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 n(h) = 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 LMS. 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.