Optimal metallicity diagnostics for MUSE observations of low-z galaxies

• Published in: Monthly notices of the Royal Astronomical Society Vol. 527; no. 3; pp. 5484 - 5502
• Main Authors: Easeman, Bethan, Schady, Patricia, Wuyts, Stijn, Yates, Robert M
• Format: Journal Article
• Language: English
• Published: London Oxford University Press 01.01.2024
• Subjects: Emission; Galaxy distribution; H II regions; Ionization; Metallicity; Parameter identification; H; regions; galaxies: abundances; ISM: abundances
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/stad3464

ABSTRACT 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 [${\rm O\, {\small II}}$]λλ3726,29 doublet. We assess various strong line diagnostics by comparing to sulphur $T_{\rm {\it e}}$-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 ∼0.3 dex below $T_{\rm {\it e}}$-based measurements, with a clear dependence on both metallicity and ionization parameter. The N2S2H α diagnostic shows a near-linear relation with the $T_{\rm {\it e}}$-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 $12 + \log (\rm {O/H})$ > 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 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.