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

A large sample of Mg n emitting star-forming galaxies with low metallicity [O/H] = log(O/H) - log(O/H)(circle dot) 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 lambda 2797, lambda 2803 emission lines in the redshift range z similar to 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] similar or equal to -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.