Whirling Galaxies Are Orderly and Neat, Defying Chaotic Dark Matter Cosmology

2_1_Centaurus A
The Centaurus A galaxy Christian Wolf and the SkyMapper team / Australian National University

New research could undermine the standard cold dark matter model of cosmology. Scientists have found the galactic arrangement of Centaurus A, a massive galaxy 12 million light-years away from Earth, does not match up to predictions, joining the two other best-studied galaxies—the Milky Way and Andromeda—as so-called outliers.

This latest discovery, published today in Science, suggests these galaxies aren't unusual after all. Instead, they could be the norm.

Standard cold dark matter cosmology

"Large galaxies, like our own Milky Way, are usually surrounded by dwarf galaxies, which are smaller and dimmer," Federico Lelli of the European Southern Observatory and one of the authors of the study explained in a press video.

Scientists can simulate the arrangement of galaxies by using supercomputers to model their formation. According to the standard model, large galaxies should be mostly made up of invisible dark matter, which is as yet undetected. Smaller satellite galaxies should be dispersed randomly around the larger galaxies.

Lead author Oliver Müller, a doctoral student at the University of Basel, Switzerland and lead author on the study, explained in the video: "Satellite galaxies should be distributed randomly around the Milky Way and, also, the motions of the satellites should be quite chaotic, like bees around a beehive."

Both the Milky Way and Andromeda galaxies defy this model: Disk-shaped planes of smaller galaxies rotate in an orderly manner around their respective host. Astronomers had assumed these whirling galaxies arrangements were outliers. Models predicted they should exist only once in every thousand cases.

A clockwork Centaurus A

The Centaurus A galaxy sits within the Centaurus constellation. The team studied the motion and velocity of satellite galaxies around Centaurus A. Like the Milky Way and Andromeda, their observations showed a rotating disk-shaped plane of orderly satellites.

Müller said: "This work shows the Milky Way system and the Andromeda system are not so odd at all. A similar co-rotating plane or satellite is also found around Centaurus A."

"We do not know how these satellite planes form but we do know that they should be rare in the standard cosmological model based on dark matter." Lelli added. "However, the three best studied galaxies in the universe—Milky Way, Andromeda and Centaurus A—all show co-rotating planes of satellites."

These may not be the only outlying galaxies. Future research may reveal even more challenges to the standard model. Müller said: "There may be many more dwarf satellites systems out there which still need to be discovered."