Large galaxies such as the Milky Way appear to have been built by repeated mergers of smaller ones, but not every small galaxy has ended up being swallowed completely by a large one. The Milky Way is orbited by dozens of dwarf galaxies, some of which have been disrupted and stripped of stars, while others may have slipped into orbit largely intact. Similar dwarf galaxies orbit our nearby neighbors, including Andromeda.
Based on what we know about these mergers and computer modeling of galaxy formation and growth, the collection of dwarfs should be an unruly lot, having approached the galaxy they orbit from directions that are essentially random. Yet the dwarfs orbiting the Milky Way largely inhabit a single plane, orbiting in a manner analogous to moons around a giant planet.
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and also the concluding paragraph--Overall, the disks of dwarf galaxies probably play a significant role in the physics of the local environment. The authors estimate that their collective angular momentum is roughly the same as all the stars in the main galaxy. As for how they got there, that's less clear. The fact that the orientation of these disks is correlated with the nearby galaxies suggests that it may be dictated by the larger-scale structural organization of the Universe. But how large-scale structure can influence the dwarfs isn't obvious, and it will be difficult to determine until we can start getting our models to reproduce it.
