The SAMI Galaxy Survey: reconciling strong emission line metallicity diagnostics using metallicity gradients

We measure the gas-phase metallicity gradients of 248 galaxies selectedfrom Data Release 2 of the SAMI Galaxy Survey. We demonstrate that thereare large systematic discrepancies between the metallicity gradientsderived using common strong emission line metallicity diagnostics. Wedetermine which pairs of diagnostics have Spearman's rank coefficientsgreater than 0.6 and provide linear conversions to allow the accuratecomparison of metallicity gradients derived using different strongemission line diagnostics. For galaxies within the mass range 8.5 <log (M/M⊙) <11.0, we find discrepancies of up to 0.11dex/Re between seven popular diagnostics in the metallicitygradient-mass relation. We find a suggestion of a break in themetallicity gradient-mass relation, where the slope shifts from negativeto positive, occurs between 9.5 <log (M/M⊙) <10.5for the seven chosen diagnostics. Applying our conversions to themetallicity gradient-mass relation, we reduce the maximum dispersionfrom 0.11 dex/Re to 0.02 dex/Re. These conversionsprovide the most accurate method of converting metallicity gradientswhen key emission lines are unavailable. We find that diagnostics thatshare common sets of emission line ratios agree best, and thatdiagnostics calibrated through the electron temperature provide moreconsistent results compared to those calibrated through photoionizationmodels.