Abstract 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 O 3+ /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 O 3+ /H + abundances from a theoretical ionization correction factor (ICF) against observed values and found that the oxygen ICF underestimates the O 3+ /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.
Vaught et al. (Fri,) studied this question.