New light on the search for low-metallicity galaxies -: I.: The N2 calibrator

We present a simple metallicity estimator based on the logarithmic [N II] lambda6584/Halpha ratio, hereafter N2, which we envisage will become very useful for ranking galaxies in a metallicity sequence from redshift survey-quality data even for moderately low spectral resolution. We have calibrated the N2 estimator using a compilation of H II galaxies having accurate oxygen abundances. plus photoionization models covering a wide range of abundances. The comparison of models and observations indicates that both primary and secondary nitrogen are important for the relevant range of metallicities. The N2 estimator follows a linear relation with log(O/H) that holds for the whole abundance range covered by the sample, from approximately 1/50th to twice the Solar value [7.2 < 12 + log(O/H) < 9.1]. We suggest that the ([S II] lambdalambda6717,6731/Halpha) ratio (hereafter S2) can also be used as a rough metallicity indicator. Because of its large scatter the S2 estimator will be useful only in systems with very low metallicity. where [N II] lambda6584 is not detected or in low-resolution spectra where [N II] lambda6584 is blended with Halpha.