Mg II λ2797, λ2803 emission in a large sample of low-metallicity star-forming galaxies from SDSS DR14

A large sample of Mg II emitting star-forming galaxies with low metallicity [O/H] = log(O/H) – log(O/H)⊙ between –0.2 and –1.2 dex is constructed from Data Release 14 of the Sloan Digital Sky Survey. We selected 4189 galaxies with Mg II λ2797, λ2803 emission lines in the redshift range z ∼ 0.3–1.0 or 35% of the total Sloan Digital Sky Survey star-forming sample with redshift z ≥ 0.3. We study the dependence of the magnesium-to-oxygen and magnesium-to-neon abundance ratios on metallicity. Extrapolating this dependence to [Mg/Ne] = 0 and to solar metallicity we derive a magnesium depletion of [Mg/Ne] ≃ –0.4 (at solar metallicity). We prefer neon instead of oxygen to evaluate the magnesium depletion in the interstellar medium because neon is a noble gas and is not incorporated into dust, contrary to oxygen. Thus, we find that more massive and more metal abundant galaxies have higher magnesium depletion. The global parameters of our sample, such as the mass of the stellar population and star formation rate, are compared with previously obtained results from the literature. These results confirm that Mg II emission has a nebular origin. Our data for interstellar magnesium-to-oxygen abundance ratios relative to the solar value are in good agreement with similar measurements made for Galactic stars, for giant stars in the Milky Way satellite dwarf galaxies, and with low-metallicity damped Lyman-alpha systems.

Oxygen and zinc abundances in 417 Galactic bulge red giants

Context. Oxygen and zinc in the Galactic bulge are key elements for the understanding of the bulge chemical evolution. Oxygen-to-iron abundance ratios provide a most robust indicator of the star formation rate and chemical evolution of the bulge. Zinc is enhanced in metal-poor stars, behaving as an α-element, and its production may require nucleosynthesis in hypernovae. Most of the neutral gas at high redshift is in damped Lyman-alpha systems (DLAs), where Zn is also observed to behave as an α-element. Aims. The aim of this work is the derivation of the α-element oxygen, together with nitrogen, and the iron-peak element zinc abundances in 417 bulge giants, from moderate resolution (R ~ 22 000) FLAMES-GIRAFFE spectra. For stars in common with a set of UVES spectra with higher resolution (R ~ 45 000), the data are intercompared. The results are compared with literature data and chemodynamical models. Methods. We studied the spectra obtained for a large sample of red giant stars, chosen to be one magnitude above the horizontal branch, using FLAMES-GIRAFFE on the Very Large Telescope. We computed the O abundances using the forbidden [OI] 6300.3 Å and Zn abundances using the Zn I 6362.34 Å lines. Stellar parameters for these stars were established in a previous work from our group. Results. We present oxygen abundances for 358 stars, nitrogen abundances for 403 stars and zinc abundances were derived for 333 stars. Having oxygen abundances for this large sample adds information in particular at the moderate metallicities of −1.6 < [Fe/H] < −0.8. Zn behaves as an α-element, very similarly to O, Si, and Ca. It shows the same trend as a function of metallicity as the α-elements, i.e., a turnover around [Fe/H] ~ − 0.6, and then decreasing with increasing metallicity. The results are compared with chemodynamical evolution models of O and Zn enrichment for a classical bulge. DLAs also show an enhanced zinc-to-iron ratio, suggesting they may be enriched by hypernovae.

On the kinematics of neutral gas in GRB host galaxies

We analyze a sample of 20 absorption systems intrinsic to long duration GRB host galaxies at z > 2 forwhich the metallicities are known. We compare the relation between the metallicity and cold gas velocity width for thissample to that of the Damped Lyman-alpha systems (DLAs) in the sight-lines of quasars (QSOs), and find completeagreement. We then compare the redshift evolution of the velocity-metallicity relation of our sample to that of QSODLAsand find that also GRB hosts favour a late onset of this evolution, around a redshift of ~2.6. We compute predicted stellar masses for the GRB host galaxies using the prescription determined from QSO-DLAsamples and compare the measured stellar masses for the four hosts where stellar masses have been determinedfrom SED fits. We find excellent agreement and conclude that, on basis of all available data and tests, long durationGRB-DLA hosts and intervening QSO-DLAs are consistent with being drawn from the same underlying population.