Zhang, KaiKaiZhangYan, RenbinRenbinYanBundy, KevinKevinBundyBershady, MatthewMatthewBershadyHaffner, L. MatthewL. MatthewHaffnerWalterbos, RenéRenéWalterbosMaiolino, RobertoRobertoMaiolinoTremonti, ChristyChristyTremontiThomas, DanielDanielThomasDrory, NivNivDroryJones, AmyAmyJonesBELFIORE, FRANCESCO MICHEL CONCETTOFRANCESCO MICHEL CONCETTOBELFIORESánchez, Sebastian F.Sebastian F.SánchezDiamond-Stanic, Aleksandar M.Aleksandar M.Diamond-StanicBizyaev, DmitryDmitryBizyaevNitschelm, ChristianChristianNitschelmAndrews, BrettBrettAndrewsBrinkmann, JonJonBrinkmannBrownstein, Joel R.Joel R.BrownsteinCheung, EdmondEdmondCheungLi, ChengChengLiLaw, David R.David R.LawRoman Lopes, AlexandreAlexandreRoman LopesOravetz, DanielDanielOravetzPan, KaikeKaikePanStorchi Bergmann, ThaisaThaisaStorchi BergmannSimmons, AudreyAudreySimmons2021-01-132021-01-1320170035-8711http://hdl.handle.net/20.500.12386/29762Diffuse ionized gas (DIG) is prevalent in star-forming galaxies. Using a sample of 365 nearly face-on star-forming galaxies observed by Mapping Nearby Galaxies at APO, 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. At fixed metallicity, DIG-dominated low Σ<SUB>Hα</SUB> regions display enhanced [S II]/Hα, [N II]/Hα, [O II]/Hβ and [O I]/Hα. The gradients in these line ratios are determined by metallicity gradients and Σ<SUB>Hα</SUB>. In line ratio diagnostic diagrams, contamination by DIG moves H II regions towards composite or low-ionization nuclear emission-line region (LI(N)ER)-like regions. A harder ionizing spectrum is needed to explain DIG line ratios. Leaky H II region models can only shift line ratios slightly relative to H II region models, and thus fail to explain the composite/LI(N)ER line ratios displayed by DIG. Our result favours ionization by evolved stars as a major ionization source for DIG with LI(N)ER-like emission. DIG can significantly bias the measurement of gas metallicity and metallicity gradients derived using strong-line methods. Metallicities derived using N2O2 are optimal because they exhibit the smallest bias and error. Using O3N2, R<SUB>23</SUB>, N2 = [N II]/Hα and N2S2Hα to derive metallicities introduces bias in the derived metallicity gradients as large as the gradient itself. The strong-line method of Blanc et al. (IZI hereafter) cannot be applied to DIG to get an accurate metallicity because it currently contains only H II region models that fail to describe the DIG.STAMPAenArticle10.1093/mnras/stw33082-s2.0-85018285808000398418600050https://academic.oup.com/mnras/article/466/3/3217/27265922017MNRAS.466.3217ZFIS/05 - ASTRONOMIA E ASTROFISICAERC sectors::Physical Sciences and Engineering::PE9 Universe sciences: astro-physics/chemistry/biology; solar systems; stellar, galactic and extragalactic astronomy, planetary systems, cosmology, space science, instrumentation::PE9_6 Stars and stellar systems