Centaurus A
Centaurus A, an elliptical galaxy 13 million light-years from Earth, hosts a group of dwarf satellite galaxies co-rotating in a narrow disk, a distribution not predicted by dark-matter-influenced cosmological models. Christian Wolf and the SkyMapper team/Australian National University

After years of thinking that the Milky Way — our home galaxy and also the one most studied — and the neighboring Andromeda were outliers to the standard cosmological model, when it came to the motion of satellite galaxies around them, astronomers are now finding that it is standard model itself which may need tweaking.

Researchers looked at a third large galaxy, Centaurus A — about 13 million light-years away — and found the smaller satellite galaxies that orbit it do so in an orderly, synchronous way. These smaller satellites, which are found around most large galaxies, were seen following the same pattern of movement, and their rotation was largely confined to a plane that was perpendicular to Centaurus A.

Similar behavior had also been observed from most of the satellite galaxies that orbit the Milky Way and Andromeda (they are both spiral galaxies, while Centaurus A shows traits of both spiral and elliptical galaxies but is usually classified as elliptical). About 50 satellite galaxies have been identified around our home galaxy and over 25 around Andromeda, which lies about 2.5 million light-years away. And most Milky Way satellites and over half the Andromeda satellites move in similar orbits in disc-planes around the galaxies. In the case of Centaurus A, 14 of the 16 known satellites exhibited the orderly, coordinated movement.

This flies in the face of the standard cosmological model, which predicts less than 1 percent of all satellite galaxies should show this synchronous behavior, the rest moving in random orbits and directions, due to the effects of factors such as the presence of dark matter.

“Coherent movement seems to be a universal phenomenon that demands new explanations,” Oliver Müller from University of Basel, who was lead author of a new study on the subject, said in a statement Thursday. “The astronomical observations challenge the simulations. The possibility of coincidence can be ruled out, as this result — already seen in the Milky Way and the Andromeda galaxy — has now been detected a third time, in Centaurus A,” he added.

Marcel Pawlowski, a coauthor on the paper, from University of California, Irvine, spoke of the relevance of the study in another statement: “The significance of this finding is that it calls into question the validity of certain cosmological models and simulations as explanations for the distribution of host and satellite galaxies in the universe.”

To explain their observations, the researchers theorized most large galaxies in the universe likely had some sort of close interaction with another galaxy, like a merger, at least once in their lifetimes. This interaction could have birthed the orbiting satellite galaxies, and in this scenario, these smaller galaxies would also lack dark matter, which could explain their observations.

The study, titled “A whirling plane of satellite galaxies around Centaurus A challenges cold dark matter cosmology,” appeared online in the journal Science on Friday. Other coauthors included Helmut Jerjen from Australian National University and Federico Lelli from European Southern Observatory.