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. 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.