A large sample of low surface brightness disc galaxies from the SDSS - II. Metallicities in surface brightness bins

We study the spectroscopic properties of a large sample of low surface brightness galaxies (LSBGs) [with B-band central surface brightness mu(0)(B) > 22 mag arcsec(-2)] selected from the Sloan Digital Sky Survey Data Release 4 (SDSS-DR4) main galaxy sample. A large sample of disc-dominated high surface brightness galaxies [HSBGs, with mu(0)(B) < 22 mag arcsec(-2)] are also selected for comparison simultaneously. To study them in more details, these sample galaxies are further divided into four subgroups according to mu(0)(B) (in units of mag arcsec(-2)): very low surface brightness galaxies (vLSBGs) (24.5-22.75), intermediate low surface brightness galaxies (iLSBGs) (22.75-22.0), intermediate high surface brightness galaxies (iHSBGs) (22.0-21.25) and very high surface brightness galaxies (vHSBGs) (< 21.25). The diagnostic diagram from spectral emission-line ratios shows that the active galactic nucleus fractions of all the four subgroups are small (< 9 per cent). The 21 032 star-forming galaxies with good-quality spectroscopic observations are further selected for studying their dust extinction, strong-line ratios, metallicities and stellar mass-metallicity relations. The vLSBGs have lower extinction values and have less metal-rich and massive galaxies than the other subgroups. The oxygen abundances of our LSBGs are not as low as those of the H II regions in LSBGs studied in literature, which could be because our samples are more luminous, and because of the different metallicity calibrations used. We find a correlation between 12 + log(O/H) and mu(0)(B) for vLSBGs, iLSBGs and iHSBGs but show that this could be a result of correlation between mu(0)(B) and stellar mass and the well-known mass-metallicity relation. This large sample shows that LSBGs span a wide range in metallicity and stellar mass, and they lie nearly on the stellar mass versus metallicity and N/O versus O/H relations of normal galaxies. This suggests that LSBGs and HSBGs have not had dramatically different star formation and chemical enrichment histories.