Far-Infrared N/O Abundance Estimates and Science with [O III] and [N III]
The Nitrogen-to-Oxygen (N/O) abundance ratio is closely tied to absolute abundance (O/H) and galaxy evolution. The study of N/O is also important for its inexplicit appearance in many metallicity indices. Based on simple physical arguments and 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. 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. Compared with optical N/O measurements, we find evidence for a systematic offset of about 0.2 dex between the far-IR and optically derived N/O values, likely due to the different regions that far-IR and optical methods are biased to. An the end We will discuss the promise of its application in early universe. We will also discuss the power of [N III] and [O III] lines in studying ionization structure of H II regions and precise measurement of physical properties of galaxies.