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

Abstract

The nature of the sources powering nebular HeII 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 HeII ionizing flux produced by hot gas and show that if its temperature is not too high (less than 2.5 MK), then the observed levels of soft diffuse X-ray radiation could explain HeII 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 HeII 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 HeII emission. We discuss observations of the template low-metallicity galaxy I Zw 18 and suggest that the HeII 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 HeII ionizing flux in low-metallicity star-forming galaxies.

6 pages, 3 figures, accepted by A&A

References in corpus (15)

• A Superbubble Feedback Model for Galaxy Simulations
• Star clusters near and far; tracing star formation across cosmic time
• X-ray binaries as the origin of nebular HeII emission in low-metallicity star-forming galaxies
• The extended HeII4686-emitting region in IZw18 unveiled: clues for peculiar ionizing sources
• Efficiently Cooled Stellar Wind Bubbles in Turbulent Clouds II. Validation of Theory with Hydrodynamic Simulations
• The Metallicity Dependence of the High-Mass X-ray Binary Luminosity Function
• The Energy Cascade from Warm Dark Matter Decays
• Catastrophic Cooling in Superwinds: Line Emission and Non-equilibrium Ionization
• On the contribution of the X-ray source to the extended nebular HeII emission in IZw18
• X-ray spectroscopy of the starburst feedback in 30 Doradus
• Keck Cosmic Web Imager Observations of He II Emission in I Zw 18
• The effects of binary stars on galaxies and metal-enriched gas during reionization
• X-ray emission from star cluster winds in starburst galaxies
• On the contribution of ULXs to stellar feedback: an intermediate mass black hole candidate and the population of ULXs in the low-metallicity starburst galaxy ESO 338-4
• Blue galaxies: modelling nebular He II emission in high redshift galaxies

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• Bringing Stellar Evolution & Feedback Together: Summary of proposals from the Lorentz Center Workshop, 2022
• Nebular C IV 1550 Imaging of the Metal-Poor Starburst Mrk 71: Direct Evidence of Catastrophic Cooling
• Low-metallicity massive single stars with rotation. III. Source of ionization and C-IV emission in I Zw 18
• Gas excitation in galaxies and active galactic nuclei with He IIλ4686 and X-ray emission
• New clues on the extended HeII ionization in IZw18 from GTC/MEGARA and JWST/MIRI
• The significant contribution of supersoft X-ray Sources to the nebular HeII line emission