Optimal metallicity diagnostics for MUSE observations of low-z galaxies

The relatively red wavelength range (4800-9300 angstrom) of the VLT Multi Unit Spectroscopic Explorer (MUSE) limits which metallicity diagnostics can be used; in particular excluding those requiring the [O II]lambda lambda 3726,29 doublet. We assess various strong line diagnostics by comparing to sulphur T-e-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 similar to 0.3 dex below T-e-based measurements, with a clear dependence on both metallicity and ionization parameter. The N2S2H alpha diagnostic shows a near-linear relation with the T-e-based metallicities, although with a systematic downward offset of similar to 0.2 dex, but no clear dependence on ionization parameter. These results imply that the N2S2H alpha 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 HII regions, contrary to expectations. The S-calibration and N2S2H alpha diagnostics show no evidence of this, and show a positive relationship between ionization parameter and metallicity at 12 + log(O/H) > 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 alpha 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.