A Multiwavelength Assessment Disfavoring the X-ray Binary Origin of He III Regions in Metal-Poor Star-Forming Dwarf Galaxies
Ivan Altunin, Christopher Ellis, Richard M. Plotkin, Roberto Soria, Ryan Tanner + 7 more
TLDR
A multiwavelength study disfavors X-ray binaries as the primary source of extreme-ultraviolet radiation creating He III regions in metal-poor dwarf galaxies.
Key contributions
- Conducted a systematic Chandra X-ray study of 21 metal-poor dwarf galaxies exhibiting He II emission.
- Quantified the extreme-ultraviolet (EUV) output from X-ray sources and the EUV required for He II.
- Revealed X-ray sources provide systematically less EUV than needed to sustain observed He II regions.
- Concludes that accreting X-ray sources alone cannot explain He III regions, pointing to other EUV sources.
Why it matters
This study resolves a long-standing puzzle regarding the origin of He III regions in early galaxies, which are crucial for understanding early galaxy formation. By disproving X-ray binaries as the primary source, it redirects future research towards alternative extreme-ultraviolet radiation mechanisms. This has significant implications for models of the early universe.
Original Abstract
Recent observations of metal-poor, star-forming dwarf galaxies reveal He III regions, traced by nebular He II 4686 emission that require a strong source of extreme-ultraviolet (EUV) radiation. The origin of this hard ionizing radiation remains poorly understood, as standard stellar populations fail to account for it, posing key implications for the understanding of early galaxy formation. We present a systematic Chandra X-ray study of 21 nearby star-forming galaxies with He II emission but lacking Wolf-Rayet spectral signatures. Using 7 new and 36 archival Chandra X-ray observations combined with optical stellar population synthesis modelling, we constrain the ionizing continuum required to sustain the observed He II line, the ionizing continuum available from X-ray objects, and the properties of the host H II regions. We find that the inferred EUV output from accreting X-ray sources in our sample is systematically lower than what is required to produce the observed He II emission. Our sample is consistent with established empirical scaling relations for X-ray luminosity, indicating that this discrepancy cannot be attributed to an anomalously low number or luminosity of X-ray sources. These results indicate that accreting X-ray sources alone cannot account for the observed He II-ionizing photon budget, pointing to additional or alternative sources of hard EUV radiation in metal-poor star-forming environments. Potential alternative or additional contributors are discussed.
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