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New light on the search for low-metallicity galaxies – I. The N2 calibrator |
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| Authors: | GlendaDenicoló RobertoTerlevich † ElenaTerlevich ‡ |
| Institution: | |
| Abstract: | We present a simple metallicity estimator based on the logarithmic N ii ] λ 6584/H α 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 ] λλ 6717,6731/H α ) 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 ] λ 6584 is not detected or in low-resolution spectra where N ii ] λ 6584 is blended with H α . |
| Keywords: | galaxies: abundances galaxies: evolution galaxies: stellar content |