The oxygen abundance calibrations and N/O abundance ratios of ∼40,000 SDSS star-forming galaxies

Using a large sample of 38,478 star-forming galaxies selected from SDSS DR2, we derive analytical calibrations for oxygen abundances from several metallicity- sensitive emission- line ratios: [ N II]/ H alpha, [ OIII]/[ N II], [ N II]/[ O II], [ N II]/[ S II], [ S II]/ H alpha, and [OIII]/ H beta. This consistent set of strong-line oxygen abundance calibrations will be useful for future abundance studies. Among these calibrations, [ N ii]/[ O ii] is the best for metal- rich galaxies due to its independence on ionization parameter and low scatter. Dust extinction must be considered properly at first. These calibrations are more suitable for metal- rich galaxies [ 8: 4 < 12 + log ( O/ H) < 9: 3] and for the nuclear regions of galaxies. The observed relations are consistent with those expected from the photoionization models of Kewley & Dopita. However, most of the observational data spread in a range of ionization parameter q from 1; 107 to 8; 107 cm s(-1), corresponding to log U - 3: 5 to - 2.5, narrower than that suggested by the models. We also estimate the ( N/ O) abundance ratios of this large sample of galaxies, and these are consistent with the combination of a primary'' and a dominant secondary'' component of nitrogen.