Ghassan T. E. Sarrouh

We present the evolution of the resolved mass-weighted age, stellar metallicity, and specific star formation rate (sSFR) of 872 Milky Way analog (MWA) progenitors up to redshift z = 5 from the Canadian Unbiased Cluster Survey. The metallicity and mass-weighted ages were obtained via spatially resolved spectral energy distribution fitting with the nonparametric code Dense Basis . We split the sample into mergers versus nonmergers using the merger parameter from the Gini– M ₂₀ plane obtained through Gini– M ₂₀ analysis of the morphology of the stellar mass maps with Statmorph . Across our redshift range, nonmergers have negative or flat average age gradients from −0.022 to 0.005 dex kpc ⁻¹ , and positive or flat sSFR gradients from −0.089 to 0.092 dex kpc ⁻¹ , consistent with inside-out assembly. The average log ( Z / Z ⊙ ) gradients for nonmergers range from −0.029 to 0.044 dex kpc ⁻¹ ; however, positive gradients only appear between 2 < z < 3. At every redshift epoch, mergers typically have flatter age gradients, more negative sSFR gradients, and similar metallicity gradients compared to nonmergers. We divide the property maps of ongoing mergers into separate regions based on their component galaxies, and find little to no difference between the components’ average ages or metallicities, but the less massive of the merging system is on average 0.1−0.4 dex higher in sSFR. Our results point to major mergers contributing some momentary disruption to the general trend of inside-out mass assembly, but this does not upend the overall picture of MWA disks growing inside-out over cosmic time.

Context. UV slopes ( β ) are a powerful diagnostic for galaxies at the epoch of reionisation, tracing star formation, interstellar medium ionisation, and the escape fraction f _esc of ionising photons. Studies at low and intermediate z suggested a gradual reddening of the UV slope with time and steeper slopes for fainter galaxies. The James Webb Space Telescope ( JWST ) enabled measurements of the UV slope for statistically significant samples of galaxies deep into the epoch of reionisation up to redshift z ≳ 10 and for increasingly faint galaxies. Recent studies of β with JWST at z > 7 revealed a flattening of the trend of the UV slope with respect to redshift and UV magnitude. Aims. We measure β at z > 7.5 using the strong lensing around massive galaxy clusters to observe high-redshift and faint galaxies. The low-brightness regime is of particular interest for reionisation, as most of the recent models of this process posit that numerous faint galaxies are the prime drivers of reionisation Methods. We used NIRCam and NIRSpec observations from CANUCS, Technicolor, JUMPS, Silver Bullet, UNCOVER, and MEGA-SCIENCE in seven strong-lensing fields in order to find galaxies down to M _UV ∼ −16 and 7.5 < z ≲ 12.5. We measured their β with a forward-modelling procedure. We estimated the f _esc of a subsample with emission line data using a relation with UV slope, galaxy size, and H β equivalent width calibrated from a low-redshift sample. Results. We found 378 visually confirmed galaxies, including 43 galaxies with a spectrum. We obtained an average UV slope < β > = −2.3 ± 0.4, an average redshift < z > = 8.5 ± 1.0, and an average AB magnitude M _UV = −18 ± 1. We found no significant evolution of β in our redshift range, suggesting a flattening of the β – z trend above z ∼ 7.5. We found a weak trend between β and M _UV . Conclusions. The flat trend of β at z > 7.5 indicates a similarity in average galaxy properties between 300 and 600 Myr after the Big Bang. The weak trend between β and M _UV suggests an analogous composition for the interstellar medium of low- and high-mass galaxies, likely due to a lack of time to build up a dust reservoir.

From the onset of observations of JWST we have discovered unexpectedly luminous galaxies at redshifts z > 10 and as high as z = 14. With their discovery, the question immediately followed as to where their progenitors are, since such progenitors should be within reach of existing surveys. However, the discovery of several bright candidates at z > 15 may indicate further discrepancies between pre-JWST model predictions and current observations. Progenitors of the bright z ∼ 14 galaxies should be visible at redshifts as high as z ∼ 20–30, showing in the data as F277W and F356W dropouts. We identify two such candidates in the Bullet Cluster JWST data; however, subsequent NIRSpec follow-up data show spectra that can be well fit with Y dwarf templates with temperatures 272–351 K and and 445–525 K (using ATMO2020 and Sonora Elf Owl models) and distances of ∼150–650 pc. The first is one of the lowest-temperature brown dwarfs known, and the lowest-temperature brown dwarf detected spectroscopically outside the solar neighborhood. With additional NIRCam imaging taken ∼1 yr later, we also detect their proper motions of (49 ± 8) mas yr ⁻¹ and (24 ± 3) mas yr ⁻¹ , further indicating that at least some F277W and F356W dropouts are substellar cold Milky Way objects such as brown dwarfs.

The discovery of a population of massive, ancient quiescent galaxies within the first 2 Gyr of the Universe’s history has led to significant tensions with models of galaxy formation. However, these analyses are often based on slit spectroscopy, which typically captures only the centermost region of these galaxies and, crucially, assumes these cores are representative of the entire galaxy. To illustrate the varying stellar populations present throughout these galaxies, we present an analysis of color gradients in four z > 3, log ( M ⋆ / M ⊙ ) > 11 quiescent galaxies which previous works have argued are in tension with models. Using medium-band photometry from MINERVA JWST observations, we measure resolved photometry in a series of elliptical annuli out to 0 . ″ 7 (∼4 R _e ). We find negative color gradients in three galaxies, and for the most extreme color gradient (Δ( U − V )/Δ R = −0.126 ± 0.030 mag kpc ⁻¹ ), we find the stellar mass is 0.1 dex lower when compared to photometry measured within NIRSpec slits. In the limiting case where these color gradients are entirely driven by age, we find lessened tensions with extreme value statistics models out to z ∼ 9.5, though different stellar population modeling choices also contribute significantly. Ultimately, these findings highlight the need for integral field unit spectroscopy. Spatially resolved spectra can provide the evidence needed to break the age–dust–metallicity degeneracy, and reliably separate the effects of the observed color gradients from the effects of different physical modeling assumptions on the formation histories of these galaxies.

The physical properties of kiloparsec-scale clumps in high-redshift star-forming galaxies (SFGs) contain crucial constraints on how they assemble. Building on recent work that indicates the presence of a metallicity offset in clumpy galaxies compared to nonclumpy SFGs, we analyze the chemical abundance in a large sample of ∼300 SFGs in the range 0.6 < z < 2.6 using LEGA-C, MOSDEF, and CANUCS near-infrared spectroscopic observations. We find that clumpy galaxies generally have lower gas-phase metallcities compared to the mass–metallicity relation, while nonclumpy galaxies have higher metallicities. We further investigate the relationship between the resolved stellar properties of clumps and the integrated gas-phase metallicity of the host galaxies using a subset of galaxies observed in the CANUCS fields. In particular, clumps in SFGs with metallicity below the mass–metallicity relation (i.e., Δ Z ≤ 0) are generally younger and have higher star-formation rates (SFRs) compared to clumps whose host galaxies have Δ Z > 0. We do not find a significant mass difference between these two clump populations. Finally, we compute the merger statistic using the Gini-M20 morphological parameters and find that the majority of clumpy galaxies are not classified as mergers based on their stellar mass maps. The results suggest that the clumpy nature of cosmic noon galaxies is linked to metal-poor gas accretion events that trigger star formation and dilute metallicities.

We investigate the properties of kiloparsec-scale star-forming clumps and their host galaxies at 0.5 < z < 5 in the JWST Canadian NIRISS Unbiased Cluster Survey fields. The fraction of clumpy galaxies is observed to peak near z ∼ 2 for galaxies with masses of log ( M g , * / M ⊙ ) ≥ 10 . Galaxies with masses of 8.5 ≤ log ( M g , * / M ⊙ ) < 10 show lower clumpy fractions with little redshift evolution. We further identify and measure individual clump masses, finding that the aggregated clump stellar mass function (cSMF) follows a power-law slope of α = −2 across all redshift bins, broadly consistent with in situ clump formation. However, when split by galaxy mass, the cSMF is found to be flatter ( α ∼ −1.6) for massive galaxies and steeper ( α ∼ −2.3) for lower-mass galaxies, with little redshift evolution. We also explore how clump formation mechanisms shape the cSMF. In particular, the cSMF slope is found to be flatter with increasing inferred gas fractions in younger clump populations (<300 Myr), suggesting that gas availability in galaxies favors the formation of more massive clumps. In addition, the cSMF slope is found to be flatter for older clumps. This may reflect several evolutionary processes such as the preferential destruction of low-mass clumps and clump mergers, which contribute to the flattening of the slopes as clumps evolve. Overall, the galaxy-mass-dependent cSMF and age distribution point to a complex history for clumps, involving different and competing mechanisms for their formation and evolution.

Context. Lyman α (Ly α ) emission is highly sensitive to dust and neutral hydrogen and is believed to be suppressed in dusty or hydrogen-rich galaxies – especially during the epoch of reionization (EoR). Yet, moderately dusty Ly α emitters (LAEs) are observed at this epoch, suggesting that complex interstellar medium (ISM) geometries and feedback-driven outflows facilitate Ly α escape. Aims. We investigated the dust, gas, and stellar properties of the gravitationally lensed LAE HCM 6A at z = 6.5676 to characterize its multiphase ISM structure and the physical conditions that regulate Ly α escape. Methods. We combined JWST/NIRISS slitless spectroscopy, HST+JWST/NIRCam imaging, and JWST/NIRSpec slit spectra from the CANUCS program. Using a customized BAGPIPES spectral-energy-distribution-fitting framework with a flexible attenuation law, we derived spatially resolved stellar, nebular, and dust properties on integrated (≈1 kpc), slit-level (≈0.1 kpc), and pixel-level (≈25 pc) measurements, thanks to strong lensing with a magnification of μ ≈ 8.3 − 9.1. A high-quality Ly α map from SLEUTH , a tool for extracting spatially resolved emission-line maps from slitless spectroscopy, traces the spatial distribution of Ly α emission. Results. We measure an unlensed stellar mass of log M _* = 8.3 − 8.4 and an intrinsic UV magnitude of M _UV = −19.8 ± 0.1. Slit-level measurements show that the oldest, most massive region (S1) is moderately dusty with consistent stellar ( A _V , β ) and nebular ( A _V ^B , line-emission) indicators, implying a uniform ISM geometry, while the dust tracers of the youngest region (S3) are highly mismatched, revealing a complex, feedback-shaped multiphase ISM. Ly α emission arises primarily from S3. Pixel-level maps reveal a dust-cleared central clump (C3) where Ly α emerges, encircled by dustier outskirts, consistent with a very recent (≲10 Myr) starburst that created Ly α escape channels. Next, slit-level maps show Calzetti-like attenuation curves with a UV bump that increases with stellar age and decreases with V -band attenuation ( A _V ), with a tentative detection of a UV bump in S1 at ∼25% of the Milky Way amplitude. Pixel-level maps reveal that both the curve slope ( S ) and the UV bump ( B ) peak in the region between two clumps (C1 and C2), indicating dust-grain processing in a merger-driven starburst. Conclusions. Our observations provide a uniquely detailed, spatially resolved view of a moderately dusty LAE at the EoR, demonstrating how the interplay between multiphase ISM geometry and feedback governs Ly α escape. Constraining some key quantities requires fully resolved spectroscopy, which future JWST/NIRSpec integral field unit observations will provide.

The little red dot (LRD) paradigm comprises three main unknowns that are intrinsically connected: (1) What is the nature of these sources? (2) How do they form? (3) How do they evolve? Larger spectroscopic samples and high-resolution data are needed to delve deeper into the mechanisms governing these sources. Understanding their formation and evolution requires identifying the rise and fall of the key features that characterize these systems, such as their compactness and “V”-shaped spectral energy distributions. In this work, we present a galaxy system nicknamed “The Stingray” that was identified in the Canadian NIRISS Unbiased Cluster Survey (CANUCS). This group contains three sources at z _spec = 5.12, including an active galactic nucleus (AGN), a Balmer break galaxy, and a star-forming satellite. The latter resembles a building blocks system in which interactions boost stellar mass and black hole mass growth beyond what is expected from secular processes alone. The AGN in this system exhibits features indicative of a transitional object, bridging a normal AGN and an LRD phase. These features include a blue rest-frame UV slope, compact size, and a broad H α line (all of which are characteristic of LRDs), but a flatter rest-frame optical slope compared to that observed in LRDs. The features in this source point to the emergence or fading of an LRD, potentially triggered by environmental effects.

We present a sample of z _phot ∼ 10–16 galaxies by exploiting some of the richest JWST NIRCam imaging data, taken in the CANUCS survey in Cycle 1 and the Technicolor (TEC) survey in Cycle 2. The combination of the CANUCS+TEC provides multiepoch, deep NIRCam images in all medium bands (MBs) and broad bands (BBs) onboard NIRCam (22 filters in total), over ∼ 23 arcmin 2 in three independent lines of sight. We select high- z galaxy candidates based on photometric redshifts, and obtain eight candidates at z ∼ 10–16, including a very robust candidate at z ∼ 15.4. The ultraviolet (UV) luminosity function (LF) from our sample is consistent with previous JWST studies showing a scatter of ∼0.6 dex across the literature, marking the significance of the field-to-field variance in interpreting galaxy abundance measurements at z > 10. We find that the UV LF moderately evolves at z > 10, and the LF normalization and the luminosity density decline by a factor of ∼7 from z ∼ 11 to z ∼ 15, indicating less steep evolution than z < 11. We highlight the importance of MB filters, not only to minimize the contamination by low- z interlopers but also to maximize the completeness. In particular, faint and less blue galaxies could be missed when the sample is built solely on BB data. The contamination and incompleteness of BB-only selected samples can bias our views of earliest galaxy evolution at z > 10, including the UV LF by ∼0.6 dex, the size–magnitude relation by ∼0.6 dex, and the UV slope-magnitude relation by Δ β _UV ∼ −0.3.