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

Journal
Date Issued
2017
Author(s)
Zhang, Kai
•
Yan, Renbin
•
Bundy, Kevin
•
Bershady, Matthew
•
Haffner, L. Matthew
•
Walterbos, René
•
Maiolino, Roberto
•
Tremonti, Christy
•
Thomas, Daniel
•
Drory, Niv
•
Jones, Amy
•
•
Sánchez, Sebastian F.
•
Diamond-Stanic, Aleksandar M.
•
Bizyaev, Dmitry
•
Nitschelm, Christian
•
Andrews, Brett
•
Brinkmann, Jon
•
Brownstein, Joel R.
•
Cheung, Edmond
•
Li, Cheng
•
Law, David R.
•
Roman Lopes, Alexandre
•
Oravetz, Daniel
•
Pan, Kaike
•
Storchi Bergmann, Thaisa
•
Simmons, Audrey
DOI
Abstract
Diffuse 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 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 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, R23, 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.
Volume
466
Issue
3
Start page
3217
Uri
Url
Issn Identifier
0035-8711
Ads BibCode
2017MNRAS.466.3217Z
Rights
open.access
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