Spectroscopic study of blue compact galaxies. V. Oxygen abundance and the metallicity-luminosity relation

This is the fifth paper in a series studying the stellar components, star formation histories, star formation rates and metallicities of a blue compact galaxy (BCG) sample. Based on our high-quality ground-based spectroscopic observations, we have determined the electron temperatures, electron densities, nitrogen abundances and oxygen abundances for 72 star-forming BCGs in our sample, using different oxygen abundance indicators. The oxygen abundance covers the range 7.15 < 12 + log (O/H)< 9.0, and nitrogen is found to be mostly a product of secondary nucleosynthesis for 12 + log (O/H)>8.2 and apparently a product of primary nucleosynthesis for 12 + log (O/H)< 8.2. To assess the possible systematic differences among different oxygen abundance indicators, we have compared oxygen abundances of BCGs obtained with the T_e method, R₂₃ method, P method, N2 method and O3N2 method. The oxygen abundances derived from the T_e method are systematically lower by 0.1-0.25 dex than those derived from the strong line empirical abundance indicators, consistent with previous studies based on region samples. We confirm the existence of the metallicity-luminosity relation in BCGs over a large range of abundances and luminosities. Our sample of galaxies shows that the slope of the metallicity-luminosity relation for the luminous galaxies (~-0.05) is slightly shallower than that for the dwarf galaxies (~-0.17). An offset was found in the metallicity-luminosity relation of the local galaxies and that of the intermediate redshift galaxies. It shows that the metallicity-luminosity relation for the emission line galaxies at high redshift is displaced to lower abundances, higher luminosities, or both.