X-ray binaries as the origin of nebular He II emission in low-metallicity star-forming galaxies

• Published in: Astronomy and astrophysics (Berlin) Vol. 622; p. L10
• Main Authors: Schaerer, D., Fragos, T., Izotov, Y. I.
• Format: Journal Article
• Language: English
• Published: Heidelberg EDP Sciences 01.02.2019
• Subjects: Galaxies; galaxies: high-redshift; galaxies: ISM; Metallicity; Star formation; X ray binaries; X ray stars; X-rays: binaries
• ISSN: 0004-6361; 1432-0746
• DOI: 10.1051/0004-6361/201935005

The origin of nebular He II emission, which is frequently observed in low-metallicity (O/H) star-forming galaxies, remains largely an unsolved question. Using the observed anticorrelation of the integrated X-ray luminosity per unit of star formation rate (LX/SFR) of an X-ray binary population with metallicity and other empirical data from the well-studied galaxy I Zw 18, we show that the observed He II λ4686 intensity and its trend with metallicity is naturally reproduced if the bulk of He+ ionizing photons are emitted by the X-ray sources. We also show that a combination of X-ray binary population models with normal single and/or binary stellar models reproduces the observed I(4686)/I(Hβ) intensities and its dependency on metallicity and age. We conclude that both empirical data and theoretical models suggest that high-mass X-ray binaries are the main source of nebular He II emission in low-metallicity star-forming galaxies.