New clues on the extended HeII ionization in IZw18 from GTC/MEGARA and JWST/MIRI

Abstract: IZw18 is one of the lowest-metallicity star-forming galaxies known at z$\sim$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\textsc{ii} emission lines, remains intensely debated and challenging to explain. Here we combine optical (GTC/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\textsc{v}]14.32 $\mu$m line in IZw18. Its emission is spatially extended and coincident with the He\textsc{ii} peak, revealing the presence of highly energetic ionizing sources that surpass mechanisms previously proposed on the basis of He\textsc{ii} alone. Our kinematic studies highlight that the He\textsc{ii}$\lambda$4686-emitting gas displays higher velocity dispersions and a different velocity pattern compared to the H$\beta$ emission, suggesting the presence of energetic processes such as shocks or stellar-driven feedback. Additionally, integrated spectra show asymmetric blueshifted profiles in the He\textsc{ii}$\lambda$4686 line, possibly indicating \textbf{early-stage} stellar-driven outflows potentially facilitating future ionizing photon leakage. Our spatial analysis also reveals differences in structure between the emission of H$\beta$ and He\textsc{ii}$\lambda$4686, with the He\textsc{ii}$\lambda$4686 peak offset by a projected distance of 140 pc from the peak H$\beta$ 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 \textbf{high-ionized} gas, offering new insights into the conditions prevailing in the early galaxies.