During the interaction/merger of two massive galaxies, tidal forces strip material out of their disk and form spectacular tidal tidals. Gas and stars within the tidal debris can collapse under self-gravity and form new objects: Tidal Dwarf Galaxies (TDGs). The picture below show three interacting systems with bona-fide TDGs (Lelli et al. 2015). Deep optical images are overlaid with HI emission from the VLA (blue) and UV emission from GALEX (pink).