Empirical strong-line oxygen abundance calibrations from galaxies with electron-temperature measurements

Abstract

Aims. Our aims are to estimate the validity of empirical methods, such as R 23 , R 23 − P, log([N ii]/Hα) (N2), log[([O iii]/Hβ)/ ([N ii]/Hα)] (O3N2), and log([S ii]/Hα) (S2), and to re-derive (or add) the calibrations of R 23 , N2, O3N2, and S2 indices for oxygen abundances on the basis of a large sample of galaxies with T e -based abundances.Methods. We determined the gas-phase oxygen abundance for a sample of 695 galaxies and H ii regions with reliable detections of [O iii]4363, using the reliable and direct temperature-sensitive (T e ) method of measuring metallicity. We selected 531 star-forming galaxies from the SDSS-DR4 database with strong emission lines, including [O iii]4363 detected at a signal-to-noise ratio higher than 5σ, as well as 164 galaxies and H ii regions from the literature with T e measurements. The O/H abundances were derived from a two-zone model for the temperature structure, assuming a relationship between high ionization and low ionization species. Results. We compare our (O/H) Te measurements of the SDSS sample with the abundances obtained by the MPA/JHU group who used multiple strong emission lines and Bayesian techniques (Tremonti et al. 2004). For roughly half of the sample the Bayesian abundances are overestimated ∼0.34 dex, possibly due to the treatment of nitrogen enrichment in the models they used. The R 23 and R 23 − P methods systematically overestimate the O/H abundance by a factor of ∼0.20 dex and ∼0.06 dex, respectively. The N2 index, rather than the O3N2 index, provides more consistent O/H abundances with the T e -method, but with some scatter. The relations of N2, O3N2, and S2 with log(O/H) are consistent with the photoionization model calculations of Kewley & Doptita (2002), but R 23 does not match well. We derive analytical calibrations for O/H from R 23 , N2, O3N2, and S2 indices on the basis of this large sample, including the excitation parameter P as an additional parameter in the N2 calibration. These empirical calibrations are free of the systematic problems inherent in abundance calibrations based on photoionization models. Conclusions. We conclude that the N2, O3N2, and S2 indices are useful indicators for calibrating metallicities of galaxies with 12 + log(O/H) < 8.5 and that the R 23 index works well for the metal-poor galaxies with 12+log(O/H) < 7.9. For the intermediate metallicity range (7.9 < 12 + log(O/H) < 8.4), the R 23 and R 23 − P methods are unreliable for characterizing the O/H abundances,

Massive star formation in Wolf-Rayet galaxies

“…These empirical calibrations also have a higher dispersion than that estimated for Pilyugin (2001a,b) or Pilyugin & Thuan (2005) calibrations. Yin et al (2007) also found high discrepancies when comparing the theoretical metallicities using the theoretical models of Tremonti et al (2004) with the T e -based metallicites obtained from Pilyugin (2001a,b) and Pilyugin & Thuan (2005).…”

“…The evolution of such relationships are now predicted by semianalytic models of galaxy formation within the Λ-cold dark matter framework that include chemical hydrodynamic simulations (De Lucia et al 2004;Tissera et al 2005;De Rossi et al 2006;Davé & Oppenheimer 2007). Ironically, today the main problem is not to estimate the mass of a galaxy but its real metallicity, so that different methods involving direct estimates of the oxygen abundance, empirical calibrations using bright emissionline ratios or theoretical methods based on photoionization models yield very different values (i.e., Yin et al 2007;Kewley & Elisson 2008). …”

Integral field spectroscopy of a sample of nearby galaxies

“…In general it is considered that this estimator is valid for the range between −1 < log O3N2 < 1.9 (e.g. Yin et al 2007). However it becomes less reliable for metallicities lower than 12 + log (O/H) < 8.3 (e.g.…”

Influence of physical galaxy properties on Lyαescape in star-forming galaxies

“…A given R 23 value could correspond to high or low metallicity. Kewley & Ellison (2008) Several studies have demonstrated that the R 23 method generally overestimates the actual metallicity when compared to the direct method (e.g., Kennicutt et al 2003;Bresolin et al 2005;Yin et al 2007). This discrepancy prevents us from having an accurate measurement with R 23 for intermediate metallicities, i.e.…”