The Mg II λ2797, λ2803 emission in low-metallicity star-forming galaxies from the SDSS⋆

We present 65 Sloan Digital Sky Survey (SDSS) spectra of 62 star-forming galaxies with oxygen abundances 12 + log O/H ∼ 7.5–8.4. Redshifts of selected galaxies are in the range z ∼ 0.36–0.70. This allows us to detect the redshifted Mg ii λ2797, λ2803 emission lines. Our aim is to use these lines for the magnesium abundance determination. The Mg ii emission was detected in ∼2/3 of the galaxies. We find that the Mg ii λ2797 emission-line intensity follows a trend with the excitation parameter x = O2+/O that is similar to that predicted by CLOUDY photoionised H ii region models, suggesting a nebular origin of Mg ii emission. The Mg/O abundance ratio is lower by a factor ∼2 than the solar ratio. This is probably the combined effect of interstellar Mg ii absorption and depletion of Mg onto dust. However, the effect of dust depletion in selected galaxies, if present, is small, by a factor of ∼2 lower than that of iron.