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

Research output: Contribution to journalArticlepeer-review

  • Kai Zhang
  • Renbin Yan
  • Kevin Bundy
  • Matthew Bershady
  • L. Matthew Haffner
  • René Walterbos
  • Roberto Maiolino
  • Christy Tremonti
  • 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 Cheung
  • Cheng Li
  • David R. Law
  • Alexandre Roman Lopes
  • Daniel Oravetz
  • Kaike Pan
  • Thaisa Storchi-Bergmann
  • Audrey Simmons
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


  • MNRAS-2016-Zhang-mnras-stw3308

    Rights statement: This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society ©: 2016 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.

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