The [O II]λ3727/Hα ratio of emission-line galaxies in the 2dF galaxy redshift survey

We investigate the systematic variation of the [OII]lambda 3727/H alpha flux line ratio as a function of various galaxy properties, i.e. luminosity, metallicity, reddening and excitation state, for a sample of 1124 emission-line galaxies, with a mean redshift z similar to 0.06, drawn from the 2dF Galaxy Redshift Survey. The mean observed and extinction-corrected emission-line flux ratios agree well with the values derived from the B-band selected Nearby Field Galaxy Survey galaxy sample, but are significantly different from the values obtained from the Ha-selected Universidad Complutense de Madrid Survey galaxy sample. This is because the different selection criteria applied in these surveys lead to a significant difference in the mean extinction and metallicity of different samples. We use the R-23 parameter to estimate the gas-phase oxygen abundance and find that the extinction-corrected [OII]lambda 3727/H alpha ratio depends on the oxygen abundance. For 12 + log(O/H) greater than or similar to 8.4, we confirm that the emission-line ratio decreases with increasing metallicity. We have extended the relationship further to the metal-poor regime, 12 + log(O/H) less than or similar to 8.4, and find that the correlation between the extinction-corrected [OII]lambda 3727/H alpha ratio and the metallicity reverses in comparison to the relationship for metal-rich galaxies. For metal-poor galaxies, in contrast with metal-rich galaxies, the variation of extinction-corrected [OII]lambda 3727/H alpha ratio is correlated with the ionization states of the interstellar gas. The relative importance of the metallicity or the excitation state in determining the extinction-corrected [OII]lambda 3727/H alpha ratio depends on galaxy luminosity.