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We describe the selection of the James Webb Space Telescope (JWST) North Ecliptic Pole (NEP) Time-domain Field (TDF), a 14′ diameter field located within JWST's northern continuous viewing zone (CVZ) and centered at (R.A., decl.)J2000 = (17:22:47.896, +65:49:21.54). We demonstrate that this is the only region in the sky where JWST can observe a clean (i.e., free of bright foreground stars and with low Galactic foreground extinction) extragalactic deep survey field of this size at arbitrary cadence or at arbitrary orientation, and without a penalty in terms of a raised zodiacal background. This will crucially enable a wide range of new and exciting time-domain science, including high-redshift transient searches and monitoring (e.g., SNe), variability studies from active galactic nuclei (AGNs) to brown dwarf atmospheres, as well as proper motions of possibly extreme scattered Kuiper Belt and Inner Oort Cloud Objects, and of nearby Galactic brown dwarfs, low-mass stars, and ultracool white dwarfs. A JWST/NIRCam+NIRISS GTO program will provide an initial 0.8-5.0 m spectrophotometric characterization to m AB ∼ 28.8 0.3 mag of four orthogonal \"spokes\" within this field. The multi-wavelength (radio through X-ray) context of the field is in hand (ground-based near-UV-visible-near-IR), in progress (VLA 3 GHz, VLBA 5 GHz, HST UV-visible, Chandra X-ray, and IRAM 30 m 1.3 and 2 mm), or scheduled (JCMT 850 m). We welcome and encourage ground- and space-based follow-up of the initial GTO observations and ancillary data, to realize its potential as an ideal JWST time-domain community field.

The Cosmic Gems arc is among the brightest and highly magnified galaxies observed at redshift z  ≈ 10.2 (ref.  1 ). However, it is an intrinsically ultraviolet faint galaxy, in the range of those now thought to drive the reionization of the Universe 2 – 4 . Hitherto the smallest features resolved in a galaxy at a comparable redshift are between a few hundreds and a few tens of parsecs (pc) 5 , 6 . Here we report JWST observations of the Cosmic Gems. The light of the galaxy is resolved into five star clusters located in a region smaller than 70 pc. They exhibit minimal dust attenuation and low metallicity, ages younger than 50 Myr and intrinsic masses of about 10 6 M ⊙ . Their lensing-corrected sizes are approximately 1 pc, resulting in stellar surface densities near 10 5 M ⊙  pc −2 , three orders of magnitude higher than typical young star clusters in the local Universe 7 . Despite the uncertainties inherent to the lensing model, they are consistent with being gravitationally bound stellar systems, that is, proto-globular clusters. We conclude that star cluster formation and feedback likely contributed to shaping the properties of galaxies during the epoch of reionization. JWST observations of the Cosmic Gems arc resolve the light of an infant galaxy into five gravitationally bound star clusters located in a region smaller than 70 pc.

Star cluster formation in the early universe and its contribution to reionization remains largely unconstrained to date. Here we present JWST/NIRCam imaging of the most highly magnified galaxy known at z ∼ 6, the Sunrise arc. We identify six young massive star clusters (YMCs) with measured radii spanning from ∼20 down to ∼1 pc (corrected for lensing magnification), estimated stellar masses of ∼106–7 M ⊙, and ages of 1–30 Myr based on SED fitting to photometry measured in eight filters extending to rest frame 7000 Å. The resulting stellar mass surface densities are higher than 1000 M ⊙ pc−2 (up to a few 105 M ⊙ pc−2), and their inferred dynamical ages qualify the majority of these systems as gravitationally bound stellar clusters. The star cluster ages map the progression of star formation along the arc, with two evolved systems (≳10 Myr old) followed by very young clusters. The youngest stellar clusters (<5 Myr) show evidence of prominent Hβ+[O iii] emission based on photometry with equivalent widths larger than >1000 Å rest frame and are hosted in a 200 pc sized star-forming complex. Such a region dominates the ionizing photon production with a high efficiency log(ξion[Hzerg−1])∼25.7 . A significant fraction of the recently formed stellar mass of the galaxy (10%–30%) occurred in these YMCs. We speculate that such sources of ionizing radiation boost the ionizing photon production efficiency, which eventually carves ionized channels that might favor the escape of Lyman continuum radiation. The survival of some of the clusters would make them the progenitors of massive and relatively metal-poor globular clusters in the local universe.

We present the spectroscopic confirmation of a protocluster at z = 7.88 behind the galaxy cluster Abell 2744 (hereafter A2744-z7p9OD). Using JWST NIRSpec, we find seven galaxies within a projected radius of 60 kpc. Although the galaxies reside in an overdensity around ≳20× greater than a random volume, they do not show strong Lyα emission. We place 2σ upper limits on the rest-frame equivalent width <16–28 Å. Based on the tight upper limits to the Lyα emission, we constrain the volume-averaged neutral fraction of hydrogen in the intergalactic medium to be x HI > 0.45 (68% C i). Using an empirical M UV–M halo relation for individual galaxies, we estimate that the total halo mass of the system is ≳4 × 1011 M ⊙. Likewise, the line-of-sight velocity dispersion is estimated to be 1100 ± 200 km s−1. Using an empirical relation, we estimate the present-day halo mass of A2744-z7p9OD to be ∼2 × 1015 M ⊙, comparable to the Coma cluster. A2744-z7p9OD is the highest redshift spectroscopically confirmed protocluster to date, demonstrating the power of JWST to investigate the connection between dark-matter halo assembly and galaxy formation at very early times with medium-deep observations at <20 hr total exposure time. Follow-up spectroscopy of the remaining photometric candidates of the overdensity will further refine the features of this system and help characterize the role of such overdensities in cosmic reionization.

We estimate the UV continuum slope (β) of 465 galaxies (with luminosities of 0.028–3.3 Lz=0.5* ) in the Great Observatories Origins Survey Northern field in the redshift range z = 0.40–0.75. We use two AstroSat/UVIT (N242W, N245M) bands, two Hubble Space Telescope (F275W, F336W) bands, and a KPNO (U) band to sample the UV continuum slope of selected galaxies between 1215 and 2600 Å. The mean (median) and 1σ scatter in the observed β are found to be −1.33 ± 0.07 ( − 1.32) and 0.60 within the considered redshift range. We do not find any significant evolution in the mean β within our redshift window. Our measurements add new data points to the global β–z relation in the least-explored redshift regime, further reinforcing the gradual reddening of galaxy UV continuum with cosmic time. We notice no strong consistent trend between β and M 1500 for the entire luminosity range −21 − 16 mag) tend to be redder, which indicates they were less actively forming stars during this cosmic time interval. Our study highlights the unique capability of UVIT near-UV imaging to characterize the rest-frame far-UV properties of galaxies at redshift z ∼ 0.5.

We report the discovery of four galaxy candidates observed 450–600 Myr after the Big Bang with photometric redshifts between z ∼ 8.3 and 10.2 measured using James Webb Space Telescope (JWST) NIRCam imaging of the galaxy cluster WHL0137−08 observed in eight filters spanning 0.8–5.0 μm, plus nine Hubble Space Telescope filters spanning 0.4–1.7 μm. One candidate is gravitationally lensed with a magnification of μ ∼ 8, while the other three are located in a nearby NIRCam module with expected magnifications of μ ≲ 1.1. Using SED fitting, we estimate the stellar masses of these galaxies are typically in the range logM⋆/M⊙ = 8.3–8.7. All appear young, with mass-weighted ages <240 Myr, low dust content A V < 0.15 mag, and specific star formation rates sSFR ∼0.25–10 Gyr−1 for most. One z ∼ 9 candidate is consistent with an age <5 Myr and an sSFR ∼10 Gyr−1, as inferred from a strong F444W excess, implying [O iii ]+H β rest-frame equivalent width ∼2000 Å, although an older z ∼ 10 object is also allowed. Another z ∼ 9 candidate is lensed into an arc 2.″4 long with a magnification of μ ∼ 8. This arc is the most spatially resolved galaxy at z ∼ 9 known to date, revealing structures ∼30 pc across. Follow-up spectroscopy of WHL0137−08 with JWST/NIRSpec will be useful to spectroscopically confirm these high-redshift galaxy candidates and to study their physical properties in more detail.

The first James Webb Space Telescope (JWST) Near InfraRed Camera imaging in the field of the galaxy cluster PLCK G165.7+67.0 (z = 0.35) uncovered a Type Ia supernova (SN Ia) at z = 1.78, called “SN H0pe.” Three different images of this one SN were detected as a result of strong gravitational lensing, each one traversing a different path in spacetime, thereby inducing a relative delay in the arrival of each image. Follow-up JWST observations of all three SN images enabled photometric and rare spectroscopic measurements of the two relative time delays. Following strict blinding protocols which oversaw a live unblinding and regulated postunblinding changes, these two measured time delays were compared to the predictions of seven independently constructed cluster lens models to measure a value for the Hubble constant, H0 = 71.8 + 9.2 − 8.1 km s−1 Mpc−1. The range of admissible H0 values predicted across the lens models limits further precision, reflecting the well-known degeneracies between lens model constraints and time delays. It has long been theorized that a way forward is to leverage a standard candle, but this has not been realized until now. For the first time, the lens models are evaluated by their agreement with the SN absolute magnifications, breaking degeneracies and producing our best estimate, H0 = 75.7−5.5+8.1 km s−1 Mpc−1. This is the first precise measurement of H0 from a multiply imaged SN Ia and only the second from any multiply imaged SN.

The first deep field images from the James Webb Space Telescope (JWST) of the galaxy cluster SMACS J0723.3-7327 reveal a wealth of new lensed images at uncharted infrared wavelengths, with unprecedented depth and resolution. Here we securely identify 14 new sets of multiply imaged galaxies totaling 42 images, adding to the five sets of bright and multiply imaged galaxies already known from Hubble Space Telescope data. We find examples of arcs crossing critical curves, allowing detailed community follow-up, such as JWST spectroscopy for precise redshift determinations, and measurements of the chemical abundances and of the detailed internal gas dynamics of very distant, young galaxies. One such arc contains a pair of compact knots that are magnified by a factor of hundreds, and features a microlensed transient. We also detect an Einstein cross candidate only visible thanks to JWST’s superb resolution. Our parametric lens model is available through the following link (https://www.dropbox.com/sh/gwup2lvks0jsqe5/AAC2RRSKce0aX-lIFCc9vhBXa?dl=0) and will be regularly updated using additional spectroscopic redshifts. The model is constrained by 16 of these sets of multiply imaged galaxies, three of which have spectroscopic redshifts, and reproduces the multiple images to better than an rms of 0.″5, allowing for accurate magnification estimates of high-redshift galaxies. The intracluster light extends beyond the cluster members, exhibiting large-scale features that suggest a significant past dynamical disturbance. This work represents a first taste of the enhanced power JWST will have for lensing-related science.

High-resolution imaging of galaxies in rest-frame UV has revealed the existence of giant star-forming clumps prevalent in high-redshift galaxies. Studying these substructures provides important information about their formation and evolution and informs theoretical galaxy evolution models. We present a new method to identify clumps in galaxies’ high-resolution rest-frame UV images. Using imaging data from CANDELS and UVCANDELS, we identify star-forming clumps in an HST/F160W ≤ 25 AB mag sample of 6767 galaxies at 0.5 ≤ z ≤ 3 in four fields, GOODS-N, GOODS-S, EGS, and COSMOS. We use a low-passband filter in Fourier space to reconstruct the background image of a galaxy and detect small-scale features (clumps) on the background-subtracted image. Clumpy galaxies are defined as those having at least one off-center clump that contributes a minimum of 10% of the galaxy’s total rest-frame UV flux. We measure the fraction of clumpy galaxies (f clumpy) as a function of stellar mass, redshift, and galaxy environment. Our results indicate that f clumpy increases with redshift, reaching ∼65% at z ∼ 1.5. We also find that f clumpy in low-mass galaxies ( 9.5≤log(M*/M⊙)≤10 ) is 10% higher compared to that of their high-mass counterparts ( log(M*/M⊙)>10.5 ). Moreover, we find no evidence of significant environmental dependence of f clumpy for galaxies at the redshift range of this study. Our results suggest that the fragmentation of gas clouds under violent disk instability remains the primary driving mechanism for clump formation, and incidents common in dense environments, such as mergers, are not the dominant processes.

We present a study of Lyman continuum (LyC) emission in a sample of ∼150 Lyα emitters (LAEs) at z ≈ 3.1 in the Subaru-XMM Deep Survey field. These LAEs were previously selected using the narrowband technique and spectroscopically confirmed with Lyα equivalent widths (EWs) ≥ 45 Å. We obtain deep UV images using a custom intermediate-band filter U J that covers a wavelength range of 3330 ∼ 3650 Å, corresponding to 810 ∼ 890 Å in the rest frame. We detect five individual LyC galaxy candidates in the U J band, and their escape fractions (f esc) of LyC photons are roughly between 40% and 80%. This supports a previous finding that a small fraction of galaxies may have very high f esc. We find that the f esc values of the five LyC galaxies are not apparently correlated with other galaxy properties such as Lyα luminosity and EW, UV luminosity and slope, and star formation rate (SFR). This is partly due to the fact that these galaxies only represent a small fraction (∼3%) of our LAE sample. For the remaining LAEs that are not detected in U J, we stack their U J-band images and constrain their average f esc. The upper limit of the average f esc value is about 16%, consistent with the results in the literature. Compared with the non-LyC LAEs, the LyC LAEs tend to have higher Lyα luminosities, Lyα EWs, and SFRs, but their UV continuum slopes are similar to those of other galaxies.