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 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 more than 2 • away from the H i emission. By comparing I Hα and I [O i] , we find that regions with log N H i / 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 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 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. 1 1 Rayleigh = 10 6 /4π photons cm −2 sr −1 s −1 , which is ∼ 2.41 × 10 −7 erg cm −2 s −1 sr −1 at Hα.

The Leading Arm (LA) of the Magellanic Stream is a vast debris field of H I clouds connecting the Milky Way and the Magellanic Clouds. It represents an example of active gas accretion onto the Galaxy. Previously only one chemical abundance measurement had been made in the LA. Here we present chemical abundance measurements using Hubble Space Telescope/Cosmic Origins Spectrograph and Green Bank Telescope spectra of four AGN sightlines passing through the LA and three nearby sightlines that may trace outer fragments of the LA. We find low oxygen abundances, ranging from 4.0 +2.0 −2.0 % solar to 12.6 +6.0 −4.1 % solar, in the confirmed LA directions, with the lowest values found in the region known as LA III, farthest from the LMC. These abundances are substantially lower than the single previous measurement, S/H=35±7% solar (Lu et al. 1998), but are in agreement with those reported in the SMC filament of the trailing Stream, supporting a common origin in the SMC (not the LMC) for the majority of the LA and trailing Stream. This provides important constraints for models of the formation of the Magellanic System. Finally, two of the three nearby sightlines show high-velocity clouds with H I columns, kinematics, and oxygen abundances consistent with LA membership. This suggests that the LA is larger than traditionally thought, extending at least 20 • further to the Galactic northwest.

The Leading Arm (LA) is a tidal feature that is in front of the Magellanic Clouds (MCs) on their orbit through the Galaxy’s halo. Many physical properties of the LA, such as its mass and size, are poorly constrained because it has few distance measurements. While Hα measurements have been used to estimate the distances to halo clouds, many studies have been unsuccessful in detecting Hα from the LA. In this study, we explore a group of H i clouds which lie 75°–90° from the MCs. Through ultraviolet and 21 cm radio spectroscopy, this region, dubbed the LA Extension, was found to have chemical and kinematic similarities to the LA. Using the Wisconsin Hα Mapper, we detect Hα emission in four out of seven of our targets. Assuming that this region is predominantly photoionized, we use a radiation model that incorporates the contributions of the Galaxy, MCs, and the extragalactic background at z = 0 to derive a heliocentric distance of d ⊙ ≥ 13.4 . We also use this model to rederive Hα distances of d ⊙ ≥ 5.0 kpc and d ⊙ ≥ 22.9 to two clouds in the literature that might also be associated with the LA. Using these new measurements, and others in the literature, we provide a general trend of the variation of LA heliocentric distance as a function of Magellanic Stream longitude, and explore its implications for the origin and closest point of approach of the LA.