The extended He II λ4686-emitting region in IZw 18 unveiled: Clues for peculiar ionizing sources

Abstract

New integral field spectroscopy has been obtained for IZw 18, the nearby lowest-metallicity galaxy considered to be our best local analog of systems forming at high redshift (z). Here we report the spatially resolved spectral map of the nebular He ii λ4686 emission in IZw 18, from which we derived for the first time its total He ii-ionizing flux. Nebular He ii emission implies the existence of a hard radiation field. He ii-emitters are observed to be more frequent among high-z galaxies than for local objects. Therefore, investigating the He ii-ionizing source(s) in IZw 18 may reveal the ionization processes at high z. He ii emission in star-forming galaxies has been suggested to be mainly associated with Wolf-Rayet stars (WRs), but WRs cannot satisfactorily explain the He ii-ionization at all times, particularly at the lowest metallicities. Shocks from supernova remnants, or X-ray binaries, have been proposed as additional potential sources of He ii-ionizing photons. Our data indicate that conventional He ii-ionizing sources (WRs, shocks, X-ray binaries) are not sufficient to explain the observed nebular He iiλ4686 emission in IZw 18. We find that the He ii-ionizing radiation expected from models for either low-metallicity super-massive O stars or rotating metal-free stars could account for the He ii-ionization budget measured, while only the latter models could explain the highest values of He iiλ4686/Hβ observed. The presence of such peculiar stars in IZw 18 is suggestive and further investigation in this regard is needed. This letter highlights that some of the clues of the early universe can be found here in our cosmic backyard.