AU Scale Structures in Extra-planar Gas

Recent spectroscopic observations of intermediate- and high-velocity clouds (IVCs and HVCs) in the Milky Way halo have unveiled the presence of diffuse interstellar molecular hydrogen (H_2) several kpc away from the Galactic disk. Most of this H_2 gas appears to reside in relatively small (~0.1 pc), dense (n_H~30 cm^-3) gaseous filaments that probably are part of the cold neutral medium (CNM) in IVCs and HVCs. Also much smaller structures at AU scale and very high densities (n_H~800 cm^-3) have been observed, suggesting the presence of tiny-scale atomic structures (TSAS) in the Milky Way's extra-planar gas. It is not yet understood how such objects can form and exist in the Milky Way halo, but the high detection rate of H_2 absorption in IVCs implies that the CNM represents a gas phase that is characteristic for neutral clouds in the lower halo.