Thirteen dwarf galaxies are playing a cosmic-scale game of Ring Around Andromeda, forming an enormous structure astronomers have never seen before and are hard-pressed to explain with current theories of how galaxies form and evolve.

According to current theories, the small galaxies, which contain as many as a few tens of billions of stars each, should be randomly arranged around the Andromeda galaxy.

Instead, they orbit Andromeda within a plane more than 1 million light-years across and about 30,000 light-years thick. For comparison, the latest estimates of Andromeda's girth put its diameter at more than 220,000 light-years.

The ring, if it can be called that, represents "the largest organized structure in what we call the local group of galaxies," says Michael Rich, a research astronomer at the University of California at Los Angeles and a member of the team reporting the results in the Jan. 3 issue of the journal Nature. The local group consists of more than 54 galaxies, including dwarfs, about 10 million light-years across.

Such rings don't appear when astrophysicists run their models of galaxy evolution, or when they model the local group's formation, he says. In addition, Andromeda and the Milky Way, the two most massive galaxies in the group, appear to be headed for a collision in about 4.5 billion years. The two galaxies are but 2.5 million light-years away and closing.

"Given all of this, we don't have a clear explanation for why this structure exists," Dr. Rich says.

Coming up with an explanation will be challenging. Andromeda was the only galaxy close enough to make the observation possible. But researchers would like to find more of these extended rings.

Larger numbers would provide increasingly rigorous real-world tests of any explanations scientists devise, notes Chris Stoughton, an astronomer at the Fermi National Accelerator Laboratory in Batavia, Ill., who was not a member of the team that discovered the ring.

In particular, he says, an understanding of these structures could help researchers unravel the mysteries of dark matter – a form of matter that provides the cocoons in which galaxies form and grow, as well as the scaffolding along which galaxies are distributed in the cosmos.

Dark matter earned its "dark" label because it emits no light or any other form of directly detectable radiation. Its presence is inferred by its gravitational effect on the matter astronomers can see.

**The team discovering the rings – led by Rodrigo Ibata of the Strasbourg Astronomical Observatory in France and Geraint Lewis at the University of Sydney in Australia – identified 27 dwarf galaxies in all orbiting Andromeda, also called M31. Thirteen of the dwarf galaxies shared a common orbital plane around Andromeda, and one was offset from the plane of M31's spiral arms by a significant degree.

Other teams had seen hints of the structure in the past, but this new work appears to build the most convincing case.

"They found a beautiful structure ... and did a very nice job of data analysis," Dr. Stoughton says.

Based on the distance from M31, the dwarfs orbit once every 5.5 billion years, the team estimates. Moreover, the stars in the dwarf galaxies are old, suggesting that if the dwarfs formed where they are, "the structure is ancient."

Dr. Ibata's team has offered up two broad explanations for the presence of Andromeda's ring of dwarfs.

One posits that M31's gravity attracted a group of dwarf galaxies in a single event, and perhaps the team just caught a lucky viewing angle as the dwarfs filed filament-like into the gravitational grasp of their new mistress.

The other is that they formed in place during the merger of two ancient gas-rich galaxies – a process that can form coherent streamers of stars in lesser mergers. Or perhaps during M31's birth, smaller halos of gas-bearing dark matter were captured by the more massive halo in which M31 formed.

Each explanation has problems, however, the researchers say.

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With galactic 13 dwarfs on the same quest, the research appears to have put Ibata and his team their own unexpected journey.

**The discovery is a result of the Pan-Andromeda Archaeological Survey, an international effort at galactic exploration – focusing on M31. The team made its optical observations with the 4 meter Canada-France-Hawaii Telescope on the summit of Mauna Kea in Hawaii. Studies of the dwarfs' motions required a sensitive spectrometer bolted to the back of one of two 10-meter telescopes at the Keck Observatory, which shares the same summit.