The Magellanic Stream System: I. Ram-pressure tails and the relics of
  the collision between the Magellanic Clouds
  We have analyzed the Magellanic Stream (MS) using the deepest and the most
resolved H I survey of the Southern Hemisphere (the Galactic All-Sky Survey).
The overall Stream is structured into two filaments, suggesting two
ram-pressure tails lagging behind the Magellanic Clouds (MCs), and resembling
two close, transonic, von Karman vortex streets. The past motions of the Clouds
appear imprinted in them, implying almost parallel initial orbits, and then a
radical change after their passage near the N(H I) peak of the MS. This is
consistent with a recent collision between the MCs, $200-300$ Myr ago, which
has stripped their gas further into small clouds, spreading them out along a
gigantic bow shock, perpendicular to the MS. The Stream is formed by the
interplay between stellar feedback and the ram pressure exerted by hot gas in
the Milky Way (MW) halo with $\rho_{hot}$= $10^{-4}$ $cm^{-3}$ at 50-70 kpc, a
value necessary to explain the MS multiphase high-velocity clouds. The
corresponding hydrodynamic modeling provides the currently most accurate
reproduction of the whole H I Stream morphology, of its velocity, and column
density profiles along $L_{MS}$. The 'ram pressure plus collision' scenario
requires tidal dwarf galaxies, which are assumed to be the Cloud and dSph
progenitors, to have left imprints in the MS and the Leading Arm, respectively.
The simulated LMC and SMC have baryonic mass, kinematics and proper motions
consistent with observations. This supports a novel paradigm for the MS System,
which could have its origin in material expelled toward the MW by the ancient
gas-rich merger that formed M31.
