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

Abstract

Using integral field spectroscopic data of 24 nearby spiral galaxies obtained with the Multi Unit Spectroscopic Explorer, we derive empirical calibrations to determine the metallicity of the diffuse ionized gas (DIG) and/or of the low-ionization emission region (LI(N)ER) in passive regions of galaxies. To do so, we identify a large number of H II-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 H II region companion when applying the same strong line calibrations. The O3N2 diagnostic (=log [([O III]/H β)/([N II]/H α)]) shows a minimal offset (0.01-0.04 dex) between DIG/LIER and H II regions and little dispersion in the metallicity differences (0.05 dex), suggesting that the O3N2 metallicity calibration for H II 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 that further reduce the scatter (0.03-0.04 dex) in the differential metallicity of H II-DIG/LIER pairs. Similarly, we explore other metallicity diagnostics such as O3S2 (=log([O III]/H β + [S II]/H α)) and N2S2H α (= log([N II]/[S II]) + 0.264log([N II]/H α)) 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.`