The Coldest Phase in Halo High-velocity Gas: Dust and Molecules

Abstract

In this chapter, we discuss the dust content and the abundance and distribution of molecular gas in the Galactic Halo. During the last two decades, significant progress has been made in observing dust and molecules in halo clouds using various instruments and observing techniques. Infrared dust emission and ultraviolet absorption by molecular hydrogen, H2, now have been detected in a number of neutral halo clouds, but only in regions where the overall metal abundance is close to that of the Milky Way or the Magellanic Clouds. The column densities of the molecular gas in the halo generally are very low, but the presence of H₂ both indicates the presence of dust and implies the existence of small-scale structure at sub-parsec/AU scales. Moreover, the H₂ measurements provide important density and temperature estimates for the absorbing structures and thus reveal important new aspects of the physical conditions in the Galactic Halo clouds.