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1. Metallicity Gradients in Modern Cosmological Simulations. II. The Role of Bursty versus Smooth Feedback at High Redshift

   https://doi.org/10.3847/1538-4357/ae561e  

   Garcia, Alex M; Torrey, Paul; Bhagwat, Aniket; Shen, Xuejian; Vogelsberger, Mark; McClymont, William; Nagarajan-Swenson, Jaya; Ridolfo, Sophia G; Zhu, Peixin; Zimmerman, Dhruv T; et al (April 2026, The Astrophysical Journal)

   Abstract The distribution of gas-phase metals within galaxies encodes the impact of stellar feedback on galactic evolution. At high redshift, when galaxies are rapidly assembling, feedback-driven outflows and turbulence can strongly reshape radial metallicity gradients. In this work, we use the FIRE-2, SPICE, Thesan Box, and Thesan Zoom cosmological simulations—spanning a range of stellar feedback, from bursty (time-variable) to smooth (steady)—to investigate how these feedback modes shape gas-phase metallicity gradients at 3 < z ≲ 11. Across all models, we find that galaxies with bursty feedback (FIRE-2, SPICE Bursty, and Thesan Zoom) develop systematically flatter (by factors of ∼2–10) metallicity gradients than those with smooth feedback (SPICE Smooth and Thesan Box), particularly at stellar massesM_⋆ > 10⁹M_⊙. These results demonstrate that bursty stellar feedback provides sufficient turbulence to prevent strong negative gradients from forming, while smooth stellar feedback does not generically allow for the efficient radial redistribution of metals, thereby keeping gradients steep. Finally, we compare with recent observations, finding that the majority—but, notably, not all—of the observed gradients may favor a bursty stellar feedback scenario. In all, these results highlight the utility of high-resolution observations of gas-phase metallicity at high redshift as a key discriminator of these qualitatively different feedback types. 

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2. 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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3. The High-Redshift Gas-Phase Mass–Metallicity Relation in FIRE-2

   https://doi.org/10.3847/2041-8213/ad4cee  

   Marszewski, Andrew; Sun, Guochao; Faucher-Giguère, Claude-André; Hayward, Christopher C; Feldmann, Robert (May 2024, The Astrophysical Journal Letters)

   Abstract The unprecedented infrared spectroscopic capabilities of JWST have provided high-quality interstellar medium metallicity measurements and enabled characterization of the gas-phase mass–metallicity relation (MZR) for galaxies atz≳ 5 for the first time. We analyze the gas-phase MZR and its evolution in a high-redshift suite of FIRE-2 cosmological zoom-in simulations atz= 5–12 and for stellar massesM_*∼ 10⁶–10¹⁰M_⊙. These simulations implement a multichannel stellar feedback model and produce broadly realistic galaxy properties, including when evolved toz= 0. The simulations predict very weak redshift evolution of the MZR over the redshift range studied, with the normalization of the MZR increasing by less than 0.01 dex as redshift decreases fromz= 12 toz= 5. The median MZR in the simulations is well approximated as a constant power-law relation across this redshift range given by $\log \left(Z/Z_{\odot }\right)=0.37\log \left(M_{*}/{\mathrm{M}}_{\odot }\right)-4.3$ . We find good agreement between our best-fit model and recent observations made by JWST at high redshift. The weak evolution of the MZR atz> 5 contrasts with the evolution atz≲ 3, where increasing normalization of the MZR with decreasing redshift is observed and predicted by most models. The FIRE-2 simulations predict increasing scatter in the gas-phase MZR with decreasing stellar mass, in qualitative agreement with some observations. 

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4. LEGA-C stellar population scaling relations: II. Dissecting mass-complete archaeological trends and their evolution since z  ∼ 0.7 with LEGA-C and SDSS

   https://doi.org/10.1051/0004-6361/202554981  

   Gallazzi, Anna R; Zibetti, Stefano; van_der_Wel, Arjen; Nersesian, Angelos; Kaushal, Yasha; Bezanson, Rachel; Mattolini, Daniele; Bell, Eric F; Scholz-Diaz, Laura; Leja, Joel; et al (April 2026, Astronomy & Astrophysics)

   We analysed a sample of 552 galaxies from the LEGA-C spectroscopic survey (0.6 < z < 0.77), for which we estimated the stellar population parameters by a Bayesian analysis of the stellar absorption features and photometry. We investigated the effect of the current star formation activity on light-weighted mean stellar ages and metallicities and their median trends with stellar mass or velocity dispersion. The bimodality in the global age–mass relation stems from the different age distributions in the quiescent and star-forming populations. No bimodality is observed in the stellar metallicity-mass relation, although quiescent and star-forming galaxies have different distributions in this parameter space. We identified a high-metallicity sequence populated by quiescent and weakly star-forming galaxies. At masses lower than 10^10.8 M_⊙, the median stellar metallicity–mass relation of star-forming galaxies steepens as a consequence of the increasing scatter towards lower stellar metallicities for galaxies with an increasing specific star formation rate at fixed mass. Relying on a consistent analysis of SDSS DR7 spectra and accounting for aperture corrections, we quantified the evolution of the volume-weighted stellar age and stellar metallicity scaling relations betweenz = 0.7 and the present. We found negligible evolution in the stellar metallicity–mass relation of quiescent galaxies and forM_* > 10¹¹ M_⊙galaxies in general. Lower-mass star-forming galaxies instead have typically lower metallicities than their local counterparts, indicating significant enrichment sincez ∼ 0.7 in the low-mass regime. Notably, the median of the stellar ages of the general population and of quiescent galaxies has changed by only 2 Gyr betweenz = 0.7 andz = 0.1, which is less than expected from cosmic ageing. Some quiescent galaxies must evolve passively to reach the old boundary of the local population. In order to explain the evolution of the median trends, however, both individual evolution through rejuvenation and/or minor merging that affects the outer galaxy regions and population evolution through quenching of massive metal-rich star-forming galaxies are required. 

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5. MSA-3D: Metallicity Gradients in Galaxies at z ∼ 1 with JWST/NIRSpec Slit-stepping Spectroscopy

   https://doi.org/10.3847/2041-8213/ada150  

   Ju, Mengting; Wang, Xin; Jones, Tucker; Barišić, Ivana; Nanayakkara, Themiya; Bundy, Kevin; Faucher-Giguère, Claude-André; Feng, Shuai; Glazebrook, Karl; Henry, Alaina; et al (January 2025, The Astrophysical Journal Letters)

   Abstract The radial gradient of gas-phase metallicity is a powerful probe of the chemical and structural evolution of star-forming galaxies, closely tied to disk formation and gas kinematics in the early Universe. We present spatially resolved chemical and dynamical properties for a sample of 25 galaxies at 0.5 ≲ z ≲ 1.7 from theMSA-3Dsurvey. These innovative observations provide 3D spectroscopy of galaxies at a spatial resolution approaching JWST’s diffraction limit and a high spectral resolution ofR ≃ 2700. The metallicity gradients measured in our galaxy sample range from −0.03 to 0.02 dex kpc⁻¹. Most galaxies exhibit negative or flat radial gradients, indicating lower metallicity in the outskirts or uniform metallicity throughout the entire galaxy. We confirm a tight relationship between stellar mass and metallicity gradient atz ∼ 1 with small intrinsic scatter of 0.02 dex kpc⁻¹. Our results indicate that metallicity gradients become increasingly negative as stellar mass increases, likely because the more massive galaxies tend to be more “disky.” This relationship is consistent with the predictions from cosmological hydrodynamic zoom-in simulations with strong stellar feedback. This work presents the effort to harness the multiplexing capability of the JWST NIRSpec microshutter assembly in slit-stepping mode to map the chemical and kinematic profiles of high-redshift galaxies in large samples and at high spatial and spectral resolution. 

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