster fields. The majority of the systems (~60%) are observed in the very deep NIRCam observations of the cluster AbellS1063 (GLIMPSE program), showing the power that deep observations, combined with lensing, has to reveal these primordial stellar structures. We performed simultaneous size–flux estimates in all available NIRCam filters and spectral energy distribution (SED) fitting analysis to recover star-cluster physical properties. All star-cluster candidates have very high magnification (µ>10). Star clusters and clumps show similar ages and redshift distributions, although noticeable differences are seen in their masses, sizes and stellar surface densities inherent to the lack of resolution in the latter group. We reconstructed the formation redshift of star clusters and find that the vast majority of the observed star clusters show young ages (z < 4). A small sample of CN star clusters are about 1 Gyr old; these potential globular clusters formed well within cosmic reionization. Star clusters have stellar densities in the range of 10 ² to 10 ⁶ M _⊙ /pc ² , with median values around 10 ⁴ M _⊙ /pc ² . Their sizes and densities better overlap with those of nuclear star clusters in the local Universe. These intrinsic properties make high-z star clusters a viable channel to grow intermediate-mass black holes. We used Bayesian inference to make the first direct measurement of the starcluster mass function at z > 1 based on a subsample of 60 star clusters younger than 100 Myr and with masses above 2 × 10 ⁶ M _⊙ . The star cluster mass function is well described by a power law with a slope of β _50% = −1.89 _−0.12 ^+0.13 , suggesting that a power-law -2 function might already be in place in the distant Universe.

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 present ultra-deep (≃20 − 30 hours) rest-frame UV spectroscopy with NIRSpec of two UV-bright galaxies at z ∼ 8.7, CEERS-1019 and CEERS-1025 ( Z _neb ≃ 0.1 Z _⊙ ), obtained as part of the JWST Cycle 4 SPURS large program. The spectra reveal exceptionally strong P Cygni profiles in the wind lines (N V λ 1240 and C IV λ 1550) and significant broad and strong He II λ 1640 emission (EW ≃ 2 − 4 Å). We compared the observations with synthetic stellar population models at Z _★ ≃ 0.1 Z _⊙ , with and without very massive stars (VMSs). Models with VMSs provide a markedly improved fit to the data relative to non-VMS models (ΔAIC and ΔBIC > 70), which fail to reproduce the observed strengths of the wind features. A comparison with empirical spectra of VMS-dominated systems in the local Universe further supports this interpretation. The best-fit VMS models imply extremely young ages of the stellar populations (≃1.5–2.0 Myr) and highly ionizing photon production efficiencies (log ξ _ion [Hz erg ⁻¹ ] ≳ 25.8) that exceed those inferred from non-VMS models by ∼0.1–0.2 dex. These results provide evidence of an overabundance of VMSs at high- z with an initial mass function extending well beyond 100 M _⊙ , and they highlight the potential role of VMSs in shaping the rest-frame UV spectra, chemical enrichment, and ionizing output of galaxies in the early Universe.

Context. Recent James Webb Space Telescope (JWST) observations have revealed a small number of galaxies with UV spectra exhibiting intense nitrogen emission lines, which are not typically detected in galaxy spectra. The observations indicate super-solar N/O abundances at low metallicity, a phenomenon whose nature is currently intensely debated. Aims. To better understand these enigmatic objects and provide new constraints on proposed scenarios, we did a systematic search for galaxies with UV nitrogen emission lines (N IV ] λ 1486 and/or N III ] λ 1750). Methods. Using public JWST NIRSpec data, we identified 45 N-emitters with robust N III ] or N IV ] detections, including four previously known objects. We then classified these objects, determined relative C, N, O, and H chemical abundance ratios, determined other properties, and carried out a statistical analysis of the N-emitter population. Results. We find N-emitters at redshifts z ∼ 3 − 11 spanning a broad diversity of galaxies in terms of morphology, UV magnitude, stellar mass, star formation rate, metallicity, and rest-optical emission line strengths. The UV nitrogen lines show typical equivalent widths (EW) between ∼5 Å and up to ∼100 Å in a few cases. Diverse ionisation conditions, as traced by N IV ] λ 1486/N III ] λ 1750, are observed. The carbon lines (C IV λ 1550 and C III ] λ 1909) are generally fainter than the N lines. Using strong line calibrations established at high-redshift, we find metallicities 12 + log(O/H)∼7.15 − 8.5, spanning up to high metallicity values. The H β EW of N-emitters varies strongly, and sources with low EWs show clear signs of a Balmer break, indicative of composite stellar populations combining both young (≲10 Myr) stars responsible for the UV emission lines and an older population contributing to the rest-optical spectrum. Super-solar N/O ratios are found in all N-emitters. C/O abundances are comparable to those of galaxies at the same metallicity, and all N-emitters show either high N/C ratios or lower limits (log(N/C) ≳ 0.5), regardless of metallicity. The observed abundance ratios are compatible with ejecta from H-burning and show no signs of carbon enhancements, even at higher metallicities. Finally, we find that the fraction of N-emitters increases with redshift, and we quantify this evolution. Conclusions. Our study increases the sample of known N-emitters by a factor of ∼3, reveals a diversity of properties among N-emitters, and provides new constraints on their nature.

We present gas-phase radial metallicity profiles for 20 main-sequence galaxies at 4 < z < 6 primarily based on JWST NIRSpec IFU observations obtained as part of the JWST-ALPINE-CRISTAL programme. Our study aims to connect the metallicity gradients of these galaxies with their kinematic properties from [C II ] 158 μm ALMA observations. We mapped the radial profiles of oxygen abundance using the strong-line method, leveraging the rich set of rest-frame optical emission lines. Linear fits to the annular-binned radial profiles show that, on average, the metallicity gradients are slightly positive with a median of +0.039 ± 0.010 dex kpc ⁻¹ . However, only three galaxies show a gradient > 0.05 dex kpc ⁻¹ at 1 σ , and none have a significant negative gradient. There are no substantial systematic offsets in gradients when using different line diagnostics. We investigated the correlation between the metallicity gradients and the intrinsic gas velocity dispersion σ ₀ as well as the V _rot / σ ₀ ratio of the disks. Combining our sample with mass-matched literature samples at 3 ≲ z ≲ 7, we found a negative shallow correlation between V _rot / σ ₀ and the metallicity gradients, but no strong relationships with σ ₀ . As V _rot / σ ₀ increases towards later cosmic times, the observed negative trend with V _rot / σ ₀ is consistent with the overall cosmic evolution of metallicity gradients from high to low redshifts. This suggests that disk maturity plays a crucial role in shaping the radial metallicity gradients. We do not find the metallicity gradients of disk galaxies to be significantly different from non-disk galaxies, which could be attributed to the frequent accretion events that took place in these gas-rich systems. Additionally, we find no strong dependence of metallicity gradients on stellar mass and only a marginal positive dependence on specific star-formation rate. Our study extends the efforts to connect the internal kinematics of galaxies with their gas-phase chemical enrichment at kiloparsec scales from cosmic noon to z > 4.

We report the discovery of a doubly imaged little red dot (LRD) candidate behind the galaxy cluster Abell 383, which we dub A383-LRD1. Initially classified as a dropout galaxy in HST imaging with several ground-based emission line detections placing it at z _spec = 6.027, new JWST/NIRCam observations taken as part of the cycle 4 VENUS survey now reveal that the source consists of two underlying components. These consist of a red point-source with a V-shaped SED consistent with LRD selection criteria and a nearby (∼380 pc) compact blue companion, which was the main contributor to the previous rest-frame UV detections. Based on lensing symmetry and its SED, the LRD appears to lie at a similar redshift as well. The magnification of the two images of A383-LRD1 is μ _A = 16.2 ± 1.2 and μ _B = 9.0 ± 0.6, respectively, and the predicted time delay between them is Δ t _grav = 5.20 ± 0.14 yr (∼0.7 yr in the rest-frame). After correcting for the lensing magnification, we derived an absolute magnitude of M _UV, LRD = −16.8 ± 0.3 for the LRD and M _UV, BC = −18.2 ± 0.2 for the blue companion. We performed SED fits to both components, revealing the LRD to show the best fit with a black hole star (BH*) model and a substantial host galaxy, along with the blue companion with an extremely young, emission-line dominated star-forming nebula. If it is verified spectroscopically in the future, A383-LRD1 would represent the second known multiply imaged LRD detected to date, following A2744-QSO1. To our knowledge, it would also stand as the first LRD system with a confirmed detection of [C II ] λ 158 μm emission from ALMA observations. Thanks to the lensing magnification, this system opens a unique door to studying the relation between a LRD, its host galaxy, and its environment. It represents a prime candidate for deep JWST spectroscopy and high-resolution ALMA follow-up observations.