@article{2022ApJ...926...55S,
 abstract = {We present new and archival SOFIA FIFI-LS far-IR spectroscopic
observations of the [O III] 52 \ensuremathμm and/or the [N
III] 57 \ensuremathμm lines of 25 local galaxies.
Including 31 other galaxies from Herschel-PACS, we discuss a
local sample of 47 galaxies, including the H II galaxies,
luminous IR galaxies, low-metallicity dwarfs, and Seyfert
nuclei. Analyzing the mid- to far-IR fine-structure lines of
this sample, we assess the metallicity and compare it with the
optical spectroscopy estimates. Using the IR, we find an O/H-N/O
relation similar to that known in the optical. Conversely, we
find systematically lower N/O IR abundances when compared to the
optical determinations, especially at high values of N/O ( ?\tex
tbackslashmathrm\log\(\m̊\N\\/\\textbacksla
shrm\O\\) $ -0.8). We explore various hypotheses to account
for this difference: (i) difference in ionization structure
traced by optical (O<SUP>+</SUP>, N<SUP>+</SUP> regions) versus
IR lines (O<SUP>2+</SUP>, N<SUP>2+</SUP> regions), (ii)
contamination of diffuse ionized gas affecting the optical lines
used to compute the N/O abundance, and (iii) dust obscuration
affecting the optical-based determinations. However, we have not
found any correlation of ?\̊Δ (\rN̊/\rm̊)=(
\rm̊/\rmO)̊_\mathrmOPT-(\rmN̊\rmO_̊\mathrm
IR$ with ionization, or electron density, or optical
extinction. We speculatively suggest that the accretion of
metal-poor gas from the circumgalactic medium could provide an
explanation for this difference because the rapid decrease of
total abundances during infall is followed by a N/O ratio
decrease due to the primary production of young-possibly
embedded-massive stars, which are preferentially traced by the
IR diagnostics, while optical diagnostics would better trace the
secondary production, when both N/O and O/H abundance ratios
increase.},
 adsnote = {Provided by the SAO/NASA Astrophysics Data System},
 adsurl = {https://ui.adsabs.harvard.edu/abs/2022ApJ...926...55S},
 archiveprefix = {arXiv},
 author = {Spinoglio, Luigi and Fernández-Ontiveros, Juan Antonio and Malkan, Matthew A. and Kumar, Suyash and Pereira-Santaella, Miguel and Pérez-Dı́az, Borja and Pérez-Montero, Enrique and Krabbe, Alfred and Vacca, William and Colditz, Sebastian and Fischer, Christian},
 doi = {10.3847/1538-4357/ac37b7},
 eid = {55},
 eprint = {2111.04434},
 journal = {\apj},
 keywords = {574, Astrophysics - Astrophysics of Galaxies},
 month = {February},
 number = {1},
 pages = {55},
 primaryclass = {astro-ph.GA},
 title = {SOFIA Observations of Far-IR Fine-structure Lines in Galaxies to Measure Metallicity},
 volume = {926},
 year = {2022}
}