Revealing the Ionization Properties of the Magellanic Stream using
  Optical Emission
  The Magellanic Stream, a gaseous tail that trails behind the Magellanic
Clouds, could replenish the Milky Way with a tremendous amount of gas if it
reaches the Galactic disk before it evaporates into the halo. To determine how
the Magellanic Stream's properties change along its length, we have conducted
an observational study of the H-alpha emission, along with other optical warm
ionized gas tracers, toward 39 sight lines. Using the Wisconsin H-alpha Mapper
telescope, we detect H-alpha emission brighter than 30 - 50 mR in 26 of our 39
sight lines. This H-alpha emission extends more than 2-degree away from the HI
emission. By comparing H-alpha and [OI] intensities, we find that regions with
log NHI = 19.5 - 20.0 are 16 - 67% ionized. Most of the H-alpha intensities
along the Magellanic Stream are much higher than expected if the primary
ionization source is photoionization from Magellanic Clouds, the Milky Way, and
the extragalactic background. We find that the additional contribution from
self ionization through a "shock cascade" that results as the Stream plows
through the halo might be sufficient to reproduce the underlying level of
H-alpha emission along the Stream. In the sparsely sampled region below the
South Galactic Pole, there exists a subset of sight lines with
uncharacteristically bright emission, which suggest that gas is being ionized
further by an additional source that could be a linked to energetic processes
associated with the Galactic center.
