The Interstellar Medium in I Zw 18 Seen with JWST/MIRI. III. Spatially Resolved Three Ionization State Oxygen Abundance

Journal article

We present observations of the nearby extremely metal-poor galaxy I Zw 18 using the Keck Cosmic Web Imager (KCWI) and the JWST Mid-InfraRed Instrument (MIRI) Integral Field Spectrographs. From optical and mid-IR oxygen emission lines, we measured direct-method abundances for three ionic states of oxygen, including O3+/H+. In contrast to previous studies of I Zw 18 the high spatial resolution afforded by KCWI and MIRI/MRS revealed chemical inhomogeneities on 60 pc scales in the form of metal-poor pockets and metal-enriched gas. These are located outside I Zw 18’s star-forming complexes having possibly been dispersed beyond these regions via stellar feedback effects. We found that metallicities derived using a single low-ionization density tracer, and Te([O ii]) derived from a temperature relationship commonly used in high-z galaxy studies, exhibited the largest scatter and underestimated the metallicity compared to those derived using multi-ion densities and estimated Te([N ii]). Finally, we compared O3+/H+ abundances from a theoretical ionization correction factor (ICF) against observed values and found that the oxygen ICF underestimates the O3+/H+ abundance by a factor of 2, indicating that either additional ionizing sources are needed or standard stellar population models are unable to produce the requisite ionizing flux.

Authors: Rickards Vaught, RJ; Hunt, LK; Aloisi, A; Navarro, MG; Mingozzi, M

• Publication status: Published
• Peer review status: Peer reviewed
• Publisher: American Astronomical Society
• Journal: The Astrophysical Journal
• Volume: 990
• Issue: 2
• Article number: 111
• Publication date: 2025-09-05
• Acceptance date: 2025-07-15
• DOI: 10.3847/1538-4357/adf10f
• EISSN: 1538-4357
• ISSN: 0004637X, 0004-637X
• Language: English
• Keywords: Blue compact dwarf galaxies; Interstellar medium; Metallicity