Abstract
We present 65 Sloan Digital Sky Survey (SDSS) spectra of 62 star-forming galaxies with oxygen abundances 12 + logO/H similar to 7.5-8.4. Redshifts of selected galaxies are in the range z similar to 0.36-0.70. This allows us to detect the redshifted Mg II lambda 2797, lambda 2803 emission lines. Our aim is to use these lines for the magnesium abundance determination. The Mg II emission was detected in similar to 2/3 of the galaxies. We find that the Mg II lambda 2797 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 similar to 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 similar to 2 lower than that of iron.