THE MAGELLANIC STREAM SYSTEM. I. RAM-PRESSURE TAILS AND THE RELICS OF THE COLLISION BETWEEN THE MAGELLANIC CLOUDS

Abstract

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