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 starforming 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, R-23 method, P method, N2 method and O3N2 method. The oxygen abundances derived from the Te 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 H.. 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 (similar to- 0.05) is slightly shallower than that for the dwarf galaxies (similar to- 0.17). An offset was found in the metallicity-luminocity 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.