A recalibration of strong-line oxygen abundance diagnostics via the direct method and implications for the high-redshift universe

We use direct method oxygen abundances in combination with strong optical emission lines, stellar masses (M ⋆), and star formation rates (SFRs) to recalibrate the N2, O3N2, and N2O2 oxygen abundance diagnostics. We stack spectra of ∼200 000 star-forming galaxies from the Sloan Digital Sky Survey in bins of M ⋆ and SFR offset from the star-forming main sequence ( $ {\Delta \log (SSFR)}$ ) to measure the weak emission lines needed to apply the direct method. All three new calibrations are reliable to within ±0.10 dex from log (M ⋆/M⊙) ∼ 7.5–10.5 and up to at least 200 M⊙ yr−1 in SFR. The N2O2 diagnostic is the least subject to systematic biases. We apply the diagnostics to galaxies in the local Universe and investigate the M ⋆–Z–SFR relation. The N2 and O3N2 diagnostics suggest the SFR dependence of the M ⋆–Z–SFR relation varies with both M ⋆ and ${\Delta \log (SSFR)}$ , whereas the N2O2 diagnostic suggests a nearly constant dependence on SFR. We apply our calibrations to a sample of high-redshift galaxies from the literature, and find them to be metal-poor relative to local galaxies with similar M ⋆ and SFR. The calibrations do reproduce direct method abundances of the local analogues. We conclude that the M ⋆–Z–SFR relation evolves with redshift.