We present a catalogue of spectroscopically selected [O III ]+H β emitters at 6.75 < z < 9.05 and the resulting [O III ] 5008 Å luminosity function (LF) in the COSMOS field. We leveraged the 0.33 deg 2 covered by COSMOS-3D using NIRCam/WFSS F444W to perform the largest spectroscopic search for [O III ] emitters at 6.75 < z < 9.05. We present our catalogue of 249 [O III ] emitters and their associated completeness function. The inferred constraints on the [O III ] LF enabled us to characterise the knee of the [O III ] 5008 Å LF, resulting in improved [O III ] 5008 Å LF constraints at z ∼ 7, 8. Notably, we find evidence for a decline of the [O III ] luminosity density between z ∼ 6 − 8, which could be expected if the metallicity of [O III ] emitters, as well as the cosmic star-formation rate density, is declining at these redshifts. We find that theoretical models that reproduce the z ∼ 7, 8 [O III ] 5008 Å LF do not reproduce well the [O III ] equivalent width distribution, pointing to potential challenges in the modelling of [O III ] and other nebular lines in the early Universe. Finally, we provide the first constraints on the cosmic variance of [O III ] emitters, estimating at 15% the fractional uncertainty for the z ∼ 7, 8 [O III ] 5008 Å LF in the 0.33 deg 2 field. This estimate is in good agreement with that inferred from clustering, and it shows that the [O III ] 5008 Å LF derived from smaller extragalactic legacy fields is strongly affected by cosmic variance. Our results highlight the fundamental role that wide-area JWST slitless surveys play to map the galaxy large-scale structure down into the reionisation era, serving as a springboard for a variety of science cases.

To better understand how galaxies assemble their structure and evolve over cosmic time, we present a new catalog of morphological measurements for over 340,000 sources spanning 0 < z < 12, derived from deep JWST NIRCam imaging across four major extragalactic fields (CEERS, PRIMER-UDS, PRIMER-COSMOS, GOODS) compiled in the DAWN JWST Archive (DJA). We perform two-dimensional surface brightness fitting for all galaxies in a uniform, flux-limited sample. Each galaxy is modeled with both a Sérsic profile and a two-component (bulge and disk) decomposition, yielding consistent structural parameters - including effective radius, Sérsic index, axis ratio, and bulge-to-total ratio (B/T). To demonstrate the scientific application of our morphology catalogs, we combined these measurements with DJA photometric redshifts, physical parameters and rest-frame colors, and investigated the relation between total, bulge and disk sizes, Sérsic index (n_S), star formation activity, and redshift. Bulge-dominated galaxies (high n_S and B/T) predominantly occupy the quiescent region of the UVJ diagram, while disk-dominated galaxies are mostly star-forming. A significant bimodality persists, with quiescent disks and compact, bulge-dominated star-forming galaxies observed out to z > 3. Quiescent galaxies also show significantly higher stellar mass surface densities, nearly an order of magnitude greater at z ∼ 4 than at z ∼ 1. Our results confirm a strong and evolving link between morphology and star formation activity, and support a scenario in which bulge growth and quenching are closely connected. This work is a highly valuable addition to the DJA, adding a morphological dimension to this rich dataset and thus enabling a wider scientific application.

Context. Strong gravitational lensing in galaxy clusters has become an essential tool in astrophysics, as it allows one to directly probe the dark matter distribution and study magnified background sources. The precision and reliability of strong lensing models rely heavily on the number and quality of multiple images of background sources with spectroscopic redshifts. Aims. We present an updated strong lensing model of the galaxy cluster MACS J0416.1-2403 with the largest sample of multiple images with spectroscopic redshifts in a galaxy cluster field to date. Furthermore, we aim to demonstrate the effectiveness of JWST, particularly its NIRISS camera, for strong lensing studies. Methods. We used JWST's NIRCam imaging and NIRSpec and NIRISS spectroscopy from the CAnadian NIRISS Unbiased Cluster Survey (CANUCS). The cluster mass model was constrained using Lenstool software. Results. Our new dataset, which we used for constraining the lens model, comprises 303 secure multiple images with spectroscopic redshifts from 111 background sources and includes 95 systems with previously known MUSE redshift and 16 systems (with 46 multiple images) for which we obtained spectroscopic redshift for the first time using NIRISS and NIRSpec spectroscopy. Three of the spectroscopic systems were not identified by previous JWST studies. The total number of secure spectroscopic systems is more than 20% higher than in the previous strong lensing studies of this cluster. The derived strong lensing model can reproduce multiple images with the root-mean-square distance of similar to 0 ''.52. We also provide a full catalogue with 415 multiple images, including less reliable candidates. In total, we provide 15 new multiple-image system candidates (with 38 multiple images) not reported in previous studies. Furthermore, we demonstrate the effectiveness of JWST, particularly NIRISS, for obtaining spectroscopic redshifts of multiple images. As NIRISS F115W, F150W, and F200W grism spectroscopy captures at least two of the [O II] lambda 3727, [O III] lambda lambda 4959, 5007, and H-alpha lines at 1 less than or similar to z less than or similar to 3 (a redshift range particularly relevant for strong lensing studies) without target pre-selection, it complements MUSE and NIRSpec observations extremely well.

The evolution of galaxy sizes in different wavelengths provides unique insights on galaxy build-up across cosmic epochs. Such measurements can now finally be done at z > 3 thanks to the James Webb Space Telescope's (JWST) exquisite spatial resolution and multi-wavelength capability. With the public data from the CEERS, PRIMER-UDS, and PRIMER-COSMOS surveys, we measure the sizes of similar to 3500 star-forming galaxies at 3 <= z < 9, in seven NIRCam bands using the multi-wavelength model fitting code GalfitM. The size-mass relation is measured in four redshift bins, across all NIRCam bands. We find that the slope and intrinsic scatter of the rest-optical size-mass relation are constant across this redshift range and consistent with previous studies at low-z with the Hubble Space Telescope. When comparing the relations across different wavelengths, the average rest-optical and rest-UV relations are consistent with each other up to z = 6, but the intrinsic scatter is largest in rest-UV wavelengths compared to rest-optical and redder bands. This behaviour is independent of redshift and we speculate that it is driven by bursty star formation in z > 4 galaxies. Additionally, for 3 <= z < 4 star-forming galaxies at M-& lowast; > 10(10) M-circle dot, we find smaller rest-1 mu m sizes in comparison to rest-optical (and rest-UV) sizes, suggestive of colour gradients. When comparing to simulations, we find agreement over M-& lowast; approximate to 10(9) - 10(10) M-circle dot but beyond this mass, the observed size-mass relation is significantly steeper. Our results show the power of JWST/NIRCam to provide new constraints on galaxy formation models.

Understanding how galaxies assemble their structure and evolve morphologically over cosmic time is a central goal of galaxy evolution studies. In particular, the morphological evolution of quiescent and star-forming galaxies provides key insights into the mechanisms that regulate star formation and quenching. We present a new catalog of morphological measurements for more than 340 000 sources spanning 0 < z < 12, derived from deep JWST NIRCam imaging across four major extragalactic fields (CEERS, PRIMER-UDS, PRIMER-COSMOS, GOODS) compiled in the DAWN JWST Archive (DJA). We performed two-dimensional surface brightness fitting for all galaxies in a uniform, flux-limited sample. Each galaxy was modeled with both a Sersic profile and a two-component (bulge and disk) decomposition, yielding consistent structural parameters, including effective radius, Sersic index (n(S)), axis ratio, and bulge-to-total ratio (B/T). To demonstrate the scientific application of our morphology catalogs, we combined these measurements with DJA photometric redshifts, physical parameters and rest-frame colors, and investigated the relation between total, bulge, and disk sizes, n(S), star formation activity, and redshift. Bulge-dominated galaxies (high n(S) and B/T) predominantly occupy the quiescent region of the UVJ diagram, while disk-dominated galaxies are mostly star-forming. A significant bimodality persists, with quiescent disks and compact, bulge-dominated star-forming galaxies observed out to z > 3. Quiescent galaxies also show significantly higher stellar mass surface densities, nearly an order of magnitude greater at z similar to 4 than at z similar to 1. Our results confirm a strong and evolving link between morphology and star formation activity and support a scenario in which bulge growth and quenching are closely connected. This work is a highly valuable addition to the DJA, adding a morphological dimension to this rich dataset and thus enabling a wider scientific application.

The Hα nebular emission line is an optimal tracer for recent star formation in galaxies. With the advent of JWST, this line has recently become observable at z > 3 for the first time. We present a catalog of 1050 Hα emitters at 3.7 < z < 6.7 in the GOODS fields obtained from a blind search in JWST NIRCam/grism data. We made use of the FRESCO survey’s 124 arcmin2 of observations in GOODS-North and GOODS-South with the F444W filter, probing Hα at 4.9 < z < 6.7, and the CONGRESS survey’s 62 arcmin2 of observations in GOODS-North with F356W, probing Hα at 3.8 < z < 5.1. We found an overdensity with 98 sources at z ∼ 4.4 in GOODS-N, and confirmed previously reported overdensities at z ∼ 5.2 in GOODS-N and at z ∼ 5.4 and z ∼ 5.9 in GOODS-S. We computed the observed Hα luminosity functions (LFs) in three bins centered at z ∼ 4.45, 5.30, and 6.15, which are the first such measurements at z > 3 obtained based purely on spectroscopic data, robustly tracing galaxy star formation rates (SFRs) beyond the peak of the cosmic star formation history. We compared our results with theoretical predictions from three different simulations and found good agreement at z ∼ 4 − 6. The UV LFs of this spectroscopically confirmed sample are in good agreement with pre-JWST measurements obtained with photometrically selected objects. Finally, we derived SFR functions and integrated them to compute the evolution of the cosmic SFR densities across z ∼ 4 − 6, finding values in good agreement with recent UV estimates from Lyman-break galaxies, which imply a continuous decrease in SFR density by a factor of three over z ∼ 4 to z ∼ 6. Our work shows the power of NIRCam grism observations to efficiently provide new tests for early galaxy formation models based on emission line statistics.

We use JWST/NIRISS slitless spectroscopy from the Next Generation Deep Extragalactic Exploratory Public (NGDEEP) Survey to investigate the physical condition of 178 star-forming galaxies at 1.7 < z < 3.4. At these redshifts, the deep NGDEEP NIRISS slitless spectroscopy covers the [OII]lambda lambda 3726,3729, [O III]lambda lambda 4959,5007, H beta and H alpha emission features for galaxies with stellar masses log(M-& lowast;/M-circle dot)greater than or similar to 7, nearly a factor of 100 lower than previous studies. We focus on the [OII]/[OIII] (O-32) ratio which is primarily sensitive to the ionization state and with a secondary dependence on the gas-phase metallicity of the interstellar medium. We find significant (greater than or similar to 5 sigma) correlations between the O-32 ratio and galaxy properties as O-32 increases with decreasing stellar mass, decreasing star formation rate (SFR), increasing specific SFR (sSFR equivalent to SFR/M-*), and increasing equivalent width (EW) of H alpha and H beta. These trends suggest a tight connection between the ionization parameter and these galaxy properties. Galaxies at z similar to 2-3 exhibit a higher O-32 than local normal galaxies with the same stellar masses and SFRs, indicating that they have a higher ionization parameter and lower metallicity than local normal galaxies. In addition, we observe a mild evolutionary trend in the O-32-EW(H beta) relation from z similar to 0 to z greater than or similar to 5, where higher redshift galaxies show increased O-32 and EW, with possibly higher O-32 at fixed EW. We argue that both the enhanced recent star formation activity and the higher star formation surface density may contribute to the increase in O-32 and the ionization parameter.

We study the H α equivalent width (EW(H α )) maps of 19 galaxies at 0.6 < z < 2.2 in the Hubble Ultra Deep Field using NIRISS slitless spectroscopy as part of the Next Generation Deep Extragalactic Exploratory Public Survey. Our galaxies mostly lie on the star formation main sequence with stellar masses between 10 ^9 and 10 ^11 M _⊙ , characterized as “typical” star-forming galaxies at these redshifts. Leveraging deep Hubble Space Telescope and JWST images, spanning 0.4–4.8 μ m, we perform spatially resolved fitting of the spectral energy distributions for these galaxies and construct specific star formation rate (sSFR) and stellar-mass-weighted age maps with a spatial resolution of ∼1 kpc. The pixel-to-pixel EW(H α ) increases with increasing sSFR and with decreasing age. The average trends are slightly different from the relations derived from integrated fluxes of galaxies from the literature, suggesting complex evolutionary trends within galaxies. We quantify the radial profiles of EW(H α ), sSFR, and age. The majority (84%) of galaxies show positive EW(H α ) gradients, in line with the inside-out quenching scenario. A few galaxies (16%) show inverse (and flat) EW(H α ) gradients, possibly due to merging or starbursts. We compare the distributions of EW(H α ) and sSFR to star formation history (SFH) models as a function of galactocentric radius. We argue that the central regions of galaxies have experienced at least one rapid star formation episode, which leads to the formation of the bulge, while their outer regions (e.g., disks) grow via more smoothly varying SFHs. These results demonstrate the ability to study resolved star formation in distant galaxies with JWST NIRISS.

We present the Next Generation Deep Extragalactic Exploratory Public (NGDEEP) Survey, a deep slitless spectroscopic and imaging Cycle 1 JWST treasury survey designed to constrain feedback mechanisms in low-mass galaxies across cosmic time. NGDEEP targets the Hubble Ultra Deep Field (HUDF) with NIRISS slitless spectroscopy (f~1.2e-18 erg/s/cm^2, 5sigma) to measure metallicities and star-formation rates (SFRs) for low-mass galaxies through the peak of the cosmic SFR density (0.512, constraining the slope of the faint-end of the rest-ultraviolet luminosity function. NGDEEP overlaps with the deepest HST ACS optical imaging in the sky: F435W in the HUDF (m=29.6), and F814W in HUDF-Par2 (m=30), making this a premier HST+JWST Deep Field. As a treasury survey, NGDEEP data is public immediately, and we will rapidly release data products and catalogs in the spirit of previous deep field initiatives. In this paper we present the NGDEEP survey design, summarize the science goals, and detail plans for the public release of NGDEEP reduced data products.

The changes in colors across a galaxy are intimately connected to the galaxy's formation, growth, quenching history, and dust content. A particularly important epoch in the growth of galaxies is near z similar to 2, often referred to as "cosmic noon," where galaxies on average reach the peak of their star formation. We study a population of 125 cluster galaxies at z similar to 1.6 in three Hubble Space Telescope filters, F475W, F625W, and F160W, roughly corresponding to the rest-frame far-ultraviolet, near-ultraviolet, and r band, respectively. By comparing to a control sample of 200 field galaxies at similar redshift, we reveal clear, statistically significant differences in the overall spatially resolved colors and color gradients in galaxies across these two different environments. On average, cluster galaxies have redder ultraviolet colors in both the inner and outer regions bounded by r 50, as well as an overall wider dispersion of outside-in color gradients. The presence of these observed differences, along with evidence from ancillary data from previous studies, strongly suggests that the environment drives these population-level color differences, by affecting the stellar populations and/or dust content.

Early JWST observations that targeted so-called double-break sources (attributed to Lyman and Balmer breaks at z>7), reported a previously unknown population of very massive, evolved high-redshift galaxies. This surprising discovery led to a flurry of attempts to explain these objects' unexpected existence including invoking alternatives to the standard ΛCDM cosmological paradigm. To test these early results, we adopted the same double-break candidate galaxy selection criteria to search for such objects in the JWST images of the CAnadian NIRISS Unbiased Cluster Survey (CANUCS), and found a sample of 19 sources over five independent CANUCS fields that cover a total effective area of ∼60arcmin^2 at z∼8. However, (1) our SED fits do not yield exceptionally high stellar masses for our candidates, while (2) spectroscopy of five of the candidates shows that while all five are at high redshifts, their red colours are due to high-EW emission lines in star-forming galaxies rather than Balmer breaks in massive, evolved systems. Additionally, (3) field-to-field variance leads to differences of ∼ 1.5 dex in the maximum stellar masses measured in the different fields, suggesting that the early single-field JWST observations may have suffered from cosmic variance and/or sample bias. Finally, (4) we show that the presence of even a single massive outlier can dominate conclusions from small samples such as those in early JWST observations. In conclusion, we find that the double-break sources in CANUCS are not sufficiently massive or numerous to warrant questioning the standard ΛCDM paradigm.

We present a new sample of 158 galaxies at redshift $z>7.5$ selected from deep \jwst\ NIRCam imaging of five widely-separated sightlines in the CANUCS survey. Two-thirds of the pointings and 80\% of the galaxies are covered by 12 to 14 NIRCam filters, including seven to nine medium bands, providing accurate photometric redshifts and robustness against low redshift interlopers. A sample of 28 galaxies at $z>7.5$ with spectroscopic redshifts shows a low systematic offset and scatter in the difference between photometric and spectroscopic redshifts. We derive the galaxy UV luminosity function at redshifts 8 to 12, finding a slightly higher normalization than previously seen with \hst\ at redshifts 8 to 10. We observe a steeper decline in the galaxy space density from $z=8$ to $12$ than found by most \jwst\ Cycle 1 studies. In particular, we find only eight galaxies at $z>10$ and none at $z>12.5$, with no $z>10$ galaxies brighter than F277W AB=28 or $M_{\rm UV}=-20$ in our unmasked, delensed survey area of 53.4 square arcminutes. We attribute the lack of bright $z>10$ galaxies in CANUCS compared to GLASS and CEERS to intrinsic variance in the galaxy density along different sightlines. The evolution in the CANUCS luminosity function between $z=8$ and $12$ is comparable to that predicted by simulations that assume a standard star formation efficiency, without invoking any special adjustments.

The epoch of reionization (EoR) progressed through the emission of ionizing photons from galaxies to their local intergalactic medium. In this work, we characterize the dwarf star-forming galaxies as candidates for the source of ionizing photons that drove EoR. We investigate the ionizing properties and star formation histories of star-forming dwarf galaxies at the last stages of EoR at 4.8 < z < 7 using observations from the CAnadian NIRISS Unbiased Cluster Survey (CANUCS). The magnification due to gravitational lensing allows us to probe large dynamic ranges in stellar mass (2 x 10(6) <= M-*/M-circle dot <= 5 x 10(9)) and ultraviolet (UV) magnitudes (-22.68 <= M-UV <= = -15.95). We find a median UV slope beta(1500) of -2.56 +/- 0.23 and the production efficiency of ionizing photons log(xi ion) = 25.39 +/- 0.6 for the full sample (4.8 < z < 7) with a median stellar mass of 6.3 +/- 0.5 x 10(7) M-circle dot. We find both beta(1500) and xi(ion) are marginally correlated with the stellar mass of the galaxy, indicating a possible greater contribution of dwarf galaxies to the reionization of the Universe. We find that on average, galaxies in our sample are experiencing a recent rise/burst of star formation which translates to a higher scatter in xi(ion) and a large scatter in H alpha equivalent widths (EWs). Finally, we investigate the trends of H alpha and [O III] + H beta EWs with UV magnitude and find M-UV is correlated between H alpha but not with [O III] + H beta EWs indicating low metallicities and recent burst in the UV faint galaxies.

We present galaxy MACS0416-Y1 at zspec = 8.312 as observed by the CAnadian NIRISS Unbiased Cluster Survey. MACS0416-Y1 has been shown to have extreme dust properties; thus, we study the physical properties and star formation histories of its resolved components. Overall, we find that MACS0416-Y1 is undergoing a star formation burst in three resolved clumps. The central clump is less massive compared to the other clumps and possibly formed in the merging process of the two larger clumps. Although the star formation history indicates an ongoing star formation burst, this gas-rich galaxy shows comparable star formation efficiency to cosmic noon galaxies. Using NIRSpec prism spectroscopy, we measure metallicity, 12+log(O/H)=7.76 +/- 0.03 , ionization parameter, logU=-2.48 +/- 0.03 , and electron temperature Te = 18000 +/- 4000 K. The emission line ratios of the galaxy indicate an evolved interstellar medium similar to z similar to 2 star-forming galaxies. Further, we find possible presence of ionization from an active galactic nucleus (AGN) using emission line diagnostics; however, we do not detect a broad-line component in the H beta emission line. As this gas-rich galaxy is undergoing a major merger, we hypothesize that the high dust temperature in MACS0416-Y1 is caused by the star formation burst or a possible narrow-line AGN.

With the remarkable sensitivity and resolution of JWST in the infrared, measuring rest-optical kinematics of galaxies at z > 5 has become possible for the first time. This study pilots a new method for measuring galaxy dynamics for highly multiplexed, unbiased samples by combining FRESCO NIRCam grism spectroscopy and JADES medium-band imaging. Here we present one of the first JWST kinematic measurements for a galaxy at z > 5. We find a significant velocity gradient, which, if interpreted as rotation, yields V-rot = 305 +/- 70 km s(-1), and we hence refer to this galaxy as Twister-z5. With a rest-frame optical effective radius of r(e) = 2.25 kpc, the high rotation velocity in this galaxy is not due to a compact size, as may be expected in the early Universe, but rather to a high total mass, log(M-dyn/M-circle dot) = 11.2 +/- 0.2. This is a factor of roughly 10x higher than the stellar mass within r(e). We also observe that the radial H alpha equivalent width profile and the specific star formation rate map from resolved stellar population modeling are centrally depressed by a factor of similar to 1.5 from the center to r(e). Combined with the morphology of the line-emitting gas in comparison to the continuum, this centrally suppressed star formation is consistent with a star-forming disk surrounding a bulge growing inside out. While large, rapidly rotating disks are common to z similar to 2, the existence of one after only 1 Gyr of cosmic time, shown for the first time in ionized gas, adds to the growing evidence that some galaxies matured earlier than expected in the history of the Universe.

The most distant galaxies detected by JWST are assembling in a Universe that is less than 5% of its present age. At these times, the progenitors of galaxies like the Milky Way are expected to be about 10,000 times less massive than they are now, with masses quite comparable to that of massive globular clusters seen in the local Universe. Composed today primarily of old stars and correlating with the properties of their parent dark matter halos, the first globular clusters are thought to have formed during the earliest stages of galaxy assembly. In this article we explore the connection between globular clusters and galaxy assembly by showing JWST observations of a galaxy at zspec = 8.304 ± 0.001, strongly lensed by a massive foreground cluster of galaxies, exhibiting a network of massive star clusters (the ‘Firefly Sparkle’) cocooned in a diffuse arc. The Fire-fly Sparkle exhibits the hallmarks expected of a future Milky Way-type galaxy captured during its earliest and most gas-rich stage of formation. The mass distribution of the galaxy seems to be concentrated in ten distinct clusters (∼ 49 − 57% of the total mass), with individual cluster masses (M∗,cluster ∼ 105.3 − 105.8M⊙) that straddle the boundary between low-mass galaxies and high-mass globular clusters. The cluster ages suggest that they are gravitationally bound with star formation histories showing a recent starburst possibly triggered by the interaction with a companion galaxy at the same redshift at a projected distance of ∼2 kpc away from the Firefly Sparkle. The central star cluster shows nebular-dominated spectra consistent with high temperatures (Te ∼ 40, 000K) and a top-heavy initial mass functon(0.78 < α < 2.0), the product of formation in a very metal poor environment. Combined with abundance matching that suggests that this is likely to be a progenitor of galaxies like our own, the Firefly Sparkle provides an unprecedented case study of a Milky Way-like galaxy in the earliest stages of its assembly in only a 600 million year old Universe

Spatially resolved studies are key to understanding when, where, and how stars form within galaxies. Using slitless grism spectra and broadband imaging from the CAnadian NIRISS Unbiased Cluster Survey (CANUCS) we study the spatially resolved properties of a strongly lensed ($\mu$ = 5.4$\pm$1.8) z = 0.8718 galaxy pair consisting of a blue face-on galaxy (10.2 $\pm$ 0.2 log($M/M_\odot$)) with multiple star-forming clumps and a dusty red edge-on galaxy (9.9 $\pm$ 0.3 log($M/M_\odot$)). We produce accurate H$\alpha$ maps from JWST/NIRISS grism data using a new methodology that accurately models spatially varying continuum and emission line strengths. With spatially resolved indicators, we probe star formation on timescales of $\sim$ 10 Myr (NIRISS H$\alpha$ emission line maps) and $\sim$ 100 Myr (UV imaging and broadband SED fits). Taking the ratio of the H$\alpha$ to UV flux ($\eta$), we measure spatially resolved star formation burstiness. We find that in the face-on galaxy both H$\alpha$ and broadband star formation rates (SFRs) drop at large galactocentric radii by a factor of $\sim$ 4.7 and 3.8 respectively, while SFR over the last $\sim$ 100 Myrs has increased by a factor of 1.6. Additionally, of the 20 clumps identified in the galaxy pair we find that 7 are experiencing bursty star formation, while 10 clumps are quenching and 3 are in equilibrium (either being in a state of steady star formation or post-burst). Our analysis reveals that the blue face-on galaxy disk is predominantly in a quenching or equilibrium phase. However, the most intense quenching within the galaxy is seen in the quenching clumps. This pilot study demonstrates what JWST/NIRISS data can reveal about spatially varying star formation in galaxies at Cosmic Noon.

Photometrically derived stellar masses are known to suffer from systematic uncertainties, particularly due to nebular emission contributions to the spectral energy distribution (SED). Using James Webb Space Telescope NIRCam imaging from the CAnadian NIRISS Unbiased Cluster Survey, we introduce a comparison study of photometrically derived redshifts and stellar masses based on two photometric catalogs of the same field spanning ∼0.4–4.5 μ m: one consisting solely of wide band photometry, and another employing a combination of wide and medium band photometry. We find that ∼70% of likely line emitters have consistent photometric redshifts between both catalogs, with a median stellar mass difference between the two catalogs of <0.2 dex across all redshift bins. There are, however, a subset of galaxies (5% at z ∼ 2 up to 15% at z ∼ 6) where wide bands underestimate emission line strengths and infer older stellar populations, leading to median stellar mass differences of ∼0.7 dex. Examination of the SEDs for galaxies with inconsistent photometric redshifts shows this is caused by the inability of the wide bands to distinguish continuum emission from emission lines. Computing a stellar mass density with our sample, we find that it is potentially underestimated using wide band photometry by ∼10%–20% at z < 4, and potentially overestimated by as much as a factor of 2–3 at z > 5. These systematic differences caused by the poor spectral resolution of wide bands have implications for both ongoing and future planned observing programs that determine stellar mass and other physical properties of high-redshift galaxies solely via wide band photometry.

The most distant galaxies detected were seen when the Universe was a scant 5% of its current age. At these times, progenitors of galaxies such as the Milky Way were about 10,000 times less massive. Using the James Webb Space Telescope (JWST) combined with magnification from gravitational lensing, these low-mass galaxies can not only be detected but also be studied in detail. Here we present JWST observations of a strongly lensed galaxy at zspec = 8.296 ± 0.001, showing massive star clusters (the Firefly Sparkle) cocooned in a diffuse arc in the Canadian Unbiased Cluster Survey (CANUCS)1. The Firefly Sparkle exhibits traits of a young, gas-rich galaxy in its early formation stage. The mass of the galaxy is concentrated in 10 star clusters (49-57% of total mass), with individual masses ranging from 105M⊙ to 106M⊙. These unresolved clusters have high surface densities (>103M⊙ pc-2), exceeding those of Milky Way globular clusters and young star clusters in nearby galaxies. The central cluster shows a nebular-dominated spectrum, low metallicity, high gas density and high electron temperature, hinting at a top-heavy initial mass function. These observations provide our first spectrophotometric view of a typical galaxy in its early stages, in a 600-million-year-old Universe.

ABSTRACT We use CANUCS JWST/NIRCam imaging of galaxies behind the gravitationally-lensing cluster MACS J0417.5-1154 to investigate star formation burstiness in low-mass (M⋆ ∼ 108 M⊙) galaxies at z ∼ 4.7–6.5. Our sample of 123 galaxies is selected using the Lyman break selection and photometric emission-line excess methods. Sixty per cent of the 123 galaxies in this sample have Hα-to-UV flux ratios that deviate significantly from the range of Hα-to-UV ratio values consistent with smooth and steady star formation histories. This large fraction indicates that the majority of low-mass galaxies is experiencing bursty star formation histories at high redshift. We also searched for interacting galaxies in our sample and found that they are remarkably common ($\sim 40~{{\ \rm per\ cent}}$ of the sample). Compared to non-interacting galaxies, interacting galaxies are more likely to have very low Hα-to-UV ratios, suggesting that galaxy–galaxy interactions enhance star formation burstiness and enable faster quenching (with time-scales of ≲100 Myr) that follows the rapid rise of star formation activity. Given the high frequency of galaxy–galaxy interactions and the rapid SFR fluctuations they appear to cause, we conclude that galaxy–galaxy interactions could be a leading cause of bursty star formation in low-mass, high-z galaxies. They could thus play a significant role in the evolution of the galaxy population at early cosmological times.