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JWST spectroscopy has revolutionized our understanding of galaxies in the early Universe. Covering wavelengths up to 5.3 μ m, NIRSpec can detect rest-frame optical H α emission lines out to z = 7 and [O iii ] to z = 9.5. Observing these lines in more distant galaxies requires longer wavelength spectroscopy with MIRI. Here we present MIRI Medium Resolution Spectrograph integral field unit observations of the lensed galaxy merger MACS0647–JD at z = 10.165. With exposure times of 4.2 hr in each of two bands (SHORT and LONG), we detect H α at 9 σ , [O iii ] λ 5008 at 11 σ , and [O iii ] λ 4960 at 3 σ . Combined with previously reported NIRSpec spectroscopy that yielded seven emission lines including the auroral line [O iii ] λ 4363, we present the first direct metallicity measurement of a z > 10 galaxy: $12+\mathrm{log}({\rm{O}}/{\rm{H}})=7.79\pm 0.09$ , or ${0.13}_{-0.03}^{+0.02}\,{Z}_{\odot }$ . This is similar to galaxies at z ∼ 4–9 with direct metallicity measurements, though higher than expected given the high specific star formation rate log(sSFR/yr ^−1 ) = −7.4 ± 0.3. We further constrain the ionization parameter $\mathrm{log}(U)$ = −1.9 ± 0.1, ionizing photon production efficiency log( ξ _ion ) = 25.3 ± 0.1, and SFR = 5.0 ± 0.6 M _⊙ yr ^−1 within the past 10 Myr. These observations demonstrate the combined power of JWST NIRSpec and MIRI for studying galaxies in the first 500 million years.

We present JWST/NIRSpec prism spectroscopy of MACS0647−JD, a triply lensed z ∼ 11 candidate discovered in Hubble Space Telescope imaging and spatially resolved by JWST imaging into two components, A and B. Spectroscopy of component A yields a spectroscopic redshift z = 10.17 based on seven detected emission lines: C iii ] λ λ 1907, 1909, [O ii ] λ 3727, [Ne iii ] λ 3869, [Ne iii ] λ 3968, H δ λ 4101, H γ λ 4340, and [O iii ] λ 4363. These are the second-most distant detections of these emission lines to date, in a galaxy observed just 460 million years after the Big Bang. Based on observed and extrapolated line flux ratios we derive a gas-phase metallicity 12 + log(O/H) ∼ 7.5–8.0, or Z ∼ (0.06–0.2) Z _⊙ , ionization parameter $\mathrm{log}(U)$ = −1.9 ± 0.2, and an ionizing photon production efficiency $\mathrm{log}({\xi }_{\mathrm{ion}})=25.2\pm 0.2\,$ erg ^−1 Hz. The spectrum has a softened Ly α break, evidence for a strong Ly α damping wing. The Ly α damping wing also suppresses the F150W photometry, explaining the slightly overestimated photometric redshift z = 10.6 ± 0.3. MACS0647−JD has a stellar mass log( M / M _⊙ ) = 8.1 ± 0.3, including ∼6 × 10 ^7 M _⊙ in component A, most of which formed recently (within ∼20 Myr) with a star formation rate ∼ 2 ± 1 M _⊙ yr ^−1 , all within an effective radius 70 ± 24 pc. Spectroscopy of a fainter companion galaxy C separated by a distance of ∼ 3 kpc reveals a Lyman break consistent with z ∼ 10.17. MACS0647−JD is likely the most distant galaxy merger known.

We present results of [C ii ] 158 μ m emission line observations, and report the spectroscopic redshift confirmation of a strongly lensed ( μ ∼ 20) star-forming galaxy, MACS0308-zD1 at z = 6.2078 ± 0.0002. The [C ii ] emission line is detected with a signal-to-noise ratio >6 within the rest-frame UV-bright clump of the lensed galaxy (zD1.1) and exhibits multiple velocity components; the narrow [C ii ] has a velocity full width half maximum (FWHM) of 110 ± 20 km s ^−1 , while broader [C ii ] is seen with an FWHM of 230 ± 50 km s ^−1 . The broader [C ii ] component is blueshifted (−80 ± 20 km s ^−1 ) with respect to the narrow [C ii ] component, and has a morphology that extends beyond the UV-bright clump. We find that, while the narrow [C ii ] emission is most likely associated with zD1.1, the broader component is possibly associated with a physically distinct gas component from zD1.1 (e.g., outflowing or inflowing gas). Based on the nondetection of λ _158 _μ _m dust continuum, we find that MACS0308-zD1's star formation activity occurs in a dust-free environment indicated by a strong upper limit of infrared luminosity ≲9 × 10 ^8 L _⊙ . Targeting this strongly lensed faint galaxy for follow-up Atacama Large Millimeter/submillimeter Array and JWST observations will be crucial to characterize the details of typical galaxy growth in the early Universe.

MACS0647–JD is a triply lensed z ∼ 11 galaxy originally discovered with the Hubble Space Telescope. The three lensed images are magnified by factors of ∼8, 5, and 2 to AB mag 25.1, 25.6, and 26.6 at 3.5 μ m. The brightest is over a magnitude brighter than other galaxies recently discovered at similar redshifts z > 10 with JWST. Here, we report new JWST imaging that clearly resolves MACS0647–JD as having two components that are either merging galaxies or stellar complexes within a single galaxy. The brighter larger component “A” is intrinsically very blue ( β ∼ −2.6 ± 0.1), likely due to very recent star formation and no dust, and is spatially extended with an effective radius ∼70 ± 24 pc. The smaller component “B” ( r ∼ 20 ${}_{-5}^{+8}\,$ pc) appears redder ( β ∼ −2 ± 0.2), likely because it is older (100–200 Myr) with mild dust extinction ( A _V ∼ 0.1 mag). With an estimated stellar mass ratio of roughly 2:1 and physical projected separation ∼400 pc, we may be witnessing a galaxy merger 430 million years after the Big Bang. We identify galaxies with similar colors in a high-redshift simulation, finding their star formation histories to be dissimilar, which is also suggested by the spectral energy distribution fitting, suggesting they formed further apart. We also identify a candidate companion galaxy “C” ∼3 kpc away, likely destined to merge with A and B. Upcoming JWST Near Infrared Spectrograph observations planned for 2023 January will deliver spectroscopic redshifts and more physical properties for these tiny magnified distant galaxies observed in the early universe.

We report the discovery of two extremely magnified lensed star candidates behind the galaxy cluster MACS J0647.7+015 using recent multiband James Webb Space Telescope (JWST) NIRCam observations. The star candidates are seen in a previously known, z _phot ≃ 4.8 dropout giant arc that straddles the critical curve. The candidates lie near the expected critical curve position, but lack clear counter-images on the other side of it, suggesting these are possibly stars undergoing caustic crossings. We present revised lensing models for the cluster, including multiply imaged galaxies newly identified in the JWST data, and use them to estimate background macro-magnifications of at least ≳90 and ≳50 at the positions of the two candidates, respectively. With these values, we expect effective, caustic-crossing magnifications of ∼[10 ^3 –10 ^5 ] for the two star candidates. The spectral energy distributions of the two candidates match well the spectra of B-type stars with best-fit surface temperatures of ∼10,000 K, and ∼12,000 K, respectively, and we show that such stars with masses ≳20 M _⊙ and ≳50 M _⊙ , respectively, can become sufficiently magnified to be observable. We briefly discuss other alternative explanations and conclude that these objects are likely lensed stars, but also acknowledge that the less-magnified candidate may alternatively reside in a star cluster. These star candidates constitute the second highest-redshift examples to date after Earendel at z _phot ≃ 6.2, establishing further the potential of studying extremely magnified stars at high redshifts with JWST. Planned future observations, including with NIRSpec, will enable a more detailed view of these candidates in the near future.

We present the results of 300 pc resolution Atacama Large Millimeter/submillimeter Array imaging of the [O iii ] 88 μm line and dust continuum emission from a z = 8.312 Lyman-break galaxy MACS0416_Y1. The velocity-integrated [O iii ] emission has three peaks that are likely associated with three young stellar clumps of MACS0416_Y1, while the channel map shows a complicated velocity structure with little indication of a global velocity gradient unlike what was found in [C ii ] 158 μm at a larger scale, suggesting random bulk motion of ionized gas clouds inside the galaxy. In contrast, dust emission appears as two individual clumps apparently separating or bridging the [O iii ]/stellar clumps. The cross-correlation coefficient between dust and ultraviolet-related emission (i.e., [O iii ] and ultraviolet continuum) is unity on a galactic scale, while it drops at

We present integral field, far-infrared (FIR) spectroscopy of Mrk 54, a local Lyman continuum emitter, obtained with FIFI-LS on the Stratospheric Observatory for Infrared Astronomy. This is only the second time, after Haro 11, that [C ii ] 158 μ m and [O iii ] 88 μ m spectroscopy of the known LCEs have been obtained. We find that Mrk 54 has a strong [C ii ] emission that accounts for ∼1% of the total FIR luminosity, whereas it has only moderate [O iii ] emission, resulting in the low [O iii ]/[C ii ] luminosity ratio of 0.22 ± 0.06. In order to investigate whether [O iii ]/[C ii ] is a useful tracer of f _esc (LyC escape fraction), we examine the correlations of [O iii ]/[C ii ] and (i) the optical line ratio of O _32 ≡ [O iii ] 5007 Å/[O ii ] 3727 Å, (ii) specific star formation rate, (iii) [O iii ] 88 μ m/[O i ] 63 μ m ratio, (iv) gas-phase metallicity, and (v) dust temperature based on a combined sample of Mrk 54 and the literature data from the Herschel Dwarf Galaxy Survey and the LITTLE THINGS Survey. We find that galaxies with high [O iii ]/[C ii ] luminosity ratios could be the result of high ionization (traced by O _32 ), bursty star formation, high ionized-to-neutral gas volume filling factors (traced by [O iii ] 88 μ m/[O i ] 63 μ m), and low gas-phase metallicities, which is in agreement with theoretical predictions. We present an empirical relation between the [O iii ]/[C ii ] ratio and f _esc based on the combination of the [O iii ]/[C ii ] and O _32 correlation, and the known relation between O _32 and f _esc . The relation implies that high-redshift galaxies with high [O iii ]/[C ii ] ratios revealed by the Atacama Large Millimeter/submillimeter Array may have f _esc ≳ 0.1, significantly contributing to the cosmic reionization.

Detailed observations of star-forming galaxies at high redshift are critical to understanding the formation and evolution of the earliest galaxies. Gravitational lensing provides an important boost, allowing observations at physical scales unreachable in unlensed galaxies. We present three lensed galaxies from the RELICS survey at z _phot = 6–10, including the most highly magnified galaxy at z _phot ∼ 6 (WHL 0137–zD1, dubbed the Sunrise Arc), the brightest known lensed galaxy at z _phot ∼ 6 (MACS 0308–zD1), and the only spatially resolved galaxy currently known at z _phot ∼ 10 (SPT 0615–JD). The Sunrise Arc contains seven star-forming clumps with delensed radii as small as 3 pc, the smallest spatial scales yet observed in a z > 6 galaxy, while SPT 0615–JD contains features measuring a few tens of parsecs. MACS 0308–zD1 contains an r ∼ 30 pc clump with a star formation rate (SFR) of ∼3 M _⊙ yr ^−1 , giving it an SFR surface density of Σ _SFR ∼ 10 ^3 M _⊙ yr ^−1 kpc ^−2 . These galaxies provide a unique window into small-scale star formation during the epoch of reionization. They will be excellent targets for future observations with JWST, including one approved program targeting the Sunrise Arc.

We give an overview and describe the rationale, methods, and first results from NIRCam images of the JWST “Prime Extragalactic Areas for Reionization and Lensing Science” (PEARLS) project. PEARLS uses up to eight NIRCam filters to survey several prime extragalactic survey areas: two fields at the North Ecliptic Pole (NEP); seven gravitationally lensing clusters; two high redshift protoclusters; and the iconic backlit VV 191 galaxy system to map its dust attenuation. PEARLS also includes NIRISS spectra for one of the NEP fields and NIRSpec spectra of two high-redshift quasars. The main goal of PEARLS is to study the epoch of galaxy assembly, active galactic nucleus (AGN) growth, and First Light. Five fields—the JWST NEP Time-Domain Field (TDF), IRAC Dark Field, and three lensing clusters—will be observed in up to four epochs over a year. The cadence and sensitivity of the imaging data are ideally suited to find faint variable objects such as weak AGN, high-redshift supernovae, and cluster caustic transits. Both NEP fields have sightlines through our Galaxy, providing significant numbers of very faint brown dwarfs whose proper motions can be studied. Observations from the first spoke in the NEP TDF are public. This paper presents our first PEARLS observations, their NIRCam data reduction and analysis, our first object catalogs, the 0.9–4.5 μ m galaxy counts and Integrated Galaxy Light. We assess the JWST sky brightness in 13 NIRCam filters, yielding our first constraints to diffuse light at 0.9–4.5 μ m. PEARLS is designed to be of lasting benefit to the community.

In the cold dark matter scenario, dark matter halos are assembled hierarchically from smaller subunits. Some of these subunits are disrupted during the merging process, whereas others survive temporarily in the form of subhalos. A long-standing problem with this picture is that the number of subhalos predicted by simulations exceeds the number of luminous dwarf galaxies seen in the vicinity of large galaxies like the Milky Way. Many of the subhalos must therefore have remained dark or very faint. If cold dark matter subhalos are as common as predicted, gravitational lensing may in principle offer a promising route to detection. In this paper, we describe the many ways through which lensing by subhalos can manifest itself, and summarize the results from current efforts to constrain the properties of cold dark matter subhalos using such effects.