A Study of the [OII]λ3727/Hα Flux Ratio of Emission Line Galaxies

With the sample of 105 emission line galaxies selected from the Sloan Digital Sky Survey Data Release 4 (SDSS DR4), we have investigated the relations of the [OII]λ3727/Hα flux ratio with the dust extinction, the ionization state of interstellar gas and the metal abundance of galaxies. It is found that the dust extinction correction has a significant effect on the [OII]λ3727/Hα flux ratio. Before and after the dust distinction correction is made, the mean [OII]λ3727/Hα flux ratios are 0.48 and 0.89, respectively. After the dust extinction is corrected, the dispersion of the distribution of F([OII]λ3727) as a function of F(Hα) is obviously reduced. The [OII]λ3727/Hα flux ratio of metal-poor galaxies decreases with the increasing ionization degree of interstellar gas, but this relation does not exist in metal-rich galaxies. Besides, it is found that the [OII]λ3727/Hα flux ratio is correlated with the metal abundance. When 12 + lg(O/H) > 8.5, the [OII]λ3727/Hα flux ratio decreases with the increasing metal abundance; for the galaxies of 12 + lg(O/H) > 8.5, the spectral flux ratio correlates positively with the metal abundance. Finally, by using the parameters of gas ionization degree and metal abundances of galaxies, the formulae for calculating the [OII]λ3727/Hα flux ratios of different types of galaxies are given. With the [OII]λ3727/Hα flux ratio given by these formulae, the star formation rate can be derived by using the [OII]λ3727-line flux, for the galaxies of the redshift z > 0.4, such as the large number of galaxies to be observed by the LAMOST telescope.