The [ OII ] λ 3727 / H α Ratio of Emission Line Galaxies in the 2 dF Galaxy Redshift Survey

We investigate the systematic variation of the [OII]λ3727/Hα flux line ratio as a function of various galaxy properties, i.e., luminosity, metallicity, reddening, and excitation state, for a sample of 1 124 emission-line galaxies, with a mean redshift z ∼ 0.06, drawn from the Two Degree Field 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 Hα-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 R23 parameter to estimate the gas-phase oxygen abundance and find that the extinction-corrected [OII]λ3727/Hα ratio depends on the oxygen abundance. For 12 + log(O/H) > ∼ 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) < ∼ 8.4, and find that the correlation between the extinctioncorrected [OII]λ3727/Hα ratio and the metallicity reverses in comparison to the relationship for metal-rich galaxies. For metal-poor galaxies, in contrast with metal-rich ones, the variation of extinction-corrected [OII]λ3727/Hα 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]λ3727/Hα ratio depends on galaxy luminosity.