Abstract
Faint, diffuse optical line emission has been detected from ∼ 20 high velocity clouds (HVCs), the Magellanic Stream (MS), and HI gas projected near the Sculptor dwarf spheroidal. Only one HVC is undetected at < 20 milli-Rayleighs; all others are brighter than expected from metagalactic UV ionization alone, implying that HVCs are not extragalactic nor Local Group objects. If the Hα emission is produced by photoionization from the Galaxy, HVC distances are generally tens of kpc, placing them in the Galactic halo. However, photoionization by the Galaxy is too weak to power the Magellanic Stream Hα. The MS Ha, and the O VI which FUSE has discovered in HVCs and the MS, imply a source of collisional ionization, but its cause remains undetermined.
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