SDSS-V LVM: Detectability of Wolf-Rayet stars and their He II ionizing flux in low-metallicity environments I. The weak-lined, early-type WN3 stars in the SMC
G. González-Torà, A. A. C. Sander, E. Egorova, R. R. Lefever, V. Ramachandran, O. V. Egorov, J. Josiek, E. C. Schösser, M. Bernini-Peron, K. Kreckel, A. Wofford, O. G. Telford, P. Senchyna, C. Leitherer, F. -H. Liang, G. Blanc, N. Drory, J. G. Fernández-Trincado, E. J. Johnston, A. J. Mejía-Narváez, S. F. Sanchez
TL;DR
This study investigates the sources of He II ionizing photons in a very low-metallicity context by examining six WN3h Wolf-Rayet stars in the SMC with the SDSS-V Local Volume Mapper. It shows that the broad wind signature of these stars is diluted in integrated light at apertures larger than ~24 pc, yet their He II ionizing flux can still power nebular He II emission, sometimes exceeding what is inferred from nebular lines when accounting for star-alone contributions. By comparing LVM-derived He II fluxes with PoWR atmosphere models, the work demonstrates that early-type WN stars with thin winds are viable He II suppliers in low-Z galaxies and that a substantial population may remain hidden in integrated spectra. These findings have important implications for population synthesis and galaxy evolution models across redshifts, as they suggest a missing WR contribution to hard ionizing photons in metal-poor systems.
Abstract
The Small Magellanic Cloud (SMC) is the nearest low-metallicity dwarf galaxy. Its proximity and low reddening has enabled us to detect its Wolf-Rayet (WR) star population with 12 known objects. Quantitative spectroscopy of the stars revealed half of these WR stars to be strong sources of He ii ionizing flux, but the average metallicity of the SMC is below where WR bumps are usually detected in integrated galaxy spectra showing nebular He ii emission. Utilizing the Local Volume Mapper (LVM), we investigate regions around the six SMC WN3h stars, whose winds are thin enough to avoid He recombination and allow photons with > 54 eV to escape. Focusing on He ii 4686 Å, we show that the broad stellar wind component, the strongest optical diagnostic of the WN3h stars, is diluted within 24 pc in the integrated light from LVM, making the WR stars hard to detect in low-metallicity integrated regions. In addition, we compare the He ii ionizing flux from LVM with the values inferred from the stellar atmosphere code PoWR and find that the nebular emission around them only in some cases reflects the high amounts emitted by the stars. We conclude that early-type WN stars with comparably weak winds are viable sources to produce the observed He ii ionizing flux in low-metallicity galaxies. The easy dilution of the stellar signatures can explain the rareness of WR bump detections at 12 + log O/H < 8.0, while at the same time providing major candidates for the observed excess of nebular He ii emission. This constitutes a challenge for population synthesis models across all redshifts as the evolutionary path towards this observed WR population at low metallicity remains enigmatic.