Vast ring of stars found around Milky Way galaxy

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 at the American Astronomical Society’s meeting in Seattle, Washington, on behalf of Drs Annette Ferguson, Rodrigo Ibata, Mike Irwin, Geraint Lewis and Nial Tanvir. Their observations suggest that there is a giant ring of several hundred million stars surrounding the main disk of the Milky Way. Despite its size, the ring has not 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 covering 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 100,000 light years across, with the Sun embedded in it and offset some 30,000 light years from the centre. From this vantage point, the nearest edge of the ring is about 30,000 light years away, in the direction of the constellation Monoceros, opposite the centre of the Galaxy. This region of sky is where traces of the ring were first discovered.

Further detailed surveys in the constellation Andromeda showed that stars belonging to the ring are 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 roughly to 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 of this kind of the structure of the Milky Way and other galaxies, reveal 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.