Far-Infrared N/O Abundance Estimates for Dusty Galaxies
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Bo Peng
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The Nitrogen-to-Oxygen (N/O) abundance ratio is closely tied to absolute abundance (O/H) and galaxy evolution. The fact that N/O inexplicitly appears in many metallicity indices also emphasizes the importance of study and precise measurement of N/O. Based on simple physical arguments as well as verification by photoionization models, we will show that the far-IR [N III]57/[O III]52 line ratio (N3O3) is a physically robust probe of N/O. This parameter is insensitive to the gas temperature and only weakly dependent on the electron density. Though it has a dependence on the hardness of the ionizing radiation field, it can be well corrected by including the [Ne III]15.5/[Ne II]12.8 line ratio. In order to assemble a complete collection of [O III] and [N III] lines which are not simultaneously accessible to Herschel/PACS, we carried out spectroscopic observations with SOFIA/FIFI-LS on a sample of nearby galaxies comprised of LIRGs and dwarf galaxies. We will demonstrate the application of N3O3 index on this sample of galaxies covering a large range of metallicity. We will also compare the results with various optical N/O measurements both taken from literature and calculated with N2S2 index. Although the far-IR and optical N/O estimates display a similar trend, we find evidence for a systematic offset of about 0.2 dex between the far-IR and optically derived N/O values. We argue this is likely due to the fact that our far-IR method is biased towards younger and denser HII regions, while the optical methods are biased towards older HII regions and probably affected by diffuse ionized gas. We will also discuss the promise of its application in the early universe.

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