Calibrating oxygen abundances for star-forming galaxies

We have selected a large sample of star-forming galaxies (104,088) from SDSS-DR4 to derive oxygen abundance calibrations using equivalent width (EW) ratios of three metallicity-sensitive emission-line ratios: [Nii]/Hα, ([Oiii]/Hβ)/([Nii]/Hα), and [Nii]/[Sii]. The analytic calibrations of 12 + log(O/H) versus these line-ratios have been obtained as third-order polynomial fits, appropriate for metal-rich galaxies. These observed relations are consistent with photoionization models. These can be used as calibration references for future studies about metallicities of galaxies, especially when there are problems dealing with proper flux calibration for spectra, e.g., some of the current generation of wide-field galaxy surveys that use multi-object spectrographs.