Title：SDSS-IV MaNGA: The Impact of Diffuse Ionized Gas on Emission-line Ratios, Interpretation of Diagnostic Diagrams, and Gas Metallicity Measurements

　　Speaker：张锴 (Department of Physics and Astronomy, University of Kentucky)

　　Abstract：Diffuse Ionized Gas (DIG) is prevalent in star-forming galaxies. Using a sample of 365 nearly face-on star-forming galaxies observed by MaNGA, we demonstrate how DIG in star-forming galaxies impacts the measurements of emission line ratios, hence the interpretation of diagnostic diagrams and gas-phase metallicity measurements. We show that Hα emission line surface brightness (SB) is a reasonably good proxy to separate regions dominated by Hii regions from those dominated by DIG. At fixed metallicity, DIG displays enhanced [SII]/Hα, [NII]/Hα, [OII]/Hβ, and [OI]/Hα for almost all galaxies in our sample. In line ratio diagnostic diagrams, contamination by DIG moves Hii regions towards composite or LI(N)ER-like regions. Photoionization models for Hii regions with varying metallicities and ionization parameters fail to predict the line ratios seen in DIG without any additional variables. Ionizing spectrum hardness is the most likely the needed variable. Leaky H ii region models are capable of explaining the DIG with lower [OIII]/Hβ than Hii regions, but the DIG with higher [O III]/Hβ than H iiregions needs another ionization source. Our result favors ionization by evolved stars as a major ionization source for higher [OIII]/Hβ DIG.

　　DIG can significantly bias the measurement of gas metallicity and metallicity gra- dient derived using strong line methods. Metallicities derived using N2O2 is optimal because it exhibits the smallest bias and error. Using O3N2 and N2S2Hα to derive metallicities can bias the metallicity gradient as large as ±0.05 dex Re−1 for an individual galaxy,a significant bias compared with the universal metallicity gradient of -0.1 dex Re−1 for local star-forming galaxies. Using R23 and N2=[Nii]/Hα to derive metallicities will systematically bias the metallicity gradients by ∼-0.1 dex Re−1 and ∼0.05-0.1 dex Re−1 .