Simulation of the formation of long-lived TDGs, from the Bournaud & Duc sample [11]. The two colliding spiral galaxies are initially seen face-on. Yellow-red to blue colors code old stars versus young stars; time is in megayear. Massive TDGs form at the tip of the tails, and some lower-mass ones also form in particular in the Northern tail. A multi-grid technique increases the resolution on the most massive TDG to 10 parsec. It is resolved with a small internal spiral structure, and survives several gigayears around a massive red elliptical galaxy.
