Metallicity calibrations and the mass-metallicity relation for star-forming galaxies

We investigate the effect of metallicity calibrations, AGN classification, and aperture covering fraction on the local mass-metallicity (M-Z) relation using 27,730 star-forming galaxies from the SDSS Data Release 4. We analyze the SDSS M-Z relation with 10 metallicity calibrations, including theoretical and empirical methods. We show that the choice of metallicity calibration has a significant effect on the shape and y-intercept [12 + log (O/H)] of the M-Z relation. The absolute metallicity scale (y-intercept) varies up to Δ [log (O/H)] = 0.7 dex, depending on the calibration used, and the change in shape is substantial. These results indicate that it is critical to use the same metallicity calibration when comparing different luminosity-metallicity or M-Z relations. We present new metallicity conversions that allow metallicities that have been derived using different strong-line calibrations to be converted to the same base calibration. These conversions facilitate comparisons between different samples, particularly comparisons between galaxies at different redshifts for which different suites of emission lines are available. Our new conversions successfully remove the large 0.7 dex discrepancies between the metallicity calibrations, and we reach agreement in the M-Zrelation to within 0.03 dex on average. We investigate the effect of AGN classification and aperture covering fraction on the M-Z relation. We find that different AGN classification methods have negligible effect on the SDSS M-Z relation. We compare the SDSS M-Z relation with nuclear and global relations from the NFGS. The turnover of the M-Z relation at M* ∼ 10¹⁰ M _⊙ depends on the aperture covering fraction. We find that a lower redshift limit of z < 0.04 is insufficient for avoiding aperture effects in fiber spectra of the highest stellar mass (M* > 10 ¹⁰ M_⊙) galaxies.