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1. Exploring chemical homogeneity in dwarf galaxies: a VLT- MUSE study of JKB 18

   https://doi.org/10.1093/mnras/staa1280  

   James, Bethan L ; Kumari, Nimisha ; Emerick, Andrew ; Koposov, Sergey E ; McQuinn, Kristen B ; Stark, Daniel P ; Belokurov, Vasily ; Maiolino, Roberto ( July 2020 , Monthly Notices of the Royal Astronomical Society)

   ABSTRACT Deciphering the distribution of metals throughout galaxies is fundamental in our understanding of galaxy evolution. Nearby, low-metallicity, star-forming dwarf galaxies, in particular, can offer detailed insight into the metal-dependent processes that may have occurred within galaxies in the early Universe. Here, we present VLT/MUSE observations of one such system, JKB 18, a blue diffuse dwarf galaxy with a metallicity of only 12 + log(O/H)=7.6 ± 0.2 (∼0.08 Z⊙). Using high spatial resolution integral-field spectroscopy of the entire system, we calculate chemical abundances for individual H ii regions using the direct method and derive oxygen abundance maps using strong-line metallicity diagnostics. With large-scale dispersions in O/H, N/H, and N/O of ∼0.5–0.6 dex and regions harbouring chemical abundances outside this 1σ distribution, we deem JKB 18 to be chemically inhomogeneous. We explore this finding in the context of other chemically inhomogeneous dwarf galaxies and conclude that neither the accretion of metal-poor gas, short mixing time-scales or self-enrichment from Wolf–Rayet stars are accountable. Using a galaxy-scale, multiphase, hydrodynamical simulation of a low-mass dwarf galaxy, we find that chemical inhomogeneities of this level may be attributable to the removal of gas via supernovae and the specific timing of the observations with respect to star formation activity. This study not only draws attentionmore »to the fact that dwarf galaxies can be chemically inhomogeneous, but also that the methods used in the assessment of this characteristic can be subject to bias.« less
2. A solar metallicity galaxy at z > 7? Possible detection of the [N  ii ] 122 μm and [O  iii ] 52 μm lines

   https://doi.org/10.1093/mnras/stad687  

   Killi, Meghana ; Watson, Darach ; Fujimoto, Seiji ; Akins, Hollis ; Knudsen, Kirsten ; Richard, Johan ; Harikane, Yuichi ; Rigopoulou, Dimitra ; Rizzo, Francesca ; Ginolfi, Michele ; et al ( March 2023 , Monthly Notices of the Royal Astronomical Society)

   We present the first detection of the [N ii] 122 $\mu$m and [O iii] 52 $\mu$m lines for a reionization-epoch galaxy. Based on these lines and previous [C ii] 158 $\mu$m and [O iii] 88 $\mu$m measurements, using two different radiative transfer models of the interstellar medium, we estimate an upper limit on electron density of ≲500 cm−3 and an approximate gas-phase metallicity of Z/Z⊙ ∼ 1.1 ± 0.2 for A1689-zD1, a gravitationally lensed dusty galaxy at z = 7.133. Other measurements or indicators of metallicity so far in galaxy interstellar media at z ≳ 6 are typically an order of magnitude lower than this. The unusually high metallicity makes A1689-zD1 inconsistent with the fundamental metallicity relation, although there is likely significant dust obscuration of the stellar mass, which may partly resolve the inconsistency. Given a solar metallicity, the dust-to-metals ratio is a factor of several lower than expected, hinting that galaxies beyond z ∼ 7 may have lower dust formation efficiency. Finally, the inferred nitrogen enrichment compared to oxygen, on which the metallicity measurement depends, indicates that star formation in the system is older than about 250 Myr, pushing the beginnings of this galaxy to z > 10.
3. METAL: The Metal Evolution, Transport, and Abundance in the Large Magellanic Cloud Hubble Program. III. Interstellar Depletions, Dust-to-Metal, and Dust-to-Gas Ratios versus Metallicity

   https://doi.org/10.3847/1538-4357/ac5248  

   Roman-Duval, Julia ; Jenkins, Edward B. ; Tchernyshyov, Kirill ; Clark, Christopher J. ; De Cia, Annalisa ; Gordon, Karl D. ; Hamanowicz, Aleksandra ; Lebouteiller, Vianney ; Rafelski, Marc ; Sandstrom, Karin ; et al ( March 2022 , The Astrophysical Journal)

   Abstract The metallicity and gas density dependence of interstellar depletions, the dust-to-gas (D/G), and dust-to-metal (D/M) ratios have important implications for how accurately we can trace the chemical enrichment of the universe, either by using FIR dust emission as a tracer of the ISM or by using spectroscopy of damped Ly α systems to measure chemical abundances over a wide range of redshifts. We collect and compare large samples of depletion measurements in the Milky Way (MW), Large Magellanic Cloud (LMC) ( Z = 0.5 Z ⊙ ), and Small Magellanic Cloud (SMC) ( Z = 0.2 Z ⊙ ). The relations between the depletions of different elements do not strongly vary between the three galaxies, implying that abundance ratios should trace depletions accurately down to 20% solar metallicity. From the depletions, we derive D/G and D/M. The D/G increases with density, consistent with the more efficient accretion of gas-phase metals onto dust grains in the denser ISM. For log N (H) > 21 cm −2 , the depletion of metallicity tracers (S, Zn) exceeds −0.5 dex, even at 20% solar metallicity. The gas fraction of metals increases from the MW to the LMC (factor 3) and SMC (factor 6),more »compensating for the reduction in total heavy element abundances and resulting in those three galaxies having the same neutral gas-phase metallicities. The D/G derived from depletions are respective factors of 2 (LMC) and 5 (SMC) higher than the D/G derived from FIR, 21 cm, and CO emission, likely due to the combined uncertainties on the dust FIR opacity and on the depletion of carbon and oxygen.« less
4. Infrared Spectral Energy Distributions and Dust Masses of Sub-solar Metallicity Galaxies at z ∼ 2.3

   https://doi.org/10.3847/1538-4357/ac54a9  

   Shivaei, Irene ; Popping, Gergö ; Rieke, George ; Reddy, Naveen ; Pope, Alexandra ; Kennicutt, Robert ; Mobasher, Bahram ; Coil, Alison ; Fudamoto, Yoshinobu ; Kriek, Mariska ; et al ( March 2022 , The Astrophysical Journal)

   Abstract We present results from Atacama Large Millimeter/submillimeter Array (ALMA) 1.2 mm continuum observations of a sample of 27 star-forming galaxies at z = 2.1–2.5 from the MOSFIRE Deep Evolution Field survey with metallicity and star formation rate measurements from optical emission lines. Using stacks of Spitzer, Herschel, and ALMA photometry (rest frame ∼8–400 μ m), we examine the infrared (IR) spectral energy distributions (SED) of z ∼ 2.3 subsolar-metallicity (∼0.5 Z ⊙ ) luminous infrared galaxies (LIRGs). We find that the data agree well with an average template of higher-luminosity local low-metallicity dwarf galaxies (reduced χ 2 = 1.8). When compared with the commonly used templates for solar-metallicity local galaxies or high-redshift LIRGs and ultraluminous IR galaxies, even in the most favorable case (with reduced χ 2 = 2.8), the templates are rejected at >98% confidence. The broader and hotter IR SED of both the local dwarfs and high-redshift subsolar-metallicity galaxies may result from different grain properties or a harder/more intense ionizing radiation field that increases the dust temperature. The obscured star formation rate (SFR) indicated by the far-IR emission of the subsolar-metallicity galaxies is only ∼60% of the total SFR, considerably lower than that of the local LIRGsmore »with ∼96%–97% obscured fractions. Due to the evolving IR SED shape, the local LIRG templates fit to mid-IR data overestimate the Rayleigh–Jeans tail measurements by a factor of 2–20. These templates underestimate IR luminosities if fit to the observed ALMA fluxes by >0.4 dex. At a given stellar mass or metallicity, dust masses at z ∼ 2.3 are an order of magnitude higher than z ∼ 0. Given the predicted molecular gas fractions, the observed z ∼ 2.3 dust-to-stellar mass ratios suggest lower dust-to-molecular gas masses than in local galaxies with similar metallicities.« less
5. The MOSDEF survey: direct-method metallicities and ISM conditions at z ∼ 1.5–3.5

   https://doi.org/10.1093/mnras/stz3032  

   Sanders, Ryan L. ; Shapley, Alice E. ; Reddy, Naveen A. ; Kriek, Mariska ; Siana, Brian ; Coil, Alison L. ; Mobasher, Bahram ; Shivaei, Irene ; Freeman, William R. ; Azadi, Mojegan ; et al ( November 2019 , Monthly Notices of the Royal Astronomical Society)

   We present detections of [O iii] λ4363 and direct-method metallicities for star-forming galaxies at z = 1.7–3.6. We combine new measurements from the MOSFIRE Deep Evolution Field (MOSDEF) survey with literature sources to construct a sample of 18 galaxies with direct-method metallicities at z > 1, spanning 7.5 < 12+log(O/H) < 8.2 and log(M*/M⊙) = 7–10. We find that strong-line calibrations based on local analogues of high-redshift galaxies reliably reproduce the metallicity of the z > 1 sample on average. We construct the first mass–metallicity relation at z > 1 based purely on direct-method O/H, finding a slope that is consistent with strong-line results. Direct-method O/H evolves by ≲0.1 dex at fixed M* and star formation rate from z ∼ 0 to 2.2. We employ photoionization models to constrain the ionization parameter and ionizing spectrum in the high-redshift sample. Stellar models with supersolar O/Fe and binary evolution of massive stars are required to reproduce the observed strong-line ratios. We find that the z > 1 sample falls on the z ∼ 0 relation between ionization parameter and O/H, suggesting no evolution of this relation from z ∼ 0 to z ∼ 2. These results suggest that the offset of the strong-line ratios of this sample from local excitation sequences is driven primarilymore »by a harder ionizing spectrum at fixed nebular metallicity compared to what is typical at z ∼ 0, naturally explained by supersolar O/Fe at high redshift caused by rapid formation time-scales. Given the extreme nature of our z > 1 sample, the implications for representative z ∼ 2 galaxy samples at ∼1010 M⊙ are unclear, but similarities to z > 6 galaxies suggest that these conclusions can be extended to galaxies in the epoch of reionization.

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