Tidal Tail

Long tails of material that are the result of gravitational interactions between galaxies. During the interaction, gas and stars are stripped from the outer regions of the galaxies to form two tidal tails: one trailing and one preceding each galaxy. These tails may persist long after the galaxies have merged and are a signature of recent merger activity. Perhaps most famous tidal tails are those of the Antennae (NGC4038/4039), which are ~350,000 light years long. The central regions of the merging galaxies have hundreds of star forming knots similar in size to globular clusters (some are as large as dwarf galaxies). Tidal tails will persist for several billion years after the galactic merger.

Image source: http://jumk.de/astronomie/img/antennen-galaxie.jpg.

The motions of tidal tails can be used as powerful probes of the shape of dark matter halos within galaxies. For example, the tidal stream from the Sagittarius Dwarf Galaxy has been used to show that the dark halo of the Milky Way is close to spherical.