Metallicity calibrations for diffuse ionized gas and low-ionization emission regions

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Kumari, N 
Belfiore, F 

Using integral field spectroscopic data of 24 nearby spiral galaxies obtained with the Multi-Unit Spectroscopic Explorer (MUSE), we derive empirical calibrations to determine the metallicity of the diffuse ionized gas (DIG) and/or of the low-ionisation emission region (LI(N)ER) in passive regions of galaxies. To do so, we identify a large number of HII--DIG/LIER pairs that are close enough to be chemically homogeneous and we measure the metallicity difference of each DIG/LIER region relative to its HII region companion when applying the same strong line calibrations. The O3N2 diagnostic (=log [([O III]/Hβ)/([N II]/Hα)]) shows a minimal offset (0.01--0.04 dex) between DIG/LIER and HII regions and little dispersion of the metallicity differences (0.05 dex), suggesting that the O3N2 metallicity calibration for HII regions can be applied to DIG/LIER regions and that, when used on poorly resolved galaxies, this diagnostic provides reliable results by suffering little from DIG contamination. We also derive second-order corrections which further reduce the scatter (0.03--0.04 dex) in the differential metallicity of HII-DIG/LIER pairs. Similarly, we explore other metallicity diagnostics such as O3S2 (=log([O III]/Hβ+[S II]/Hα)) and N2S2Hα (= log([N II]/[S II]) + 0.264log([N II]/Hα)) and provide corrections for O3S2 to measure the metallicity of DIG/LIER regions. We propose that the corrected O3N2 and O3S2 diagnostics are used to measure the gas-phase metallicity in quiescent galaxies or in quiescent regions of star-forming galaxies.

ISM: abundances, galaxies: abundances, galaxies: individual, galaxies: spiral
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Monthly Notices of the Royal Astronomical Society
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Oxford University Press (OUP)
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Science and Technology Facilities Council (ST/M001172/1)
European Research Council (695671)