The Interstellar Medium in IZw18 seen with JWST/MIRI: I. Highly Ionized Gas

TL;DR

This study uses JWST/MIRI spectra to analyze the highly ionized gas in the nearby dwarf galaxy IZw18, revealing extreme ionization levels likely driven by metal-poor stellar populations and X-ray sources, with new insights into its interstellar medium.

Contribution

First detailed JWST/MIRI spectroscopic analysis of IZw18's highly ionized gas, identifying multiple ionization sources and new continuum sources in a low-metallicity galaxy.

Findings

Detected 10 fine-structure lines indicating high ionization.

Ionization driven by metal-poor stars and X-ray sources, not shocks.

Discovered four new 14 micron continuum sources likely HII regions.

Abstract

We present JWST/MIRI spectra from the Medium-Resolution Spectrometer of IZw18, a nearby dwarf galaxy with a metallicity of $\sim$3% Solar. Its proximity enables a detailed study of highly ionized gas that can be interpreted in the context of newly discovered high-redshift dwarf galaxies. We derive aperture spectra centered on eleven regions of interest; the spectra show very low extinction, A_V $\lesssim 0.1$, consistent with optical determinations. The gas is highly ionized; we have detected 10 fine-structure lines, including [OIV] 25.9 micron with an ionization potential (IP) of $\sim$ 55 eV, and [NeV] 14.3 micron with an IP of $\sim$ 97 eV. The ionization state of IZw18 falls at the extreme upper end of all of the line ratios we analyzed, but not coincident with galaxies containing an accreting massive black hole (active galactic nucleus). Comparison of the line ratios with state-of-the-art photoionization and shock models suggests that the high ionization state in IZw18 is not due to shocks. Rather it can be attributed to metal-poor stellar populations with a self-consistent contribution of X-ray binaries or ultra-luminous X-ray sources. It could also be partially due to a small number of hot low-metallicity Wolf-Rayet stars ionizing the gas; a small fraction (a few percent) of the ionization could come from an intermediate-mass black hole. Our spectroscopy also revealed four 14 micron continuum sources, $\gtrsim 30-100$ pc in diameter, three of which were not previously identified. Their properties are consistent with HII regions ionized by young star clusters.