Checking the reliability of equivalent width R 23 to estimate metallicities of galaxies

Aims. We verify whether the O/H abundances of galaxies may be derived from the equivalent width (EW) R23 instead of the extinctioncorrected flux R23, and eventually propose a method to improve the reliability of this empirical method which is often used for the non-flux calibrated spectra of galaxies. Methods. We select 37,173 star-forming galaxies from the Second Data Release of the Sloan Digital Sky Survey (SDSS-DR2) which offer a wide range of properties to test the EW method. Results. The EW-R23 method is bringing a significant bias: for the bulk of SDSS galaxies, it may affect the determination of log(O/H) by factors ranging from -0.2 to 0.1 dex, and for some galaxies by factors ranging from -0.5 to 0.2 dex. We characterize this discrepancy (or bias) by α = (I[OII]/IHβ)/(EW[OII]/EWHβ) which is virtually independent of dust extinction, while tightly correlating with Dn(4000), although at a lower significance, with g − r colors. Conclusions. The EW-R23 method cannot be used as a proxy for the extinction-corrected flux R23 method. From analytical third-order polynomial fits of α versus g − r colors, we have been able to correct the EW-R23 method. With this additional and easy correction, the EW-R23 method provides O/H abundance values similar to those derived from the extinction-corrected flux R23 method within an accuracy of ±0.1 dex for >92% of the SDSS galaxies.