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 \pm 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 .