The Mg II 2797, 2803 emission in low-metallicity star-forming galaxies from the SDSS

TL;DR

This study analyzes Mg II emission lines in low-metallicity star-forming galaxies from SDSS to determine magnesium abundance and explore nebular origins, revealing a lower Mg/O ratio likely due to dust depletion.

Contribution

First detection of Mg II emission in these galaxies and analysis of its relation to nebular conditions and magnesium abundance.

Findings

Mg II emission detected in ~2/3 of galaxies.

Mg II emission correlates with excitation parameter as predicted by models.

Mg/O ratio is about half the solar value, indicating dust depletion effects.

Abstract

We present 65 Sloan Digital Sky Survey (SDSS) spectra of 62 star-forming galaxies with oxygen abundances 12 + logO/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 MgII 2797,2803 emission lines. Our aim is to use these lines for the magnesium abundance determination. The MgII emission was detected in ~2/3 of the galaxies. We find that the MgII 2797 emission-line intensity follows a trend with the excitation parameter x= O^{2+}/O that is similar to that predicted by CLOUDY photoionised HII region models, suggesting a nebular origin of MgII emission. The Mg/O abundance ratio is lower by a factor ~2 than the solar ratio. This is probably the combined effect of interstellar MgII 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.