Giant Stellar Structure Surrounds the Milky Way Galaxy

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A vast, but previously unknown structure has been discovered around our own Milky Way galaxy by an international team of astronomers. The announcement is being made on behalf of Drs. Annette Ferguson, Rodrigo Ibata, Mike Irwin, Geraint Lewis and Nial Tanvir at the American Astronomical Society in Seattle, Washington. Their observations suggest that surrounding the main disk of the Milky Way is a giant ring of several hundred million stars. Despite it's size, the ring hasn't been clearly seen before since the stars are spread around the whole sky, and are far fewer in number than the tens of billions of stars making up the rest of the galaxy.

What has made the new discovery possible are the first large area surveys of the sky with sensitive CCD cameras. "Until now we haven't been able to see the wood for the trees", said Rodrigo Ibata one of the team members "large numbers of intervening stars, not to mention clouds of dust, makes it hard to probe these regions". By comparing two major surveys pointed at different regions of the sky, the team realised that they had evidence for what looks like a complete ring of distant stars surrounding the outer disk of our Galaxy.

Although known to be warped, probably from encounters with its orbiting satellite galaxies, the disk of the Milky Way was otherwise thought to be a relatively simple structure. The disk is roughly 100000 light years across, with the Sun embedded in it and offset some 30000 light years from the centre. From this vantage point the nearest edge of the ring is about 30000 light years away, in the direction of the constellation of Monoceros, directly away from the Galactic Centre. This region of sky is where traces of the ring were first discovered.

Further detailed surveys in the constellation of Andromeda showed that the stars belonging to the ring were visible 100 degrees away from the original discovery site and that these stars closely mimic the vertical distribution of the Milky-Way's so-called thick disk. Additional survey areas also serendipitously yielded evidence of the ring's presence, allowing the astronomers to get the first hints of the immense size of the structure.

The newly discovered ring seems to roughly encircle the disk, but is considerably thicker, probably shaped like a giant doughnut. "We can't yet be sure where it's come from", commented team member Geraint Lewis, "but the stars themselves are clearly very old and one possible explanation is that this is the debris of a small satellite galaxy of the Milky Way which has been torn apart by tidal forces." "Another possibility is that the stars originally came from the disk of our galaxy and their orbits have been warped or spread over time so that they now wander far from the disk plane." added Annette Ferguson.

Ultimately detailed studies like this, of the structure of the Milky Way, and other galaxies, tell us how they came into being and have evolved over the lifetime of the universe. If the old stars in this ring-like structure are inherently part of the outer disk, they pose an interesting challenge for galaxy formation models; alternatively if they are the remnants of a disrupted satellite, they will provide a first-hand opportunity to study the effects of massive accretions on the disks of large galaxies.

Images (above)

Caption: This schematic figure illustrates the geometry of the newly discovered ring, in relation to the spiral structure of the Milky-Way. It has long been supposed that the disk of the Milky Way galaxy slowly declines in brightness, vanishing into darkness at its edge 50000 light years from its centre. This startling new discovery shows the outer regions of the disk are considerably more complicated than previously thought, and sheds new light on the evolutionary history of our galaxy.

Credits (please quote): Image by Geraint F. Lewis. Created with the POVRAY package (www.povray.org) and Chris Colefax's galaxy include files (www.geocities.com/SiliconValley/Lakes/1434/).

Important Note

This announcement has been coordinated with a press conference on related work by Newberg et al. at the American Astronomical Society meeting in Seattle, Washington, at 9:30 am PST January 6, 2003.

Our discoveries would not have been possible without access to the Wide Field Camera on the 2.5m Isaac Newton Telescope on La Palma, in the Canary Islands and the foresight of the UK and Dutch astronomical communities in promoting large scale surveys with this system.

Constellation names are based in ancient mythology. We found the following web-site an excellent source of information:

Monoceros - "The Unicorn" or the "One Horned"
Andromeda - "The Chained Maiden"

Contacts

Annette Ferguson
University of Groningen, The Netherlands
Phone: +31 50 363 8324
email: ferguson@astro.rug.nl

Rodrigo Ibata
Observatoire de Strasbourg, France
Phone: +33 3 90 24 23 91
email: ibata@newb6.u-strasbg.fr

Mike Irwin
University of Cambridge, U.K.
Phone: +44 1223 337524
email: mike@ast.cam.ac.uk

Geraint Lewis
Universiy of Sydney, Australia
Phone: +61 2 9351 5184
email: gfl@physics.usyd.edu.au

Nial Tanvir
University of Hertfordshire, U.K.
Phone: +44 1707 286299
email: nrt@star.herts.ac.uk



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