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.