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1. The NIRVANDELS survey: the stellar and gas-phase mass-metallicity relations of star-forming galaxies at z = 3.5

   https://doi.org/10.1093/mnras/stae1705  

   Stanton, T M; Cullen, F; McLure, R J; Shapley, A E; Arellano-Córdova, K Z; Begley, R; Amorín, R; Barrufet, L; Calabrò, A; Carnall, A C; et al (July 2024, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT We present determinations of the gas-phase and stellar metallicities of a sample of 65 star-forming galaxies at $$z \simeq 3.5$$ using rest-frame far-ultraviolet (FUV) spectroscopy from the VANDELS survey in combination with follow-up rest-frame optical spectroscopy from VLT/KMOS and Keck/MOSFIRE. We infer gas-phase oxygen abundances ($$Z_{\mathrm{g}}$$; tracing O/H) via strong optical nebular lines and stellar iron abundances ($$Z_{\star }$$; tracing Fe/H) from full spectral fitting to the FUV continuum. Our sample spans the stellar mass range $$8.5 \lt \mathrm{log}(M_{\star }/\mathrm{M}_{\odot }) \lt 10.5$$ and shows clear evidence for both a stellar and gas-phase mass-metallicity relation (MZR). We find that our O and Fe abundance estimates both exhibit a similar mass-dependence, such that $$\mathrm{Fe/H}\propto M_{\star }^{0.30\pm 0.11}$$ and $$\mathrm{O/H}\propto M_{\star }^{0.32\pm 0.09}$$. At fixed $$M_{\star }$$ we find that, relative to their solar values, O abundances are systematically larger than Fe abundances (i.e. α-enhancement). We estimate an average enhancement of $$\mathrm{(O/Fe)} = 2.65 \pm 0.16 \times \mathrm{(O/Fe)_\odot }$$ which appears to be independent of $$M_{\star }$$. We employ analytic chemical evolution models to place a constraint on the strength of galactic-level outflows via the mass-outflow factor ($$\eta$$). We show that outflow efficiencies that scale as $$\eta \propto M_{\star }^{-0.32}$$ can simultaneously explain the functional form of of the stellar and gas-phase MZR, as well as the degree of α-enhancement at fixed Fe/H. Our results add further evidence to support a picture in which α-enhanced abundance ratios are ubiquitous in high-redshift star-forming galaxies, as expected for young systems whose interstellar medium is primarily enriched by core-collapse supernovae. 

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2. LATIS: The Stellar Mass–Metallicity Relation of Star-forming Galaxies at z ∼ 2.5

   https://doi.org/10.3847/1538-4357/ad0554  

   Chartab, Nima; Newman, Andrew B.; Rudie, Gwen C.; Blanc, Guillermo A.; Kelson, Daniel D. (December 2023, The Astrophysical Journal)

   Abstract We present the stellar mass–stellar metallicity relation for 3491 star-forming galaxies at 2 ≲z≲ 3 using rest-frame far-ultraviolet spectra from the LyαTomography IMACS Survey (LATIS). We fit stellar population synthesis models from the Binary Population And Spectral Synthesis code (v2.2.1) to medium-resolution (R∼ 1000) and high signal-to-noise (>30 per 100 km s⁻¹over the wavelength range 1221–1800 Å) composite spectra of galaxies in bins of stellar mass to determine their stellar metallicity, primarily tracing Fe/H. We find a strong correlation between stellar mass and stellar metallicity, with stellar metallicity monotonically increasing with stellar mass at low masses and flattening at high masses (M_*≳ 10^10.3M_⊙). Additionally, we compare our stellar metallicity measurements with the gas-phase oxygen abundance of galaxies at similar redshift and estimate the average [α/Fe] ∼ 0.6. Such highα-enhancement indicates that high-redshift galaxies have not yet undergone significant iron enrichment through Type Ia supernovae. Moreover, we utilize an analytic chemical evolution model to constrain the mass loading parameter of galactic winds as a function of stellar mass. We find that as the stellar mass increases, the mass loading parameter decreases. The parameter then flattens or reaches a turning point at aroundM_*∼ 10^10.5M_⊙. Our findings may signal the onset of black-hole-driven outflows atz∼ 2.5 for galaxies withM_*≳ 10^10.5M_⊙. 

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3. The MOSDEF survey: the mass–metallicity relationship and the existence of the FMR at z ∼ 1.5

   https://doi.org/10.1093/mnras/stab1793  

   Topping, Michael W; Shapley, Alice E; Sanders, Ryan L; Kriek, Mariska; Reddy, Naveen A; Coil, Alison L; Mobasher, Bahram; Siana, Brian; Freeman, William R; Shivaei, Irene; et al (July 2021, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT We analyse the rest-optical emission-line ratios of z ∼ 1.5 galaxies drawn from the Multi-Object Spectrometer for Infra-Red Exploration Deep Evolution Field (MOSDEF) survey. Using composite spectra, we investigate the mass–metallicity relation (MZR) at z ∼ 1.5 and measure its evolution to z = 0. When using gas-phase metallicities based on the N2 line ratio, we find that the MZR evolution from z ∼ 1.5 to z = 0 depends on stellar mass, evolving by $$\Delta \rm log(\rm O/H) \sim 0.25$$ dex at M*< $$10^{9.75}\, \mathrm{M}_{\odot }$$ down to $$\Delta \rm log(\rm O/H) \sim 0.05$$ at M* ≳ $$10^{10.5}\, \mathrm{M}_{\odot }$$. In contrast, the O3N2-based MZR shows a constant offset of $$\Delta \rm log(\rm O/H) \sim 0.30$$ across all masses, consistent with previous MOSDEF results based on independent metallicity indicators, and suggesting that O3N2 provides a more robust metallicity calibration for our z ∼ 1.5 sample. We investigated the secondary dependence of the MZR on star formation rate (SFR) by measuring correlated scatter about the mean M*-specific SFR and M*−$$\log (\rm O3N2)$$ relations. We find an anticorrelation between $$\log (\rm O/H)$$ and sSFR offsets, indicating the presence of a M*−SFR−Z relation, though with limited significance. Additionally, we find that our z ∼ 1.5 stacks lie along the z = 0 metallicity sequence at fixed μ = log (M*/M⊙) − 0.6 × $$\log (\rm SFR / M_{\odot } \, yr^{-1})$$ suggesting that the z ∼ 1.5 stacks can be described by the z = 0 fundamental metallicity relation (FMR). However, using different calibrations can shift the calculated metallicities off of the local FMR, indicating that appropriate calibrations are essential for understanding metallicity evolution with redshift. Finally, understanding how [N ii]/H α scales with galaxy properties is crucial to accurately describe the effects of blended [N ii] and H α on redshift and H α fiux measurements in future large surveys utilizing low-resolution spectra such as with Euclid and the Roman Space Telescope. 

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4. THE MASS-METALLICITY RELATION AT Z~8:DIRECT-METHOD METALLICITY CONSTRAINTS AND NEAR-FUTURE PROSPECTS

   Jones, Tucker; Sanders, Ryan; Roberts-Borsani, Guido; Ellis, Richard S.; Laporte, Nicolas; Treu, Tommaso; Harikane, Yuichi (July 2020, The Astrophysical journal)

   Physical properties of galaxies at z>7 are of interest for understanding both the early phases of star formation and the process of cosmic reionization. Chemical abundance measurements offer valuable information on the integrated star formation history, and hence ionizing photon production, as well as the rapid gas accretion expected at such high redshifts. We use reported measurements of [O III] 88μm emission and star formation rate to estimate gas-phase oxygen abundances in five galaxies at z=7.1-9.1 using the direct T_e method. We find typical abundances 12+log(O/H) = 7.9 (∼0.2 times the solar value) and an evolution of 0.9±0.5 dex in oxygen abundance at fixed stellar mass from z≃8 to 0. These results are compatible with theoretical predictions, albeit with large (conservative) uncertainties in both mass and metallicity. We assess both statistical and systematic uncertainties to identify promising means of improvement with the Atacama Large Millimeter Array (ALMA) and the James Webb Space Telescope (JWST). In particular we highlight [O III] 52μm as a valuable feature for robust metallicity measurements. Precision of 0.1-0.2 dex in T_e-based O/H abundance can be reasonably achieved for galaxies at z≈5-8 by combining [O III] 52μm with rest-frame optical strong lines. It will also be possible to probe gas mixing and mergers via resolved T_e-based abundances on kpc scales. With ALMA and JWST, direct metallicity measurements will thus be remarkably accessible in the reionization epoch. 

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5. CO Emission, Molecular Gas, and Metallicity in Main-sequence Star-forming Galaxies at z ∼ 2.3*

   Sanders, Ryan L.; Shapley, Alice E.; Jones, Tucker; Shivaei, Irene; Popping, Gergö; Reddy, Naveen A.; Davé, Romeel; Price, Sedona H.; Mobasher, Bahram; Kriek, Mariska; et al (January 2023, The Astrophysical Journal)

   Abstract We present observations of CO(3−2) in 13 main-sequence z = 2.0–2.5 star-forming galaxies at log ( M * / M ⊙ ) = 10.2 – 10.6 that span a wide range in metallicity (O/H) based on rest-optical spectroscopy. We find that L CO ( 3 − 2 ) ′ /SFR decreases with decreasing metallicity, implying that the CO luminosity per unit gas mass is lower in low-metallicity galaxies at z ∼ 2. We constrain the CO-to-H 2 conversion factor ( α CO ) and find that α CO inversely correlates with metallicity at z ∼ 2. We derive molecular gas masses ( M mol ) and characterize the relations among M * , SFR, M mol , and metallicity. At z ∼ 2, M mol increases and the molecular gas fraction ( M mol / M * ) decreases with increasing M * , with a significant secondary dependence on SFR. Galaxies at z ∼ 2 lie on a near-linear molecular KS law that is well-described by a constant depletion time of 700 Myr. We find that the scatter about the mean SFR− M * , O/H− M * , and M mol − M * relations is correlated such that, at fixed M * , z ∼ 2 galaxies with larger M mol have higher SFR and lower O/H. We thus confirm the existence of a fundamental metallicity relation at z ∼ 2, where O/H is inversely correlated with both SFR and M mol at fixed M * . These results suggest that the scatter of the z ∼ 2 star-forming main sequence, mass–metallicity relation, and M mol – M * relation are primarily driven by stochastic variations in gas inflow rates. We place constraints on the mass loading of galactic outflows and perform a metal budget analysis, finding that massive z ∼ 2 star-forming galaxies retain only 30% of metals produced, implying that a large mass of metals resides in the circumgalactic medium. 

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