SOFIA Observations of Far-IR Fine-structure Lines in Galaxies to Measure Metallicity

DOI: 
10.3847/1538-4357/ac37b7
Publication date: 
01/02/2022
Main author: 
Spinoglio L.
IAA authors: 
Pérez-Díaz, Borja;Pérez-Montero, Enrique
Authors: 
Spinoglio, Luigi;Fernández-Ontiveros, Juan Antonio;Malkan, Matthew A.;Kumar, Suyash;Pereira-Santaella, Miguel;Pérez-Díaz, Borja;Pérez-Montero, Enrique;Krabbe, Alfred;Vacca, William;Colditz, Sebastian;Fischer, Christian
Journal: 
Astrophysical Journal
Publication type: 
Article
Volume: 
926.0
Pages: 
55
Number: 
55
Abstract: 
We present new and archival SOFIA FIFI-LS far-IR spectroscopic observations of the [O iii] 52 μm and/or the [N iii] 57 μm 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 ( log(N/O)> -0.8). We explore various hypotheses to account for this difference: (i) difference in ionization structure traced by optical (O+, N+ regions) versus IR lines (O2+, N2+ 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(N/O)=(N/O)OPT-(N/O)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.
Database: 
SCOPUS
ADS
URL: 
https://ui.adsabs.harvard.edu/#abs/2022ApJ...926...55S/abstract
ADS Bibcode: 
2022ApJ...926...55S