Letter to the Editor

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

¹ Zentrum für Astronomie der Universität Heidelberg, Astronomisches Rechen-Institut, Mönchhofstr. 12-14, 69120 Heidelberg, Germany
² Universität Heidelberg, Interdiszipliäres Zentrum für Wissenschaftliches Rechnen, 69120 Heidelberg, Germany
³ Instituto de Astronomía, Universidad Nacional Autónoma de México, A.P. 106, Ensenada, 22800 BC, Mexico
⁴ Department of Physics and Astronomy, University of Utah, 275 South University Street, Salt Lake City, UT, 84112, USA
⁵ Department of Astrophysical Sciences, Princeton University, 4 Ivy Lane, Princeton, NJ, 08544, USA
⁶ The Observatories of the Carnegie Institution for Science, 813 Santa Barbara Street, Pasadena, CA, 91101, USA
⁷ Space Telescope Science Institute, 3700 San Martin Dr, Baltimore, MD, 21218, USA
⁸ McDonald Observatory, The University of Texas at Austin, 1 University Station, Austin, TX, 78712-0259, USA
⁹ Universidad Católica del Norte, Instituto de Astronomía, Av. Angamos 0610, Antofagasta, Chile
¹⁰ Univ. Diego Portales, Inst. de Estudios Astrofísicos, Fac. de Ingeniería y Ciencias, Av. Ejército Libertador 441, Santiago, Chile
¹¹ Departamento de Astronomía, Universidad de Chile, Camino del Observatorio 1515, Las Condes, Santiago, Chile
¹² Instituto de Astrofísica de Canarias, La Laguna, Tenerife, E-38200, Spain
¹³ Departamento de Astrofísica, Universidad de La Laguna, Tenerife, E-38206, Spain

^⋆ Corresponding author: gemma.gonzalez-tora@uni-heidelberg.de

Received: 29 August 2025
Accepted: 28 October 2025

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) 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 II 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 II 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 II nebular emission. We conclude that early-type WN stars with comparably thin 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 is challenging for population synthesis models across all redshifts as the evolutionary path toward this observed WR population at low metallicity remains enigmatic.