2017ApJ...851..110B

The Magellanic Stream, a gaseous tail that trails behind the Magellanic Clouds, could replenish the Milky Way (MW) 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 Hα emission, along with other optical warm ionized gas tracers, toward 39 sight lines. Using the Wisconsin Hα Mapper telescope, we detect Hα emission brighter than 30-50 mR in 26 of our 39 sight lines. This Hα emission extends over 2^○ away from the H I emission. By comparing I_Hα and I_[OI], we find that regions with logN_HI/cm^–2~19.5–20.0 are 16%-67% ionized. Most of the I_Hα along the Magellanic Stream are much higher than expected if the primary ionization source is photoionization from Magellanic Clouds, the MW, 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 Hα 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.