Optimal metallicity diagnostics for MUSE observations of low-z galaxies

The relatively red wavelength range (4800-9300 Å) of the VLT Multi Unit Spectroscopic Explorer (MUSE) limits which metallicity diagnostics can be used; in particular excluding those requiring the []λλ3726,29 doublet. We assess various strong line diagnostics by comparing to sulphur -based metallicity measurements for a sample of 671 H ii regions from 36 nearby galaxies from the MUSE Atlas of Disks (MAD) survey. We find that the O3N2 and N2 diagnostics return a narrower range of metallicities that lie up to ∼0.3 dex below -based measurements, with a clear dependence on both metallicity and ionization parameter. The N2S2H α diagnostic shows a near-linear relation with the -based metallicities, although with a systematic downward offset of ∼0.2 dex, but no clear dependence on ionization parameter. These results imply that the N2S2H α diagnostic produces the most reliable results when studying the distribution of metals within galaxies with MUSE. On sub-H ii region scales, the O3N2 and N2 diagnostics measure metallicity decreasing towards the centres of H ii regions, contrary to expectations. The S-calibration and N2S2H α diagnostics show no evidence of this, and show a positive relationship between ionization parameter and metallicity at > 8.4, implying the relationship between ionization parameter and metallicity differs on local and global scales. We also present hiidentify, a python tool developed to identify H ii regions within galaxies from H α emission maps. All segmentation maps and measured emission line strengths for the 4408 H ii regions identified within the MAD sample are available to download.