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arXiv · 2311.03514

Optimal metallicity diagnostics for MUSE observations of low-z galaxies

Bethan Easeman·Patricia Schady·Stijn Wuyts·Robert Yates
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Abstract

The relatively red wavelength range (4800-9300Å) of the VLT Multi Unit Spectroscopic Explorer (MUSE) limits which metallicity diagnostics can be used; in particular excluding those requiring the [O ii]λλ3726,29 doublet. We assess various strong line diagnostics by comparing to sulphur Te-based metallicity measurements for a sample of 671 HII 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 which lie up to ~0.3 dex below Te-based measurements, with a clear dependence on both metallicity and ionisation parameter. The N2S2Hα diagnostic shows a near-linear relation with the Te-based metallicities, although with a systematic downward offset of ~0.2 dex, but no clear dependence on ionisation parameter. These results imply that the N2S2Hα diagnostic produces the most reliable results when studying the distribution of metals within galaxies with MUSE. On sub-HII region scales, the O3N2 and N2 diagnostics measure metallicity decreasing towards the centres of HII regions, contrary to expectations. The S-calibration and N2S2Hα diagnostics show no evidence of this, and show a positive relationship between ionisation parameter and metallicity at 12 + log(O/H)> 8.4, implying the relationship between ionisation parameter and metallicity differs on local and global scales. We also present HIIdentify, a python tool developed to identify HII regions within galaxies from Hα emission maps. All segmentation maps and measured emission line strengths for the 4408 HII regions identified within the MAD sample are available to download.

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Bethan Easeman, Patricia Schady, Stijn Wuyts, Robert Yates. 2023-11-06. Optimal metallicity diagnostics for MUSE observations of low-z galaxies. https://arxiv.org/abs/2311.03514

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