SDSS-IV MaNGA: the impact of Diffuse Ionized Gas on emission-line ratios, interpretation of diagnostic diagrams, and gas metallicity measurements

Kai Zhang, Renbin Yan, Kevin Bundy, Matthew Bershady, L. Matthew Haffner, René Walterbos, Roberto Maiolino, Christy Tremonti, Daniel Thomas, Niv Drory, Amy Jones, Francesco Belfiore, Sebastian F. Sánchez, Aleksandar M. Diamond-Stanic, Dmitry Bizyaev, Christian Nitschelm, Brett Andrews, Jon Brinkmann, Joel R. Brownstein, Edmond CheungCheng Li, David R. Law, Alexandre Roman Lopes, Daniel Oravetz, Kaike Pan, Thaisa Storchi-Bergmann, Audrey Simmons

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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. At fixed metallicity, DIG-dominated low ΣHα 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 ΣHα. In line ratio diagnostic diagrams, contamination by DIG moves H II regions towards composite or 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 favors 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, R23, N2=[N II]/Hα, and N2S2Hα (Dopita et al. 2016) to derive metallicities introduces bias in the derived metallicity gradients as large as the gradient itself. The strong-line method of Blanc et al. (2015; IZI hereafter) cannot be applied to DIG to get an accurate metallicity because it currently contains only H II region models which fail to describe the DIG.
Original languageEnglish
Pages (from-to)3217-3243
JournalMonthly Notices of the Royal Astronomical Society
Issue number3
Early online date20 Dec 2016
Publication statusPublished - Apr 2017


  • astro-ph.GA
  • galaxies: surveys
  • galaxies: evolution
  • galaxies: fundamental parame- ters
  • galaxies: ISM
  • galaxies: abundances
  • galaxies: active
  • RCUK
  • STFC


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