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.