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]/Hα, [O III]/[N II], [N II]/[O II], [N II]/[S II], [S II]/Hα, and [O III]/Hβ. 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.