Archer of the Galactic disk? The effect on the outer HI disk of the   Milky Way of collisional encounters with the Sagittarius dwarf galaxy
  Hydrodynamical calculations undertaken to simulate the collisional interaction between the Sgr dwarf galaxy and the Galactic outer HI disk are presented, constrained by recently derived orbital and mass parameters for this dwarf galaxy. It is found that a significant distortion to the structure of the Galactic HI disk will be induced by the collision if the mass of the dwarf exceeds ~10^9 M_Sun; this value is consistent with an estimate derived by requiring that the dwarf galaxy is sufficiently robust to survive tidal disruption until the present time. Though the precise details of the interaction are compromised in our simulations by the lack of a live Galactic halo, we find that for model masses > 5x10^9 M_Sun, prominent spiral arms and a substantial lopsidedness in the outer disk are produced. Furthermore, a noticeable warp-like structure is induced in the disk. Thus the Sgr dwarf may have significantly affected the star formation history and structure of the outer Galaxy. These simulations confirm the possibility of determining the current merging rate of low surface brightness, gas-poor dwarf galaxies of mass > 10^9 M_Sun onto giant spiral galaxies from careful analysis of observations of the structure of HI disks. 
Rodrigo A. Ibata, Alexey O. Razoumov