Spectroscopic study of blue compact galaxies

V. Oxygen abundance and the metallicity-luminosity relation

¹ Center for Astrophysics, University of Science and Technology of China, 230026, PR China e-mail: sfemail@mail.ustc.edu.cn;xkong@ustc.edu.cn
² National Astronomical Observatory, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan

Received: 3 September 2004
Accepted: 14 March 2005

Abstract

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 , and nitrogen is found to be mostly a product of secondary nucleosynthesis for and apparently a product of primary nucleosynthesis for . 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.