Ionization in the Interstellar HI Region by Low energy Cosmic ray Electrons

Extract

Recent observations of the pulsar dispersion measures show a large scale height and relatively high density for the diffuse, ionized gas in HI regions. The maintenance of the ionization requires a pervasive ionizing agent. This agent could hardly be the UV photons from O stars or the extragalactic background UV photons due to their large optical depth; nor could it be the soft X–rays from the hot, ionized gas due to a low flux indicated by observations. Studies of the Galactic diffuse low energy γ-rays provides a clue to this problem. The extraordinarily high intensity of MeV electrons, which is derived from the γ–ray flux, could account for the ionization. In this scenario the electrons do both ionization and heating in HI regions, accordingly the atomic hydrogen gas is warm (~ 10⁴ K). The ionized gas density has a radial gradient with a scale length 4 kpc, there being more ionized gas in the inner Galaxy, due to a gradient in the electron intensity.