The life and evolution of galaxies are
dramatically affected by environmental effects.
Interactions with the intergalactic medium and
collisions with companions cause major
perturbations in the morphology and contents of
galaxies: in particular stars and gas clouds may
be gravitationally pulled out from their parent
galaxies during tidal encounters, forming rings,
tails and bridges. This debris of collisions lies
at the origin of a new generation of small
galaxies, the so-called “tidal dwarf galaxies”
(hereafter TDGs). Such an exotic way of forming
galaxies was put forward by Schweizer (1978) and
by Mirabel et al. (1992), who clearly observed the
genesis of a star-forming object, out of material
tidally expelled from the interacting system NGC
4038/39 (“The Antennae”). Recent studies, based on
optical and HI observations, have shown that TDGs
actually form a class of “recycled” objects with
some properties similar to the more classical
dwarf irregulars (dIrr) and blue compact dwarf
galaxies (BCDs).