2025ApJ...990..111R

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 T_e([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 T_e([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.