Optical emission from high velocity clouds and the ionization sources in the Galactic halo

Optical emission lines have now been detected from about 20 high velocity clouds. These emission lines -- primarily H-alpha, secondarily [N II] and [S II] -- are very faint and diffuse, spread over the surfaces of the clouds. We compile emission line measurements and present a model in which the H-alpha is recombination caused by photoionizing radiation escaping the Milky Way. In such a model, we infer HVC distances of 5--30 kpc. The photoionization model fails to explain the relatively strong H-alpha emission from the Magellanic Stream, and the O VI absorption seen by FUSE in HVCs and the MS, which require a second source of ionization (likely collisional). Regardless of mechanism, the fact that HVCs are detectable in H-alpha indicates they are not far away enough to be Local Group objects. Adopting the HVC distances from the model, there appear to be two classes of HVCs: H-alpha-bright clouds with low velocity deviations from Galactic rotation, and often strong [N II], which are presumably affiliated with the Galactic disk; and H-alpha-faint clouds with high velocity deviations, which are likely to be infalling gas.