Hα Emission from High‐Velocity Clouds and Their Distances

We present deep Hα spectroscopy toward several high-velocity clouds (HVCs), which vary in structure from compact HVCs (CHVCs) to the Magellanic Stream. The clouds range from being bright (~640 mR) to having upper limits on the order of 30-70 mR. The Hα measurements are discussed in relation to their H I properties, and distance constraints are given to each of the complexes based on esc ≈ 6% of the ionizing photons escaping normal to the Galactic disk ( f esc ≈ 1%-2% when averaged over solid angle). The results suggest that many HVCs and CHVCs are within a ~40 kpc radius from the Galaxy and are not members of the Local Group at megaparsec distances. However, the Magellanic Stream is inconsistent with this model and needs to be explained. It has bright Hα emission and little [N II] emission and appears to fall into a different category than the currently detected HVCs. This may reflect the lower metallicities of the Magellanic Clouds compared to the Galaxy, but the strength of the Hα emission cannot be explained solely by photoionization from the Galaxy. The interaction of the Magellanic Stream with halo gas or the presence of yet unassociated young stars may assist in ionizing the Magellanic Stream.