  In the best observed spiral galaxies, the HI is observed to decline slowly
with radius for up to 90% of the maximum extent, before experiencing a rapid
truncation. Early models predicted that the outer parts of spirals would be
fully ionized by a metagalactic UV field at column depths of roughly 10^{19}
atoms cm^{-2}. Weak optical line emission has now been observed beyond the HI
edge in a number of cases and, furthermore, this warm gas has been used to
trace the dark halo far beyond the HI cut-off. But the observed emission
measures (40-90 mR) are higher than expected for a cosmic origin. A more likely
explanation is ionization of the warped outer disk by the blue stellar
population at smaller radius. There are many implications of ionized warped
edges, in particular, severe HI warps may be greatly under represented in
current HI surveys. We briefly discuss the implications of this model for the
Galactic warp.
