Can Wolf-Rayet stars be the missing ingredient to explain high-z He II ionizing radiation?

TL;DR

This study investigates whether low-metallicity Wolf-Rayet stars can account for the high-energy He II ionizing radiation observed in distant and nearby low-metallicity galaxies, addressing a gap in stellar population models.

Contribution

It demonstrates that WN3 Wolf-Rayet stars at low metallicity emit sufficient ionizing radiation to explain observed nebular He II emission, highlighting their potential role in galaxy evolution.

Findings

Low-Z WN3 stars emit enough ionizing photons for He II nebular emission.

Stellar emission is diluted in integrated spectra, becoming undetectable beyond 24 pc.

WN3 stars are strong candidates for He II ionizing sources in high-z and low-Z galaxies.

Abstract

Classical Wolf-Rayet (WR) stars are hot, massive stars with depleted hydrogen. At low metallicities (Z), WN3-type WR stars have relatively thin winds and are major sources of ionizing flux. The detection of high-ionization emission lines in high-redshift ($z$) galaxies as well as nearby low-Z dwarf galaxies raises questions about the origin of He II ionizing radiation and its role in galaxy evolution, as stellar population models fail to reproduce the required fluxes. Low-Z WN3 stars may provide the missing contribution but are easily hidden in integrated light. Using the Local Volume Mapper, we compare resolved optical spectra of SMC WN3 stars with integrated regions, focusing on the broad He II $\lambda4686\,\AA$ line. We find stellar emission diluted within nebular regions, becoming undetectable when integrating over areas larger than 24 pc. Nonetheless, these stars emit enough ionizing photons to explain observed He II nebular emission, being strong candidates for the He II ionizing sources in low-Z and high-$z$ galaxies.