利用从斯隆数字巡天(Sloan Digital Sky Survey,简称SDSS)第4次释放的光谱数据中选取的10^5个发射线星系样本,研究了[OII]λ3727/Hα流量比与星系尘埃消光、气体电离态和金属丰度的关系.发现尘埃消光改正对[OII]λ3727/Hα谱线流量比影响显著,消光改正前、后的[OII]λ3727/Hα谱线流量比的中值分别为0.48和0.89;尘埃消光改正后,F([OII]λ3727)-F(Hα)的弥散显著减小.贫金属星系的[OII]λ3727/Hα谱线流量比随星系气体的电离度增高而减小,而富金属星系不存在这种关系.另外,[OII]λ3727/Hα流量比与星系金属丰度相关.当12+lg(O/H)〉8.5时,星系[OII]λ3727/Hα流量比随金属丰度增加而下降;12+lg(O/H)〈8.5的星系,谱线流量比与金属丰度正相关.最后,利用气体电离度参数和星系的金属丰度,给出了计算不同类型星系[OII]λ3727/Hα流量比的公式.LAMOST望远镜将观测到大量红移z〉0.4的星系光谱,利用该公式可以给出星系的[OII]λ3727/Hα流量比,从而可以利用[OII]λ3727谱线流量计算z〉0.4星系的恒星形成率.
Although the Ha emission line provides a useful star formation rate (SFR) indicator for nearby galaxies, it is not easily observed for distant galaxies since this line shifts out of the visible band for z 〉 0.4. Instead, the [OII]λ3727 line is often used as an alternative indicator of SFR. Unfortunately, the [OII] emission line is subjected to several factors, such as reddening, excitation state and abundance dependence. To investigate the systematic dependence of the [OII]λ3727/Hα flux ratio on these different factors, 105 emission line galaxies are selected from the Sloan Digital Sky Survey Data Release 4 (SDSS DR4). The effective V-band optical depths (τν) of galaxies are calculated based on fluxes of Ha and H lines. The dust extinction correction is very important to the [OII]λ3727/Hα flux ratio. The mean observed and extinction-corrected flux ratios are 0.48 and 0.89, respectively. The extinction-corrected [OII]λ3727/Hα flux ratio correlates with the ionization states of the interstellar gas for metal-poor galaxies, but there is no strong dependence for metalrich galaxies. In additional, the [OII]λ3727/Hα ratio depends on the oxygen abundance. For 12 + lg(O/H) 〉 8.5, the emission-line ratio decreases with the increase of metallicity and conversely for metal-poor galaxies. We provide simple formulae to estimate the [OII]λ3727/Hα flux ratio as a function of metallicity and excitation state. These formulae can be used to calculate SFR of galaxies with z 〉 0.4, such as galaxies to be observed from the deep field survey of the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST).