What's going on around Andromeda? Curious structure puzzles scientists.

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.

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.

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