Issue |
A&A
Volume 661, May 2022
|
|
---|---|---|
Article Number | A67 | |
Number of page(s) | 6 | |
Section | Extragalactic astronomy | |
DOI | https://doi.org/10.1051/0004-6361/202142520 | |
Published online | 03 May 2022 |
Ionization of He II in star-forming galaxies by X-rays from cluster winds and superbubbles
1
Institute for Physics and Astronomy, University Potsdam, 14476 Potsdam, Germany
e-mail: lida@astro.physik.uni-potsdam.de
2
Observatoire de Genève, Département d’Astronomie, Université de Genève, 51 chemin Pegasi, 1290 Versoix, Switzerland
3
CNRS, IRAP, 14 avenue E. Belin, 31400 Toulouse, France
Received:
25
October
2021
Accepted:
3
March
2022
The nature of the sources powering nebular He II emission in star-forming galaxies remains debated, and various types of objects have been considered, including Wolf-Rayet stars, X-ray binaries, and Population III stars. Modern X-ray observations show the ubiquitous presence of hot gas filling star-forming galaxies. We use a collisional ionization plasma code to compute the specific He II ionizing flux produced by hot gas and show that if its temperature is not too high (≲2.5 MK), then the observed levels of soft diffuse X-ray radiation could explain He II ionization in galaxies. To gain a physical understanding of this result, we propose a model that combines the hydrodynamics of cluster winds and hot superbubbles with observed populations of young massive clusters in galaxies. We find that in low-metallicity galaxies, the temperature of hot gas is lower and the production rate of He II ionizing photons is higher compared to high-metallicity galaxies. The reason is that the slower stellar winds of massive stars in lower-metallicity galaxies input less mechanical energy in the ambient medium. Furthermore, we show that ensembles of star clusters up to ∼10−20 Myr old in galaxies can produce enough soft X-rays to induce nebular He II emission. We discuss observations of the template low-metallicity galaxy I Zw 18 and suggest that the He II nebula in this galaxy is powered by a hot superbubble. Finally, appreciating the complex nature of stellar feedback, we suggest that soft X-rays from hot superbubbles are among the dominant sources of He II ionizing flux in low-metallicity star-forming galaxies.
Key words: galaxies: ISM / galaxies: high-redshift / ISM: bubbles / X-rays: ISM
© ESO 2022
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