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Ionization of He II in star-forming galaxies by X-rays from cluster winds and superbubbles

Abstract : 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.
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https://hal-insu.archives-ouvertes.fr/insu-03672017
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Submitted on : Thursday, May 19, 2022 - 11:40:21 AM
Last modification on : Wednesday, June 1, 2022 - 4:25:09 AM

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Lidia M. Oskinova, Daniel Schaerer. Ionization of He II in star-forming galaxies by X-rays from cluster winds and superbubbles. Astronomy & Astrophysics, 2022, 661, ⟨10.1051/0004-6361/202142520⟩. ⟨insu-03672017⟩

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