An international team of researchers believe the small satellite galaxies strewn around the Milky Way are the remains of a collision with another galaxy. And the same is probably true for our next door neighbour, the Andromeda Galaxy.
Australian National University astronomer Associate Professor Helmut Jerjen said the findings of a new study overturn the traditional theory that the small galaxies grew slowly in place.
"The current best theory predicts there should be about 1,000 satellite galaxies, randomly distributed around the Milky Way. But after searching the northern skies for the past 10 years, only 13 more Milky Way companions were found, taking the total to 24," Associate Professor Jerjen said.
"All the satellite galaxies are distributed in a flat plane, almost perpendicular to the disk of our Milky Way."
The team has been studying the ferris wheel of small galaxies strewn around the Milky Way, such as the Magellanic Clouds.
Milky Way's nearest neighbour, the Andromeda galaxy, has a similar stream of satellite galaxies orbiting in a plane, although at a different angle to the Milky Way's.
Professor Jerjen argues it is likely most large galaxies have had a close encounter or merged with another galaxy at least once in their life, and points to the example of the Antenna galaxy, which shows such a collision in progress.
"With new cameras you have the ability to probe large areas around galaxies, and you start to see these stellar streams and satellite galaxies," says Jerjen. "They're quite a common feature, which is something unexpected."
He now plans to search for these elusive Milky Way satellite galaxies over the entire southern skies using the ANU SkyMapper telescope to clarify the situation.
"There is a persisting discrepancy between the best theory and observation. Nature is trying to tell us something. There must be some underlying secret and we want to understand what it is."
The research will be published in the Monthly Notices of the Royal Astronomical Journal. A preprint is available here.
A computer model of two galaxies colliding, leading to the formation of streams of galactic debris, is shown in a short video.