Dwarf galaxies around Andromeda pose questions for astronomers

An image of Andromeda, the nearest giant galaxy to the Milky Way, captured by the Hubble Space Telescope.
(University of Utah)

A discovery that many small companion galaxies surrounding Andromeda seem to orbit in concert — and aligned in a vast, thin disk — could change scientists’ understanding of how galaxies form, researchers said Wednesday.

A team of astronomers led by Rodrigo Ibata of the Observatoire Astronomique de Strasbourg in Strasbourg, France, made the discovery about the dwarf galaxies using observations collected by the Pan-Andromeda Archaeological Survey, which studied the Andromeda and Triangulum galaxies, also known as M31 and M33. The survey uses the Canada-France-Hawaii telescope in Hawaii. Scientists are interested in studying Andromeda — which, at about 2.5 million light-years away, is the Milky Way’s closest neighboring giant galaxy — because it allows them to examine structures that we can’t view in our galaxy.


The survey detected 27 dwarf galaxies in a region extending 114 to 1,305 light-years from Andromeda. Fifteen of the galaxies aligned along a single plane, and 13 of those 15 appeared to “co-rotate” in the same direction.

“The formation of this structure around M31 poses a puzzle,” Ibata and coauthors wrote in a study (subscription required) in this week’s edition of the journal Nature.

Current ideas of how galaxy formation work simply don’t explain how the dwarf galaxies, which are believed to be remnants of ancient structures that coalesced to form giant galaxies, could align in such a way, the team said. Models suggest that they would have scattered more broadly over time.

“The present detection proves that in some giant galaxies, a significant fraction of the population of dwarf satellite galaxies … are aligned,” the team wrote, noting that “the implications for the origin and dynamical history of dwarf galaxies are profound. ”

The team also wrote that “intriguingly” the band of co-rotating dwarf galaxies was also aligned with the Milky Way. That link could be a result of chance, they wrote, but was worth considering further as astronomers ponder how the current layout of our cosmic neighborhood might have come to be.

In an editorial (subscription required) in the same issue of Nature, astronomer R. Brent Tully of the University of Hawaii’s Institute for Astronomy in Honolulu marveled at the discovery.

“No theorist of galaxy formation would have dared to predict such a situation,” he wrote. “What’s more, the Milky Way is in the same plane as the 13 satellites. The discovery of this plane is a spectacular result.”