2007A&A...473..411L

Direct measurement of oxygen abundance for metal-rich galaxies from electron temperature is difficult or impossible since temperature-sensitive auroral lines generally become too weak to be measured. We aim to derive the electron temperature (T_e) in the gas of metal-rich star-forming galaxies, which can be obtained from their ratios of auroral lines [OII]λλ7320,7330 to nebular lines [OII]λ3727, in order to establish a more robust mass-metallicity relationship, and compare the T_e-based (O/H) abundances with those from empirical strong-line calibrations, such as R₂₃ (=([OII]λ3727+[OIII]λλ4959,5007)/Hβ). We obtained 27 spectra by stacking the spectra of several hundred (even several thousand) star-forming galaxies selected from the SDSS-DR4 in each of the 27 stellar mass bins from log(M_*)∼8.0 to 10.6 (in units of log(M_☉)). This ``stack'' method sufficiently improves the signal-to-noise ratio of the auroral lines [OII]λλ7320,7330. Using a two-zone model for the temperature structure, we derive the electron temperature t₂ in the low ionization region from the [OII]λλ7320,7330/[OII]λ3727 ratio, and then use a relation derived by fitting HII region models to estimate the electron temperature t₃ in the high ionization region from t₂. Then, the direct (O/H) abundances are obtained from t₂, t₃ and the related line-ratios. The emission lines have been carefully corrected for dust extinction using the Balmer line ratio after correcting for the underlying stellar absorption. Combining our results with those from the literature with lower metallicities, we are able to provide a new relationship between the direct measurements of (O/H) and R₂₃, which still shows an upper and a lower branch with the transition around 12+log(O/H)∼8.0-8.2. It also shows that the empirical R₂₃ method will overestimate log(O/H) by 0.2 to 0.6dex. The new metal-mass relationship of the galaxies with moderate metallicities is fitted by a linear fit (12+log(O/H)=6.223+0.231xlog(M_*)) confirming that empirical methods significantly overestimate (O/H). We also derived their (N/O) abundance ratios on the basis of the T_e method, which are consistent with the combination of the primary and secondary components of nitrogen. This study provides for the first time a method to calibrate direct O/H abundances (from T_e) for a large range of galaxies within a stellar mass range of ∼5x10⁸M_☉ to 4x10¹⁰M_☉.