Volume 622, February 2019
|Number of page(s)||5|
|Section||Letters to the Editor|
|Published online||04 February 2019|
Letter to the Editor
X-ray binaries as the origin of nebular He II emission in low-metallicity star-forming galaxies
Observatoire de Genève, Université de Genève, 51 Ch. des Maillettes, 1290 Versoix, Switzerland
2 CNRS, IRAP, 14 Avenue E. Belin, 31400 Toulouse, France
3 Bogolyubov Institute for Theoretical Physics, National Academy of Sciences of Ukraine, 14-b Metrolohichna str., Kyiv 03143, Ukraine
Accepted: 17 January 2019
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.
Key words: galaxies: ISM / galaxies: high-redshift / X-rays: binaries
© ESO 2019
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