Making Counter-Orbiting Tidal Debris: The Origin of the Milky Way Disc
  of Satellites
  Using stellar-dynamical calculations it is shown for the first time that
counter-orbiting material emerges naturally in tidal interactions of disc
galaxies. Model particles on both pro- and retrograde orbits can be formed as
tidal debris in single encounters with disc galaxies of 1-to-1 and 4-to-1 mass
ratios. A total of 74 model calculations are performed for a range of different
initial parameters. Interactions include fly-by and merger cases. The fraction
of counter-orbiting material produced varies over a wide range (from a few up
to 50 percent). All fly-by models show a similar two-phase behaviour, with
retrograde material forming first. Properties of the prograde and retrograde
populations are extracted to make an observational discrimination possible.
  During such encounters the tidal debris occupies a certain region in phase
space. In this material, tidal-dwarf galaxies may form. The modelling therefore
can explain why galaxies may have dwarf galaxies orbiting counter to the bulk
of their dwarf galaxies. An example is the Sculptor dwarf of the Milky Way,
which orbits counter to the bulk of the disc of satellites. The modelling thus
supports the scenario of the MW satellites being ancient tidal-dwarf galaxies
formed from gaseous material stripped from another galaxy during an encounter
with the young MW.
  A possible candidate for this galaxy is identified as the Magellanic Cloud
progenitor galaxy. Its angular motion fits the angular motion of the MW disc of
satellites objects. This scenario is in agreement with Lynden-Bell's original
suggestion for the origin of the dSph satellites and the near-unbound orbit of
the LMC.
