The formation of high-ionization species associated with Galactic, intermediate velocity H I

We report first results on the photoproduction of the species H I, Al III, Si IV, and C IV kinematically associated with the Galactic intermediate‐velocity (IV) arch. We find that photoionization of solar abundance gas cannot reproduce the observed column densities of these species, while models using stellar radiation fields and the observed x‐ray background can reproduce the observed columns in gas having depleted metal abundances. The pattern of metal depletions required is consistent with their incorporation into standard interstellar grains. Limits on the intensity of the halo radiation field indicate that the material of the IV arch is smoothly distributed, having a local density ∼10−2 cm−3 and filling factor of order unity. These results suggest that photoionization plays a key role in the production of at least some of the high‐ionization species Si IV and C IV in the Galactic halo.