Streaming into the Age of Aquarius

By Fred Watson 8 November 2013
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Anomalous stars zipping across the Milky Way are the ‘fossils’ of a remnant galaxy, says Fred Watson.

THERE’S A CERTAIN CLASS of astronomer who takes great delight in punning their way through the Universe. Notable among these are the RAVErs, a group of 60 or so scientists who are involved with a million-star survey called RAVE – the RAdial Velocity Experiment.

The RAVE survey is being carried out using the 1.2 m UK Schmidt Telescope at the Australian Astronomical Observatory at Coonabarabran in NSW.

Now, however, one of the RAVErs – New Zealand expat Dr Mary Williams – has excelled herself with puns about her new discovery.

Called the Aquarius Stream, this significant group of 15 stars has been identified as unusual from the 385,000 stars whose speeds have so far been measured by RAVE.

The stream of 15 are moving at 15,000 km/h all in the same direction and at an angle to the direction of motion of the Milky Way.

I have a stream…

Unable to resist the punning potential, Mary entitled her paper in the Astrophysical JournalThe Dawning of the Stream of Aquarius in RAVE“. She went on to announce her discovery at an international astronomy conference with the words “I have a stream…”.

Mary’s work at the Astrophysical Institute at Potsdam in Germany is involved with analysing the motions of stars in the RAVE catalogue to look for patterns that might reveal evidence of smaller galaxies having been absorbed by our own Milky Way Galaxy.

The Milky Way’s flattened disc of some 400 billion stars is our home galaxy, and is thought to have grown to its current size by consuming the dwarf galaxies that would have swarmed around it, early on in its 12 billion year history.

Galactic archaeology

The ‘fossil’ remains of those dwarf galaxies should still be detectable as groups of stars moving through the Sun’s neighbourhood with a common speed and direction. That is exactly what Mary Williams has now found, with the Aquarius Stream representing RAVE’s first major discovery in this field of ‘galactic archaeology’.

The Stream gets its name from the constellation of Aquarius, where most of the stars are located. Tracking back from the speeds measured today, it appears that the Aquarius Stream represents the remains of a dwarf galaxy that was absorbed into the Milky Way some 700 million years ago.

The discovery is particularly striking since most of the stars lie close to the disc of the Milky Way itself, making it harder to distinguish them from the general background.

With the RAVE project continuing until 2012, it is likely that further remnants of cannibalised dwarf galaxies will be discovered. It remains to be seen, however, whether any RAVErs can surpass the high bar for punning that Mary has set…

Download a video animation here of the Aquarius Stream (AVI, 7 MB).

Fred Watson is Astronomer-in-Charge of the Australian Astronomical Observatory at Coonabarabran in NSW, and is well-known to ABC radio listeners and Australian Geographic readers. He is also a fellow RAVEr.