Visualisation of the Aquarius stream of stars and its location in the Milky Way.
Credit: Credits: Arman Khalatyan, AIP
PERTH: A new stream of stars has been discovered in the Milky Way, and is a remnant of a neighbouring dwarf galaxy, according to an international team of researchers.
Although nearby, the ‘Aquarius stream’ – so-called because it lies within the Aquarius constellation – has eluded scientists due the high concentration of stars in the Milky Way which made it almost indistinguishable. The exceptionally young stream was pulled apart by the gravitational force of the Milky Way just 700 million years ago.
“It was right on our doorstep,” said astrophysicist and lead author Mary Williams from the Astrophysical Institute Postdam (AIP) in Postdam, Germany, “but we just couldn’t see it.”
Discovered using radial velocity data
Williams discovered the Aquarius stream using radial velocity measurements of stars in the Milky Way region captured by the Radial Velocity Experiment (RAVE) based at the Australian Astronomical Observatory’s UK Schmidt telescope in Siding Spring, New South Wales.
The RAVE survey measures the radial velocity – how fast a star is moving toward or away from the Earth - of stars by analysing changes in the spectrum emitted by the star over a period of time.
As a star moves towards Earth its spectrum appears to shift towards the higher frequency - or blue end - of the spectrum. Conversely, the spectrum of a star appears to shift towards the lower frequency - or red end - of the spectrum when moving away from Earth. This phenomenon is known as the Doppler shift and is also used by many speed cameras.
A very young stream of stars
Williams’ findings, published in The Astrophysical Journal, are based on the first measurements of the radial velocity of 12,000 stars in the Milky Way region that have been recorded.
Using this data Williams noticed a group of 15 stars in the constellation of Aquarius moving in a way significantly different from the surrounding stars. Williams compared the movements of the stars in the Aquarius stream to computer simulations to discover they came from a dissolved dwarf galaxy.
According to Williams, by astronomical standards, the 700 million year old Aquarius stream is surprisingly young, as other known streams of stars located on the outskirts of our galaxy are billions of years old. “Finding remnants of an accretion (formed by gravity pulling together surrounding object and gases) like the Aquarius stream further closes the gap between what is predicted by theory and what we see in our backyard,” said Williams.
RAVE to collect data from one million stars
Astrophysicist Megan Argo with the International Centre for Radio Astronomy Research, who was not involved in the study said, “cosmically speaking the Aquarius stream is right in our neighbourhood which makes accurate follow up observations much easier. This allows us to study the properties of the stars, their chemical compositions, evolutionary stages and motions in some detail."
The RAVE survey will continue collecting information on up to one million stars in the Milky Way until 2012. “RAVE has reached ‘critical mass’ we now have enough data to start doing some really exciting science, as shown by the burst of papers from RAVE in the last year,” said Williams.
The Milky Way’s next massive collision is likely to be with the Andromeda galaxy in about 3 billion years, according to astronomers.
Scientists believe the dwarf galaxies that haven’t been ripped apart by larger galaxies may be hard to find as they consist of mostly dark matter. Dark matter does not emit or scatter radiation, which is what scientists generally rely on for their observations.
