Open AccessNew light on the search for low‐metallicity galaxies – I. The N2 calibrator
Author(s) -
Denicoló Glenda,
Terlevich Roberto,
Terlevich Elena
Publication year - 2002
Publication title -
monthly notices of the royal astronomical society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.058
H-Index - 383
eISSN - 1365-2966
pISSN - 0035-8711
DOI - 10.1046/j.1365-8711.2002.05041.x
Subject(s) - physics , metallicity , astrophysics , redshift , galaxy , estimator , photoionization , spectral line , astronomy , statistics , ionization , ion , mathematics , quantum mechanics
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 .