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1. MOSDEF-3D: Keck/OSIRIS Maps of the Ionized Interstellar Medium in z ∼ 2 Galaxies

   https://doi.org/10.3847/1538-4357/ae5704  

   Lam, Natalie; Shapley, Alice E; Sanders, Ryan L; Do, Tuan; Jones, Tucker; Coil, Alison; Kriek, Mariska; Mobasher, Bahram; Reddy, Naveen A; Siana, Brian; et al (April 2026, The Astrophysical Journal)

   Abstract We present spatially resolved rest-frame optical emission line maps of four galaxies atz ∼ 2 observed with the Keck/OH-Suppressing Infra-Red Imaging Spectrograph to study the physical conditions of the interstellar medium (ISM) at cosmic noon. Our analysis of strong emission line ratios in these galaxies reveals an offset from the local star-forming locus on the Baldwin–Phillips–Terlevich diagram but agrees with other star-forming galaxies at similar redshifts. Despite the offset toward higher [Oiii]λ5008/Hβand [Nii]λ6585/Hα, these strong line ratios remain consistent with or below the maximum starburst threshold even in the inner ∼1 kpc region of the galaxies, providing no compelling evidence for central active galactic nucleus activity. The galaxies also exhibit flat radial gas-phase metallicity gradients, consistent with previous studies ofz ∼ 2 galaxies and suggesting efficient radial mixing possibly driven by strong outflows from intense star formation. Overall, our results reveal the highly star-forming nature of these galaxies, with the potential to launch outflows that flatten metallicity gradients through significant radial gas mixing. Future observations with JWST/NIRSpec are crucial to detect fainter emission lines at higher spatial resolution to further constrain the physical processes and ionization mechanisms that shape the ISM during cosmic noon. 

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2. Emission-line Diagnostics at z  ≳  2: A Probe of the Ionizing Spectrum and α -enhancement beyond Cosmic Noon

   https://doi.org/10.3847/2041-8213/ae57a6  

   Clarke, Leonardo; Lam, Natalie; Shapley, Alice E; Topping, Michael W; Brammer, Gabriel B; Sanders, Ryan L; Reddy, Naveen A; Karthikeyan, Shreya (April 2026, The Astrophysical Journal Letters)

   Abstract We analyze several key rest-optical emission-line ratios in a sample of 828 galaxies as well as composite spectra from JADES DR3 in the range 1.4 < z < 7. These emission-line ratios include: [Oiii]λ5008/Hβ, [Nii]λ6585/Hα, [Sii]λλ6718, 6733/Hα, [Oi]λ6302/Hα, O32, R23, Ne3O2, and RO2Ne3. We find evidence for a harder ionizing spectrum atz ∼ 3.5 compared toz ∼ 2 at fixed gas-phase metallicity, resulting in a pronounced shift in the star-forming galaxy locus on the [Nii]/HαBaldwin-Phillips-Terlevich (BPT) diagram and the O32 versus R23 diagram. Atz ≳ 3.5, star-forming galaxies occupy a common locus, indicating that interstellar medium (ISM) ionizing conditions at fixed gas-phase metallicity do not evolve strongly at these early cosmic times. There is a connection between ISM ionizing conditions and the chemical abundance patterns (i.e.,α/Fe) in massive stars, providing the ionizing radiation field. Therefore, the lack of evolution in ISM ionizing conditions atz ≳ 3.5, followed by evolution towards a softer ionizing spectrum at fixed nebular metallicity as cosmic time proceeds toz ∼ 2 and lower redshift mirrors the chemical abundance patterns in Milky Way stars as a function of iron abundance. Our results highlight the diagnostic power of emission-line diagrams in the era of JWST to further our understanding of the ISM conditions into the Epoch of Reionization. 

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3. The MOSDEF-LRIS Survey: The Interplay Between Massive Stars and Ionized Gas in High-Redshift Star-Forming Galaxies1

   https://doi.org/10.1093/mnras/staa1410  

   Topping, Michael W; Shapley, Alice E; Reddy, Naveen A; Sanders, Ryan L; Coil, Alison L; Kriek, Mariska; Mobasher, Bahram; Siana, Brian (July 2020, Monthly Notices of the Royal Astronomical Society)

   null (Ed.)

   Abstract We present a joint analysis of rest-UV and rest-optical spectra obtained using Keck/LRIS and Keck/MOSFIRE for a sample of 62 star-forming galaxies at z ∼ 2.3. We divide our sample into two bins based on their location in the [OIII]5007/Hβ vs. [NII]6584/Hα BPT diagram, and perform the first differential study of the rest-UV properties of massive ionizing stars as a function of rest-optical emission-line ratios. Fitting BPASS stellar population synthesis models, including nebular continuum emission, to our rest-UV composite spectra, we find that high-redshift galaxies offset towards higher [OIII]λ5007/Hβ and [NII]λ6584/Hα have younger ages ($$\log (\textrm {~Age/yr})=7.20^{+0.57}_{-0.20}$$) and lower stellar metallicities ($$Z_*=0.0010^{+0.0011}_{-0.0003}$$) resulting in a harder ionizing spectrum, compared to the galaxies in our sample that lie on the local BPT star-forming sequence ($$\log (\textrm {Age/yr})=8.57^{+0.88}_{-0.84}$$, $$Z_*=0.0019^{+0.0006}_{-0.0006}$$). Additionally, we find that the offset galaxies have an ionization parameter of $$\log (U)=-3.04^{+0.06}_{-0.11}$$ and nebular metallicity of ($$12+\log (\textrm {~O/H})=8.40^{+0.06}_{-0.07}$$), and the non-offset galaxies have an ionization parameter of $$\log (U)=-3.11^{+0.08}_{-0.08}$$ and nebular metallicity of $$12+\log (\textrm {~O/H})=8.30^{+0.05}_{-0.06}$$. The stellar and nebular metallicities derived for our sample imply that the galaxies offset from the local BPT relation are more α-enhanced ($$7.28^{+2.52}_{-2.82}\textrm {~O/Fe}_{\odot }$$) compared to those consistent with the local sequence ($$3.04^{+0.95}_{-0.54}\textrm {~O/Fe}_{\odot }$$). However, even galaxies that are entirely consistent with the local nebular excitation sequence appear to be α-enhanced – in contrast with typical local systems. Such differences must be considered when estimating gas-phase oxygen abundances at high redshift based on strong emission-line ratios. Specifically, a similarity in the location of high-redshift and local galaxies in the BPT diagram may not be indicative of a similarity in their physical properties. 

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4. A dense, solar metallicity ISM in the z = 4.2 dusty star-forming galaxy SPT 0418−47

   https://doi.org/10.1051/0004-6361/201936169  

   De Breuck, Carlos; Weiß, Axel; Béthermin, Matthieu; Cunningham, Daniel; Apostolovski, Yordanka; Aravena, Manuel; Archipley, Melanie; Chapman, Scott; Chen, Chian-Chou; Fu, Jianyang; et al (November 2019, Astronomy & Astrophysics)

   We present a study of six far-infrared fine structure lines in the z = 4.225 lensed dusty star-forming galaxy SPT 0418−47 to probe the physical conditions of its interstellar medium (ISM). In particular, we report Atacama Pathfinder EXperiment (APEX) detections of the [OI] 145 μ m and [OIII] 88 μ m lines and Atacama Compact Array (ACA) detections of the [NII] 122 and 205 μ m lines. The [OI] 145 μ m/[CII] 158 μ m line ratio is ∼5× higher compared to the average of local galaxies. We interpret this as evidence that the ISM is dominated by photo-dissociation regions with high gas densities. The line ratios, and in particular those of [OIII] 88 μ m and [NII] 122 μ m imply that the ISM in SPT 0418−47 is already chemically enriched to nearly solar metallicity. While the strong gravitational amplification was required to detect these lines with APEX, larger samples can be observed with the Atacama Large Millimeter/submillimeter Array (ALMA), and should allow observers to determine if the dense, solar metallicity ISM is common among these highly star-forming galaxies. 

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5. Galaxy Metallicity Gradients in the Epoch of Reionization from the FIRE-2 Simulations

   https://doi.org/10.3847/1538-4357/ae5f78  

   Sun, Xunda; Wang, Xin; Jiang, Fangzhou; Mo, Houjun; Ho, Luis C; Zhou, Qianqiao; Ma, Xiangcheng; Zhan, Hu; Wetzel, Andrew; Graf, Russell L; et al (May 2026, The Astrophysical Journal)

   We employ the high-redshift suite of FIRE-2 cosmological hydrodynamic zoom-in simulations to investigate the evolution of gas-phase metallicity radial gradients in galaxies in the epoch of reionization (EOR). Our sample consists of 22 galaxies spanning the redshift rangez ∼ 10–5. We find that galaxies atz ∼ 10 exhibit a median metallicity gradient of −0.15 dex kpc⁻¹with substantial scatter, which gradually flattens to −0.1 dex kpc⁻¹atz ∼ 6, accompanied by a reduction in scatter. In the EOR, metallicity gradients correlate positively with stellar mass: more massive galaxies display flatter gradients with smaller scatter, broadly consistent with recent JWST observations. At fixed stellar mass, galaxies with a higher star formation rate (SFR) exhibit steeper negative gradients, while the specific SFR shows a strong anticorrelation with gradient slope. Because EOR galaxies in FIRE-2 generally lack significant rotational support, we adopt the ratio of the peak-to-peak velocity shear to twice the velocity dispersion (Δv/2σ) as a proxy for the strength of gas flows. We find a strong positive correlation between metallicity gradients and Δv/2σ: galaxies with a lower Δv/2σ(i.e., weaker gas flows) tend to exhibit steeper negative gradients. Furthermore, galaxies with a steeper gradient display higher central SFR surface densities, suggesting localized star formation with inefficient interstellar medium mixing that drives inside-out chemical enrichment in galaxy evolution in the early Universe. 

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