Remnant of merger between two dwarf galaxies spotted in Andromeda Galaxy

Researchers at Niels Bohr Institute have detected a stream of stars in one of the Andromeda Galaxy's outer satellite galaxies, a dwarf galaxy called Andromeda II.

The movement of the stars tells us that what we are observing is the remnant of a merger between two dwarf galaxies.

Mergers between galaxies of such low mass has not been observed before.

The galaxies in the early universe started off small and the theory of the astronomers is that the baby galaxies gradually grew larger and more massive by constantly colliding with neighbouring galaxies to form new, larger galaxies.

Large, massive galaxies constantly attract smaller galaxies due to gravity and they eventually merge together and grow even larger.

But not all of the small galaxies are being 'eaten' by the large galaxies. Some of them remain in an orbit around the large galaxy.

The largest galaxy in our cosmic neighbourhood is the Andromeda Galaxy, which is about 2.3 million light years away. Like our own galaxy, the Milky Way, Andromeda is a large spiral galaxy.

Andromeda is surrounded by a swarm of small galaxies - astronomers have counted more than 20.

Mergers between such small galaxies are expected during the galaxy formation process, but are rare at present times and had hitherto not been seen.

Andromeda II is the least massive known example of merging of galaxies so far and illustrates the scale-free character of the formation of galaxies down to the lowest galactic mass scales.

The results are published in the scientific journal, Nature.