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The Nitrogen-to-Oxygen (N/O) abundance ratio is an important diagnostic ofgalaxy evolution since the ratio is closely tied to the growth of metallicityand the star formation history in galaxies. Estimates for the N/O ratio aretraditionally accomplished with optical lines that could suffer from extinctionand excitation effects, so the N/O ratio is arguably measured better throughfar-infrared (far-IR) fine-structure lines. Here we show that the [NIII]57$\mu$m/[O III]52$\mu$m line ratio, denoted $N3O3$, is a physically robustprobe of N/O. This parameter is insensitive to gas temperature and only weaklydependent on electron density. Though it has a dependence on the hardness ofthe ionizing radiation field, we show that it is well corrected by includingthe [Ne III]15.5$\mu$m/[Ne II]12.8$\mu$m line ratio. We verify the method, andcharacterize its intrinsic uncertainties by comparing the results tophotoionization models. We then apply our method to a sample of nearby galaxiesusing new observations obtained with SOFIA/FIFI-LS in combination withavailable Herschel/PACS data, and the results are compared with optical N/Oestimates. We find evidence for a systematic offset between the far-IR andoptically derived N/O ratio. We argue this is likely due to that our far-IRmethod is biased towards younger and denser H II regions, while the opticalmethods are biased towards older H II regions as well as diffuse ionized gas.This work provides a local template for studies of ISM abundance in the earlyUniverse.

Far-Infrared Line Diagnostics: Improving N/O Abundance Estimates for Dusty Galaxies