The extended HeII4686-emitting region in IZw18 unveiled: clues for peculiar ionizing sources

TL;DR

This study uses integral field spectroscopy to map the nebular HeII4686 emission in IZw18, revealing that conventional ionizing sources are insufficient and suggesting the presence of peculiar, possibly metal-free, massive stars as the ionization source.

Contribution

It provides the first spatially resolved HeII-ionizing flux map of IZw18 and demonstrates that models of low-metallicity super-massive or rotating metal-free stars better explain the observed emission.

Findings

Conventional sources like WRs, shocks, and X-ray binaries cannot fully explain the HeII emission.

Models of low-metallicity super-massive or rotating metal-free stars match the HeII-ionization budget.

Rotating metal-free star models explain the highest HeII4686/Hbeta ratios.

Abstract

New integral field spectroscopy has been obtained for IZw18, the nearby lowest-metallicity galaxy considered our best local analog of systems forming at high-z. Here we report the spatially resolved spectral map of the nebular HeII4686 emission in IZw18, from which we derived for the first time its total HeII-ionizing flux. Nebular HeII emission implies the existence of a hard radiation field. HeII-emitters are observed to be more frequent among high-z galaxies than for local objects. So investigating the HeII-ionizing source(s) in IZw18 may reveal the ionization processes at high-z. HeII emission in star-forming galaxies, has been suggested to be mainly associated with Wolf-Rayet stars (WRs), but WRs cannot satisfactorily explain the HeII-ionization at all times, in particular at lowest metallicities. Shocks from supernova remnants, or X-ray binaries, have been proposed as additional potential sources of HeII-ionizing photons. Our data indicate that conventional HeII-ionizing sources (WRs, shocks, X-ray binaries) are not sufficient to explain the observed nebular HeII4686 emission in IZw18. We find that the HeII-ionizing radiation expected from models for either low-metallicity super-massive O stars or rotating metal-free stars could account for the HeII-ionization budget measured, while only the latter models could explain the highest values of HeII4686/Hbeta observed. The presence of such peculiar stars in IZw18 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.