Galaxies have tails?

When disk galaxies interact graviationally, tidal forces peel long, thin, streamers of stars off either side of the galaxies: "Tidal Tails" (Toomre & Toomre 1972).

(Sparke & Gallagher 2000)

Simulation by Hibbard and Mihos (1995).

(Hibbard & van Gorkom 1996)

Because the outer parts of the galaxy are in a shallower potential well, and are therefore less bound, it should be easier to pull stars off of the outside of the galaxy. This means that tidal tails tell us something about the outer portions of the galaxy.