New Clues on the Extended He ii Ionization in IZw18 from GTC/MEGARA and JWST/MIRI

Abstract

IZw18 is one of the lowest-metallicity star-forming galaxies known at z ∼ 0, considered a unique local analogue of the first galaxies. The origin of its hard ionizing continuum, expected to be a common feature in the early Universe and traced by He ii emission lines, remains intensely debated and challenging to explain. Here we combine optical Gran Telescopio Canarias MEGARA and mid-infrared (JWST/MIRI) integral field spectroscopic observations for IZw18 to shed new light on the high-ionization phenomenon. This Letter reports the first detection of the high-ionization [Ne v]14.32 μm line in IZw18. Its emission is spatially extended and coincident with the He ii peak, revealing the presence of highly energetic ionizing sources that surpass mechanisms previously proposed on the basis of He ii alone. Our kinematic studies highlight that the He iiλ4686-emitting gas displays higher velocity dispersions and a different velocity pattern compared with the Hβ emission, suggesting the presence of energetic processes such as shocks or stellar-driven feedback. Additionally, integrated spectra show asymmetric blueshifted profiles in the He iiλ4686 line, possibly indicating early-stage stellar-driven outflows potentially facilitating future ionizing photon leakage. Our spatial analysis also reveals differences in structure between the emission of Hβ and He iiλ4686, with the He iiλ4686 peak offset by a projected distance of 140 pc from the peak Hβ emission. This indicates distinct locations for the most extreme ionizing sources compared to moderate ionizing sources. Our findings underscore the complex interplay of physical processes in extremely metal-poor environments with highly ionized gas, offering new insights into the conditions prevailing in the early galaxies.