H_alpha Emission from High-Velocity Clouds and their Distances

We present deep Halpha 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 (similar to640 mR) to having upper limits on the order of 30-70 mR. The Halpha measurements are discussed in relation to their H I properties, and distance constraints are given to each of the complexes based on (f) over cap (esc)approximate to6% of the ionizing photons escaping normal to the Galactic disk (f(esc)approximate to1%-2% when averaged over solid angle). The results suggest that many HVCs and CHVCs are within a similar to40 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 Halpha 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 Halpha 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.