  In several interacting systems, gas accumulations as massive as 10^9 solar
masses are observed near the tip of tidal tails, and are thought to be possible
progenitors of Tidal Dwarf Galaxies. Using N-body simulations of galaxy
interactions, we show that the existence of such features requires that dark
matter haloes around spiral galaxies extend at least ten times further than the
stellar disks. The massive gas clouds formed in our simulations have a
kinematical origin and gravitationally collapse into dwarf galaxies that often
survive for a few billion years.
