The galaxy Andromeda II is one of a swarm of small satellite galaxies surrounding the Andromeda Galaxy, a spiral galaxy like our Milky Way just 2.3 million light years away.
Movement of some stars in the small galaxy -- less than 1 percent of the Milky Way -- tells us that what we are observing is the remnant of a merger between two dwarf galaxies, scientists at the Niels Bohr Institute at the University of Copenhagen reported Sunday.
Mergers between galaxies of such low mass have not been observed before, they said.
"Stars in a dwarf galaxy often move around at random, but this is not exactly the case for Andromeda II," astrophysicist Nicola C. Amorisco said. "In particular we could see that a stream of stars is moving around differently than the rest in a very coherent way."
The rotating stream of stars in the galaxy is entirely made up of old stars and their properties and movements suggest a dramatic cosmic event, they said.
"What we are seeing is the remains of a collision between two dwarf galaxies, which had a dramatic effect on the dynamics of the remnant," Amorisco said.
Mergers between such small galaxies would have been common in the galaxy formation process in the early universe, but are rare at present times and had not been observed before, the scientists said.