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

G. González-Torà, A. A. C. Sander, E. Egorova, O. Egorov, M. Bernini-Peron, J. Josiek, K. Kreckel, R. R. Lefever, V. Ramachandran, E. C. Schösser

TL;DR

This study investigates whether low-metallicity WN3 Wolf-Rayet stars can provide the He II ionizing photons observed in high-redshift and nearby low-$Z$ galaxies. It combines PoWR non-LTE atmosphere calculations with Local Volume Mapper IFU spectroscopy to compare six resolvable WN3 stars in the SMC against integrated nebular spectra, deriving the ionizing budget $Q_{HeII}$ from both stellar and nebular data. The main results show that WN3 winds in the SMC are typically optically thin with $tau_F(R_{sonic})<1$ but can supply sufficient He II ionizing photons to explain nebular emission, although the stellar He II features are diluted in apertures larger than about 24 pc. Unresolved observations may miss WR signatures even when they drive the He II ionization budget, highlighting a gap in current population synthesis at low metallicity and motivating improved WR evolution models for early 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 $\lambda4686\,Å$ 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.

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

TL;DR

This study investigates whether low-metallicity WN3 Wolf-Rayet stars can provide the He II ionizing photons observed in high-redshift and nearby low- galaxies. It combines PoWR non-LTE atmosphere calculations with Local Volume Mapper IFU spectroscopy to compare six resolvable WN3 stars in the SMC against integrated nebular spectra, deriving the ionizing budget from both stellar and nebular data. The main results show that WN3 winds in the SMC are typically optically thin with but can supply sufficient He II ionizing photons to explain nebular emission, although the stellar He II features are diluted in apertures larger than about 24 pc. Unresolved observations may miss WR signatures even when they drive the He II ionization budget, highlighting a gap in current population synthesis at low metallicity and motivating improved WR evolution models for early 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 () 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 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- galaxies.

Paper Structure

This paper contains 3 sections, 2 equations, 2 figures.

Table of Contents

  1. Introduction
  2. Atmosphere analyses and IFU observations
  3. Results and conclusions

Figures (2)

  • Figure 1: Left: AB7 fiber apertures studied from the LVM. Center: Normalized 4686 $\AA$ flux of the star and the different fiber apertures of the LVM. Right: Intrinsic flux of the WN3h component in the AB 7 binary.
  • Figure 2: Ratio of Q$_{\mathrm{HeII}}$ for the different stars and the corresponding LVM integrated region with respect to the diameter of the LVM aperture. The Q$_{\mathrm{HeII, star}}$ ratio is obtained by the PoWR models, while the Q$_{\mathrm{HeII, LVM}}$ is calculated using Eq. \ref{['eq:flux']}, using only for the narrow nebular component at each aperture.