Metallicity Calibrations and the Mass‐Metallicity Relation for Star‐forming Galaxies

Abstract

(Abridged) We investigate the effect of metallicity calibrations, AGN\nclassification, and aperture covering fraction on the local mass-metallicity\n(MZ) relation using 27,730 star-forming galaxies from the Sloan Digital Sky\nSurvey (SDSS) Data Release 4. We analyse the SDSS MZ relation with 10\nmetallicity calibrations, including theoretical and empirical methods. We show\nthat the choice of metallicity calibration has a significant effect on the\nshape and y-intercept(12+log(O/H)) of the MZ relation. The absolute metallicity\nscale (y-int) varies up to 0.7 dex, depending on the calibration used, and the\nchange in shape is substantial. These results indicate that it is critical to\nuse the same metallicity calibration when comparing different\nluminosity-metallicity or mass-metallicity relations. We present new\nmetallicity conversions that allow metallicities that have been derived using\ndifferent strong-line calibrations to be converted to the same base\ncalibration. These conversions facilitate comparisons between different\nsamples, particularly comparisons between galaxies at different redshifts for\nwhich different suites of emission-lines are available.Our new conversions\nsuccessfully remove the large 0.7 dex discrepancies between the metallicity\ncalibrations, and we reach agreement in the MZ relation to within 0.03 dex on\naverage. We find that different AGN classification methods have negligible\neffect on the SDSS MZ relation. We compare the SDSS MZ relation with nuclear\nand global relations from the Nearby Field Galaxy Survey (NFGS). The turn over\nof the MZ relation depends on aperture covering fraction. We find that a lower\nredshift limit of z<0.04 is insufficient for avoiding aperture effects in fiber\nspectra of the highest stellar mass galaxies.\n