  Among those objects formed out of collisional debris during galaxy mergers,
the prominent gaseous accumulations observed near the tip of some long tidal
tails are the most likely to survive long enough to form genuine recycled
galaxies. Using simple numerical models, Bournaud, Duc & Masset (2003) claimed
that tidal objects as massive as 10^9 Msun could only form, in these
simulations, within extended dark matter (DM) haloes. We present here a new set
of simulations of galaxy collisions to further investigate the structure of
tidal tails. First of all, we checked that massive objects are still produced
in full N-body codes that include feedback and a large number of particles.
Using a simpler N-body code with rigid haloes, we noticed that dissipation and
self-gravity in the tails, although important, are not the key factors.
Exploiting toy models, we found that, for truncated DM haloes, material is
stretched along the tail, while, within extended haloes, the tidal field can
efficiently carry away from the disk a large fraction of the gas, while
maintaining its surface density to a high value. This creates a density
enhancement near the tip of the tail. Only later-on, self-gravity takes over;
the gas clouds collapse and start forming stars. Thus, such objects were
fundamentally formed following a kinematical process, according to a top-down
scenario, contrary to the less massive Super Star Clusters that are also
present around mergers. This conclusion leads us to introduce a restrictive
definition for Tidal Dwarf Galaxies (TDGs) and their progenitors, considering
only the most massive ones, initially mostly made of gas, that were able to
pile up in the tidal tails. More simulations will be necessary to precisely
determine the fate of these proto--TDGs and estimate their number.
