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 ( \sim 0.1 pc ) , dense ( n _ { H } \approx 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 } \approx 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 .