Astronomers have uncovered the shredded remains of a dwarf galaxy buried deep inside the stellar disk of our own galaxy the Milky Way.
The discovery will help scientists understand how galaxies consume smaller satellite galaxies, also known as galactic cannibalism.
The findings reported in the Astrophysical Journal, were made by the RAVE (RAdial Velocity Experiment) collaboration, an international team of scientists led by Professor Fred Watson from the Australian Astronomical Observatory.
Watson says models of galactic evolution predict big galaxies should be surrounded by a lot of smaller dwarf galaxies.
"But we don't see as many of these small galaxies as we expect. One theory is they've been cannibalised by the bigger galaxies."
According to Watson the stars of the dwarf galaxy are stretched out in a long trail as they orbit a larger galaxy. They remain in this formation, even after they have been swallowed up.
"It's this stream of stars moving differently compared to stars around them which signals their inter-galactic origins," he says.
RAVE astronomer Dr Mary Williams from the Astrophysical Institute in Potsdam, Germany, used the UK Schmidt Telescope at Siding Spring in New South Wales to identify 15 stars moving in a direction different to the group of 12,000 surrounding it.
Watson says, while more than a dozen star streams have been detected so far, this one is rare because it's so close.
"Being no more than 10 kiloparsecs (32,620 light years) from our solar system, places it within our Sun's neighbourhood," he says.
"These stars were discovered mostly in the disc of the galaxy rather than the galactic halo, a spherical swarm of stars surrounding the galaxy where we thought these sorts of stars should have been."
All in the motion
Watson says there was nothing unusual about these individual stars to make them stand out other than their common motion, moving at about 15,000 kilometres per hour relative to other stars in the galaxy.
"The stars are in the direction of the constellation Aquarius, so we called them the Aquarius Stream," he says.
"Their chemical composition indicates they're about ten billion years old, that's a sizeable fraction of the age of the universe so this galaxy had a independent existence for most of its life."
Watson says extrapolating the observations backwards allowed the team to determine the galactic collision generating this star stream was well underway 700 million years ago.
"It's only in the last decade that we've realised these star streams are caused by galactic collisions in the Milky Way.
"It's a science that's just coming of age."