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1. The Physical Conditions of Emission-line Galaxies at Cosmic Dawn from JWST/NIRSpec Spectroscopy in the SMACS 0723 Early Release Observations

   https://doi.org/10.3847/1538-4357/acba8a  

   Trump, Jonathan_R; Arrabal_Haro, Pablo; Simons, Raymond_C; Backhaus, Bren_E; Amorín, Ricardo_O; Dickinson, Mark; Fernández, Vital; Papovich, Casey; Nicholls, David_C; Kewley, Lisa_J; et al (March 2023, The Astrophysical Journal)

   Abstract We present rest-frame optical emission-line flux ratio measurements for fivez> 5 galaxies observed by the James Webb Space Telescope Near-Infared Spectrograph (NIRSpec) in the SMACS 0723 Early Release Observations. We add several quality-control and post-processing steps to the NIRSpec pipeline reduction products in order to ensure reliablerelativeflux calibration of emission lines that are closely separated in wavelength, despite the uncertainabsolutespectrophotometry of the current version of the reductions. Compared toz∼ 3 galaxies in the literature, thez> 5 galaxies have similar [Oiii]λ5008/Hβratios, similar [Oiii]λ4364/Hγratios, and higher (∼0.5 dex) [NeIII]λ3870/[OII]λ3728 ratios. We compare the observations to MAPPINGS V photoionization models and find that the measured [NeIII]λ3870/[OII]λ3728, [Oiii]λ4364/Hγ, and [Oiii]λ5008/Hβemission-line ratios are consistent with an interstellar medium (ISM) that has very high ionization ( $\log \left(Q\right)\simeq 8-9$, units of cm s⁻¹), low metallicity (Z/Z_⊙≲ 0.2), and very high pressure ( $\log \left(P/k\right)\simeq 8-9$, units of cm⁻³). The combination of [Oiii]λ4364/Hγand [Oiii]λ(4960 + 5008)/Hβline ratios indicate very high electron temperatures of $4.1<\log \left(T_e/\mathrm{K}\right)<4.4$, further implying metallicities ofZ/Z_⊙≲ 0.2 with the application of low-redshift calibrations for “T_e-based” metallicities. These observations represent a tantalizing new view of the physical conditions of the ISM in galaxies at cosmic dawn. 

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2. Efficient Ionizers with Low H β + [O iii ] Equivalent Widths: JADES Spectroscopy of a Peculiar High-redshift Population

   https://doi.org/10.3847/1538-4357/adddb5  

   Laseter, Isaac H; Maseda, Michael V; Simmonds, Charlotte; Endsley, Ryan; Stark, Daniel; Bunker, Andrew J; Bhatawdekar, Rachana; Boyett, Kristan; Cameron, Alex J; Carniani, Stefano; et al (July 2025, The Astrophysical Journal)

   Abstract Early JWST photometric studies discovered a population of UV-faint ( $<{\mathit{L}}_{\mathrm{UV}}^{*}$)z ∼ 6.5–8 Lyman break galaxies with spectral energy distributions implying young ages (∼10 Myr) yet relatively weak Hβ+ [Oiii] equivalent widths (EW_{Hβ+ [Oiii]} ≈ 400 Å). These galaxies seemingly contradict the implicit understanding that young star-forming galaxies are ubiquitously strong Hβ+ [Oiii] emitters, i.e., extreme emission line galaxies (EW ≳750 Å). Low metallicities, high Lyman continuum escape fractions, and rapidly declining star formation histories have been proposed as primary drivers behind low Hβ+ [Oiii] EWs, but the blend of Hβ+ [Oiii] in photometric studies makes proving one of these scenarios difficult. We aim to characterize this peculiar population with deep spectroscopy from the JWST Advanced Deep Extragalactic Survey. We find that a significant subset of these galaxies atz ≳ 2 with modest Hβ+ [Oiii] EWs (≈300–600 Å) have high ionization efficiencies ( $\log {\xi }_{\mathrm{ion}}\gtrsim 25.5\mathrm{Hz}{\mathrm{erg}}^{-1}$). Suppressed [Oiii] EW values yet elevated Hαand HβEW values imply that the level of chemical enrichment is the primary culprit, supported by spectroscopic measurements of metallicities below 12 + log(O/H) ≈ 7.70 (0.1Z_⊙). We demonstrate that integrated Hβ+ [Oiii] selections (e.g., Hβ+ [Oiii] EW > 700 Å) exclude the most metal-poor efficient ionizers and favor (1) more chemically enriched systems with comparable extreme radiation fields and (2) older starbursting systems. In contrast, metallicity degeneracies are reduced in Hαspace, enabling the identification of these metal-poor efficient ionizers by their specific star formation rate. 

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3. The Resolved Structure of a Low-metallicity Photodissociation Region

   https://doi.org/10.3847/1538-4357/adef38  

   Clark, Ilyse Y; Sandstrom, Karin; Wolfire, Mark; Bolatto, Alberto D; Chastenet, Jérémy; Dale, Daniel A; Gaches, Brandt_A L; Glover, Simon_C O; Goicoechea, Javier R; Gordon, Karl D; et al (September 2025, The Astrophysical Journal)

   Abstract Photodissociation regions (PDRs) are key to understanding the feedback processes that shape interstellar matter in galaxies. One important type of PDR is the interface between Hiiregions and molecular clouds, where far-ultraviolet radiation from massive stars heats gas and dissociates molecules. Photochemical models predict that as metallicity decreases, the C/CO transition occurs at greater depths in the PDR compared to the H/H₂transition, increasing the extent of CO-dark H₂gas in low-metallicity environments. This prediction has been difficult to test outside the Milky Way due to the lack of high-spatial-resolution observations tracing H₂and CO. This study examines a low-metallicity PDR in the N13 region of the Small Magellanic Cloud (SMC), where we spatially resolve the ionization front, the H₂dissociation front, and the C/CO transition using¹²COJ= 2−1, 3−2, and [CI] 1–0 observations from the Atacama Large Millimeter/submillimeter Array and near-infrared spectroscopy of the H₂2.12 1–0 S(1) vibrational line, and H recombination lines from the James Webb Space Telescope. Our analysis shows that the separation between the H/H₂and C/CO boundaries is approximately 0.043 ± 0.013(stat.) ± 0.0036(syst.) pc (equivalent to $0\stackrel{″}{.}146\pm 0\stackrel{″}{.}042\left(\mathrm{stat.}\right)\pm 0\stackrel{″}{.}012\left(\mathrm{syst.}\right)$at the SMC’s distance of 62 kpc), defining the spatial extent of the CO-dark H₂region. Compared to our plane-parallel PDR models, we find that a constant-pressure model matches the observed structure better than a constant-density one. Overall, we find that the PDR model does well at predicting the extent of the CO-dark H₂layer in N13. This study represents the first resolved benchmark for low-metallicity PDRs. 

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4. A Metallicity Dependence on the Occurrence of Core-collapse Supernovae

   https://doi.org/10.3847/2041-8213/acf7c6  

   Pessi, Thallis; Anderson, Joseph P; Lyman, Joseph D; Prieto, Jose L; Galbany, Lluís; Kochanek, Christopher S; Sánchez, Sebastian F; Kuncarayakti, Hanindyo (September 2023, The Astrophysical Journal Letters)

   Abstract Core-collapse supernovae (CCSNe) are widely accepted to be caused by the explosive death of massive stars with initial masses ≳8M_⊙. There is, however, a comparatively poor understanding of how properties of the progenitors—mass, metallicity, multiplicity, rotation, etc.—manifest in the resultant CCSN population. Here, we present a minimally biased sample of nearby CCSNe from the All-Sky Automated Survey for Supernovae survey whose host galaxies were observed with integral-field spectroscopy using MUSE at the Very Large Telescope. This data set allows us to analyze the explosion sites of CCSNe within the context of global star formation properties across the host galaxies. We show that the CCSN explosion site oxygen abundance distribution is offset to lower values than the overall Hiiregion abundance distribution within the host galaxies. We further split the sample at $12+{\log }_{10}\left(\mathrm{O}/\mathrm{H}\right)=8.6$dex and show that within the subsample of low-metallicity host galaxies, the CCSNe unbiasedly trace the star formation with respect to oxygen abundance, while for the subsample of higher-metallicity host galaxies, they preferentially occur in lower-abundance star-forming regions. We estimate the occurrence of CCSNe as a function of oxygen abundance per unit star formation and show that there is a strong decrease as abundance increases. Such a strong and quantified metallicity dependence on CCSN production has not been shown before. Finally, we discuss possible explanations for our result and show that each of these has strong implications not only for our understanding of CCSNe and massive star evolution but also for star formation and galaxy evolution. 

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5. GLIMPSE: An Ultrafaint ≃10 ⁵ M _⊙ Pop III Galaxy Candidate and First Constraints on the Pop III UV Luminosity Function at z  ≃  6–7

   https://doi.org/10.3847/1538-4357/ade9a1  

   Fujimoto, Seiji; Naidu, Rohan P; Chisholm, John; Atek, Hakim; Endsley, Ryan; Kokorev, Vasily; Furtak, Lukas J; Pan, Richard; Liu, Boyuan; Bromm, Volker; et al (August 2025, The Astrophysical Journal)

   Abstract Detecting the first generation of stars, Population III (Pop III), has been a long-standing goal in astrophysics, yet they remain elusive even in the JWST era. Here we present a novel NIRCam-based selection method for Pop III galaxies, and carefully validate it through completeness and contamination simulations. We systematically search ≃ 500 arcmin²across JWST legacy fields for Pop III candidates, including GLIMPSE, which, assisted by gravitational lensing, has produced JWST’s deepest NIRCam imaging thus far. We discover one promising Pop III galaxy candidate (GLIMPSE-16043) at $z=6.50_{-0.24}^{+0.03}$, a moderately lensed galaxy ( $\mu =2.9_{-0.2}^{+0.1}$) with an intrinsic UV magnitude of $M_{\mathrm{UV}}=-15.89_{-0.14}^{+0.12}$. It exhibits key Pop III features: strong Hαemission (rest-frame EW 2810 ± 550 Å); a Balmer jump; no dust (UV slopeβ = −2.34 ± 0.36); and undetectable metal lines (e.g., [Oiii]; [Oiii]/Hβ < 0.44), implying a gas-phase metallicity ofZ_gas/Z_⊙ < 0.5%. These properties indicate the presence of a nascent, metal-deficient young stellar population (<5 Myr) with a stellar mass of ≃10⁵M_⊙. Intriguingly, this source deviates significantly from the extrapolated UV–metallicity relation derived from recent JWST observations atz= 4–10, consistent with UV enhancement by a top-heavy Pop III initial mass function or the presence of an extremely metal-poor active galactic nucleus. We also derive the first observational constraints on the Pop III UV luminosity function atz ≃ 6–7. The volume density of GLIMPSE-16043 (≈10⁻⁴cMpc⁻³) is in excellent agreement with theoretical predictions, independently reinforcing its plausibility. This study demonstrates the power of our novel NIRCam method to finally reveal distant galaxies even more pristine than the Milky Way’s most metal-poor satellites, thereby promising to bring us closer to the first generation of stars than we have ever been before. 

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