The Detection of [O III] λ4363 in a Lensed, Dwarf Galaxy at z = 2.59: Testing Metallicity Indicators and Scaling Relations at High Redshift and Low Mass

We present Keck/MOSFIRE (Multi-Object Spectrometer for InfraRed Exploration) and Keck/LRIS (Low Resolution Imaging Spectrometer) spectroscopy of A1689-217, a lensed (magnification ~7.9), star-forming (SFR ~ 16 M_☉ yr⁻¹), dwarf (log(M_★/M_☉) = 8.07–8.59) Lyα emitter (EW₀ ~ 138 Å) at z = 2.5918. Dwarf galaxies similar to A1689-217 are common at high redshift and likely responsible for reionization, yet few have been studied with detailed spectroscopy. We report a 4.2σ detection of the electron-temperature-sensitive [O iii] λ4363 emission line and use this line to directly measure an oxygen abundance of 12 + log(O/H) = 8.06 ± 0.12 (~1/4 Z_☉). A1689-217 is the lowest-mass galaxy at z > 2 with an [O iii] λ4363 detection. Using the rest-optical emission lines, we measure A1689-217's other nebular conditions, including electron temperature (T e ([O iii]) ~ 14,000 K), electron density (n e ~ 220 cm⁻³), and reddening (E(B-V) ~ 0.39). We study relations between strong-line ratios and direct metallicities with A1689-217 and other galaxies with [O iii] λ4363 detections at z ~ 0–3.1, showing that the locally calibrated, oxygen-based, strong-line relations are consistent from z ~ 0 to 3.1. We also show additional evidence that the O₃₂ versus R₂₃ excitation diagram can be utilized as a redshift-invariant, direct-metallicity-based, oxygen abundance diagnostic out to z ~ 3.1. From this excitation diagram and the strong-line ratio–metallicity plots, we observe that the ionization parameter at fixed O/H is consistent with no redshift evolution. Although A1689-217 is metal-rich for its M_★ and star formation rate, we find it to be consistent within the large scatter of the low-mass end of the fundamental metallicity relation

A Comprehensive Analysis of the UV Spectral Properties of Dwarf Galaxies at z∼2z\sim2

Galaxy evolution is concerned with the mechanisms that determine the distributions of gas, dust, and metals, and with the efficiency with which galaxies produce stars. These components are the primary visible building blocks of a galaxy. Throughout cosmic history the escape of ionizing radiation from a galaxy out into the intergalactic medium has been a notoriously difficult measurement to make, and one that informs the ionizing background radiation. This is relevant for the development of galaxies generally, but also plays a significant role in understanding the last great phase change of the universe, reionization ($z\sim6$). It is unclear whether it is massive galaxies or dwarf galaxies which are able to expel sufficient amounts of ionizing radiation to ionize neutral hydrogen in the intergalactic medium during this epoch of the universe. Any attempts to measure the ionizing escape fraction during reionization is untenable even for newer instruments like the James Webb space telescope due to the neutral intergalactic medium obscuring large amounts of ionizing radiation. As such, lower redshift galaxies are studied both for the sake of constraining the ionizing background throughout cosmic history and as benchmarks for understanding reionization. Often, studies will select galaxies based on their Ly$\alpha$ emission as it has a possible connection with Lyman continuum escape. However, since Ly$\alpha$ is a resonant line transition it is absorbed and scattered by even small amounts of neutral hydrogen. Because of this, it is unclear what galaxy properties may be correlated with Ly$\alpha$ escape. To understand the biases behind selecting on Ly$\alpha$ we need to better understand what causes Ly$\alpha$ to escape and the distribution of Ly$\alpha$ properties in galaxy samples. Ly$\alpha$ also serves as a proxy of the neutral fraction in the intergalactic medium at high redshift. Finally, outflows and feedback are best understood by comparing empirical measures of outflows and covering fractions with models from simulations. These models can be better constrained when including empirical markers in the dwarf regime. Many studies have looked into these measurements at higher masses (\rm log(M^*/M_\odot)>9), or for samples biased towards strong line emitters. There has yet to be a complete sample of low mass galaxies studied at $z\sim2$.In this work we present a sample of 32 galaxies at $z\sim2$ during the peak of star-formation in the history of the universe. We focus on low-UV luminosity (\rm M_{UV} > -19) and low mass (\rm log(M^*/M_\odot)<9) sub-samples of this parent sample. A unique characteristic of this sample is the complete spectroscopy of the galaxies. Each galaxy has rest-UV and rest-optical spectra from the Keck/LRIS and keck/MOSFIRE instruments respectively. This allows us to confirm the redshift of galaxies for which we do not observe any apparent spectral features in the rest-UV spectra. This, along with improved S/N from gravitational lensing, allows us to to produce a complete sample of dwarf galaxies from which to measure proxies of the ionizing escape fraction of galaxies. The primary proxies used in the literature and in this work are the Ly$\alpha$ EW, the Ly$\alpha$ escape fraction, and the low-ionization interstellar absorption line EW \citep{Saldana-Lopez2023}. The Ly$\alpha$ EW we measure for individual galaxies and find that generally dwarf galaxies tend to have Ly$\alpha$ in emission in contrast to higher mass samples which show Ly$\alpha$ EWs have Ly$\alpha$ in absorption on average \citep{Du2018}. The volumetric escape fraction of dwarf galaxies at $z\sim2$ is consistent with the literature value of $5\%$ \citep{Hayes2010,Ciardullo2014,Sobral2017,Weiss2021}, placing an upper limit on the volumetric escape fraction of ionizing photons. The LIS absorption lines show about a factor of 2 weaker EWs for low mass galaxies when compared with higher mass samples \citep{jones2012,Du2018}. A lower EW of LIS absorption lines implies a lower covering fraction of neutral hydrogen and therefore greater avenues of escape on average for ionizing photons. Collectively, our measurements imply greater amounts of ionizing radiation escaping from dwarf galaxies on average when compared with higher mass samples of star-forming galaxies

The Direct-method Oxygen Abundance of Typical Dwarf Galaxies at Cosmic High Noon

We present a Keck/MOSFIRE rest-optical composite spectrum of 16 typical gravitationally lensed star-forming dwarf galaxies at 1.7 ≲ z ≲ 2.6 ( z _mean = 2.30), all chosen independent of emission-line strength. These galaxies have a median stellar mass of $\mathrm{log}{({M}_{* }/{M}_{\odot })}_{\mathrm{med}}={8.29}_{-0.43}^{+0.51}$ and a median star formation rate of ${{\rm{S}}{\rm{F}}{\rm{R}}}_{{\rm{H}}\alpha }^{{\rm{m}}{\rm{e}}{\rm{d}}}={2.25}_{-1.26}^{+2.15}\,{M}_{\odot }\,{{\rm{y}}{\rm{r}}}^{-1}$ . We measure the faint electron-temperature-sensitive [O iii ] λ 4363 emission line at 2.5 σ (4.1 σ ) significance when considering a bootstrapped (statistical-only) uncertainty spectrum. This yields a direct-method oxygen abundance of $12+\mathrm{log}{({\rm{O}}/{\rm{H}})}_{\mathrm{direct}}={7.88}_{-0.22}^{+0.25}$ ( ${0.15}_{-0.06}^{+0.12}\ {Z}_{\odot }$ ). We investigate the applicability at high z of locally calibrated oxygen-based strong-line metallicity relations, finding that the local reference calibrations of Bian et al. best reproduce (≲0.12 dex) our composite metallicity at fixed strong-line ratio. At fixed M _* , our composite is well represented by the z ∼ 2.3 direct-method stellar mass—gas-phase metallicity relation (MZR) of Sanders et al. When comparing to predicted MZRs from the IllustrisTNG and FIRE simulations, having recalculated our stellar masses with more realistic nonparametric star formation histories $(\mathrm{log}{({M}_{* }/{M}_{\odot })}_{\mathrm{med}}={8.92}_{-0.22}^{+0.31})$ , we find excellent agreement with the FIRE MZR. Our composite is consistent with no metallicity evolution, at fixed M _* and SFR, of the locally defined fundamental metallicity relation. We measure the doublet ratio [O ii ] λ 3729/[O ii ] λ 3726 = 1.56 ± 0.32 (1.51 ± 0.12) and a corresponding electron density of ${n}_{e}={1}_{-0}^{+215}\ {\mathrm{cm}}^{-3}$ ( ${n}_{e}={1}_{-0}^{+74}\ {\mathrm{cm}}^{-3}$ ) when considering the bootstrapped (statistical-only) error spectrum. This result suggests that lower-mass galaxies have lower densities than higher-mass galaxies at z ∼ 2

Vie Économique : Dimensions politiques

Revue Vie Économique Sujet : La Revue Vie Économique est une initiative des Éditions vie économique (EVE), coopérative de solidarité. Lieu de débat et de réflexion sur les enjeux économiques, la revue veut aller au-delà de la pensée économique traditionnelle, trop souvent réduite aux seules transactions marchandes, en abordant les dimensions politique et sociétale des activités économiques contemporaines. Ceci inclut, évidemment, les enjeux écologiques qui imposent de changer nos manières..