Mysterious orbits of dwarf galaxies found all over the nearby Universe

A rare dwarf galaxy that initiated a burst of star formation within the last billion years.

NASA

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.

It's easy to dismiss that as a fluke of chance, but that became a bit harder to do as evidence built over the past several years that most of Andromeda's dwarf galaxies were also organized into a single plane. Stranger still, that plane's edge is oriented toward the Milky Way. Now, a French-Australian team of astronomers has figured out a way to search existing data for the presence of planes farther out from the Milky Way, finding that Andromeda's setup is actually quite common.