Radio observations at metre-centimetre wavelengths shed light on the nature of the emission of HII regions. Usually, this category of objects is dominated by thermal radiation produced by ionised hydrogen, namely protons and electrons. A number of observational studies have revealed the existence of HII regions with a mixture of thermal and non-thermal radiation. The latter represents a clue to the presence of relativistic electrons. However, neither the interstellar cosmic-ray electron flux nor the flux of secondary electrons produced by primary cosmic rays through ionisation processes are high enough to explain the observed flux densities.
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Source: Phys.org