New research led by Nicola Cristiano Amorisco from the Niels Bohr Institute in Copenhagen yields evidence that dwarf galaxy Andromeda II was formed from the merger of two smaller galaxies. There has never before been confirmation of a cosmic collision between such small galaxies. The results from the study have been published in Nature.
Andromeda is the closest spiral galaxy to our Milky Way, located around 2.4 million light years away in the constellation Andromeda. It is surrounded by over two dozen satellite galaxies, one of which is a dwarf spheroidal galaxy named Andromeda II, which is less than one percent of the size of the Milky Way.
Scientists theorize that large galaxies are created from collisions with several smaller galaxies. The larger the galaxies get, the more gravity pulls in small galaxies and grows even larger. Small galaxies that are not pulled in to connect with the larger galaxy may be affected enough to become satellites, just like a planet can have orbiting moons.
Through analysis of stars in Andromeda II, Amorisco et al. determined that there was a stream of stars along the dwarf galaxy’s border. These stars are considerably older than the rest of the stars, which indicates that they originated in a different galaxy and were mixed in. The old stars almost fully encircle the galaxy and actually rotate around, which is unusual in a dwarf galaxy. Typically, star movement in these small galaxies is more random, but the merger must have impacted the stars’ dynamics.
While it was assumed that these collisions among smaller galaxies had to happen at some point, it was believed that they happened at the onset of large galaxy formation. Andromeda II is now the lowest mass galaxy merger ever discovered. Because most galaxy formation happened early in the Universe’s history and doesn’t happen very often anymore, there isn’t a lot for scientists to observe on the issue. Astronomers will be able to use this information to try to understand how small galaxies came together to form some of the largest galaxies we can see today.