SDSS-V LVM: Detectability of Wolf-Rayet stars and their He II ionizing flux in low-metallicity environments

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 <ns0:sc>II</ns0:sc> ionizing flux, but the average metallicity of the SMC is below where WR bumps are usually detected in integrated galaxy spectra showing nebular He <ns0:sc>II</ns0:sc> emission. Utilizing the Local Volume Mapper (LVM) integral-field spectroscopic survey, we investigate regions around the six SMC WN3h stars, whose winds are optically thin at ≥54 eV, allowing these energetic photons to escape. Focusing on He <ns0:sc>II</ns0:sc> 4686 Å, we show that the broad stellar wind component, the strongest optical diagnostic of WN3h stars, is diluted within 24 pc in the integrated spectra, making such WR stars hard to detect in unresolved low-metallicity regions. In addition, we compare the He <ns0:sc>II</ns0:sc> ionizing flux from LVM with the values inferred from the stellar atmosphere code PoWR and find that in nearly all cases, the stars emit more than enough hard ionizing photons to explain the observed He <ns0:sc>II</ns0:sc> nebular emission. We conclude that early-type WN stars with comparably thin winds are viable sources to produce the observed He <ns0:sc>II</ns0:sc> 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 <ns0:sc>II</ns0:sc> emission. This is challenging for population synthesis models across all redshifts as the evolutionary path toward this observed WR population at low metallicity remains enigmatic.