Astronomers dig up a 'buried dwarf'

Mary Williams Aquarius Stream

Dr Mary Williams (AIP).
Photo: Dr Mary Williams

Visualization of the Aquarius
Stream of stars in our Galaxy.
Credit: Arman Khalatyan, AIP


Astronomers using a telescope in eastern Australia have identified the shredded remains of a dwarf galaxy 'buried' inside our own as they seek to better understand how our Galaxy has grown over time.

"Models of how galaxies evolve predict that big galaxies like ours will be surrounded by lots of little ones," said Professor Fred Watson of the Australian Astronomical Observatory.

"But we don?t see enough of those little ones," he said. "Why? Well, perhaps many of them have been eaten up by the big galaxies. So, we're looking at our Milky Way Galaxy to try to find little galaxies that it's swallowed."

Professor Watson is Project Manager for the 10-nation RAVE (Radial Velocity Experiment) collaboration that found the 'buried' galaxy. RAVE runs on a 1.2-m telescope of the Australian Astronomical Observatory and has so far measured the velocities of 385 000 stars.

Led by New Zealand scientist Dr Mary Williams, some RAVE team members analysed the movements of 12 000 stars in the disk of our Galaxy ? the flat, starry part away from the Galaxy's centre. Among the stars, they noticed 15 that were moving anomalously, at speeds of up to 15,000 kilometres per hour.

Analysis showed that they were part of a large stream of stars, originating from a small galaxy that ours had dismembered about 700 million years ago.

Most of the stars in the stream lie in the direction of the constellation of Aquarius, so the group has been called the Aquarius Stream. It covers an area of the sky about 1300 times the size of the full Moon.

About 15 other star streams have been found in our Galaxy. Most arc up out of the Galactic plane into much emptier space, and so have been easier to spot. By contrast, the Aquarius Stream was revealed only by careful 'digging'. "It was right on our doorstep," said Dr Williams, "but we just couldn't see it."

Professor Joss Bland-Hawthorn of the University of Sydney, a member of the RAVE team, said "RAVE is the first of a new class of million-star surveys that will tell us a huge amount about the ancient origins of the Galaxy. We expect to find more such infalling dwarf galaxies."

The countries taking part in RAVE are Australia, Canada, Germany, France, Italy, the Netherlands, Slovenia, Switzerland, the UK and the USA. Professor Matthias Steinmetz, Director of the Astrophysical Institute Potsdam (AIP) in Germany leads the collaborative effort.

The Australian Astronomical Observatory is our national optical observatory, and is part of the Commonwealth Department of Innovation, Industry, Science and Research. It operates the 3.9-m Anglo-Australian Telescope and the 1.2-m UK Schmidt Telescope at Siding Spring Observatory in New South Wales.

More information

Dr Amanda Bauer (ARC Super Science Fellow, Australian Astronomical Observatory)
T: +61 2 9372 4852
M: +61 447 029 368

Media assistance: Helen Sim 
T: +61 2 9372 4251
M: +61 419 635 905

Professor Fred Watson, Australian Astronomical Observatory
Contact via Helen Sim
Mob: 0419 635 905
Office tel: 02 9372 4251

Professor Joss Bland-Hawthorn, University of Sydney
Mob: 0406 973 133
Office tel: 9351 2621

Dr Mary Williams, Astrophysical Institute Potsdam, Germany
Tel.: +44 139 242 1964 (office phone - with UK country code)
Mobile (with international roaming): +49 1767 7585 354 ("+49" is the country code for Germany)


Astrophysical Journal, 728-2, 2011.
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Visualization of the Aquarius Stream of stars in our Galaxy.
Credit: Arman Khalatyan, AIP


Visualization of the Aquarius Stream of stars in our Galaxy.
Credit: Arman Khalatyan, AIP