Title:The Disc of Satellites: The origin of Counter-Rotating Tidal Debris

Abstract:
Using numerical calculations, we show for the first time that counter-rotating material is a natural 
occurrence in tidal interactions of disc galaxies. Model particles on both pro- and retrograde orbits can be 
formed as tidal debris in single encounters of disc galaxies of 1-to-1 and 1-to-4 mass ratios. Calculations 
are performed for a range of different initial parameters, interactions include fly-by and merger cases. The 
fraction of counter-rotating material produced varies over a wide range (from a few up to 50 per cent). All 
fly-by models show a similar two-phase behaviour, with retrograde material forming first. Properties of the 
pro- and retrograde populations are extracted to make an observational discrimination possible.
Tidal-dwarf galaxies formed during such encounters will occupy the phase-space region of the tidal debris 
found in our calculations. This could be a reasonable explanation for the counter-rotation of the Sculptor 
dwarf galaxy with respect to the bulk motion of the disc of satellites. It would support the scenario of the 
MW satellites being ancient tidal-dwarf galaxies formed during an encounter of the young MW with another 
galaxy. A probable candidate for this galaxy is identified: the Magellanic Cloud progenitor galaxy, being in 
agreement with Lynden-Bell''s original suggestion for the origin of the dSph satellites.