A recent study of the satellite galaxies surrounding the Milky Way casts doubt on existing models of dark matter — according to its authors in Germany, Austria and Australia. The locations of the galaxies suggest that they should not contain any dark matter — but the motions of their constituent stars cannot be explained without invoking the elusive dark stuff. According to the researchers, this contradiction could provide support for alternative theories of gravity such as modified Newtonian dynamics (MOND).

The need for dark matter came to light when astronomers realized that galaxies were rotating at abnormally high speeds – and would otherwise be torn apart in the absence of hidden mass to provide ‘gravitational glue’. Dark matter is fundamentally different from normal “luminous” matter because it seems to interact only through gravity. However, direct proof of its existence has not yet been found.

As a result some physicists have proposed alternative theories to explain galactic rotation — theories that dispense with dark matter and assume that our current understanding gravity is not complete. Now Manuel Metz and Pavel Kroupa at the German Aerospace Centre in Bonn along with colleagues at the University of Vienna and Australian National University have found new evidence that could support such theories.

Where are the dwarfs?

The team looked at satellite companions of the Milky Way — dwarf galaxies that contain as a few as a thousand stars. Simulations that assume the existence of cold dark matter – the leading theory of dark matter — predict that the Milky Way should be surrounded by about 500 satellites, which are more or less spherically distributed. But according to Metz, “We instead observe about 30 satellite galaxies that are arranged in a remarkably thin disc-like structure, perpendicular to the Milky Way”. In addition, the galaxies seem to rotate in one direction, a feature that is not predicted by dark matter theory according to Metz.

While the low satellite count could have occurred simply because other galaxies are too faint to see, Kroupa told physicsworld.com that these spatial and rotational properties are inconsistent with the satellites being dark–matter dominated.

Instead, the results imply the satellites are ancient “tidal dwarf galaxies” that formed when a very young Milky Way collided with another young galaxy. Astronomers believe that long arms of hot gaseous and stellar material were thrown out and new dwarf galaxies were formed within these by condensing out of the hot gas.

However, this hot material should not include cold dark matter, because it does not participate in the collision process. Indeed, the dark matter from both galaxies should carry on as if the collision never occurred — something that has been famously seen in bullet clusters.

However, this presumed lack of dark matter appears to be in direct contradiction to another observation made by the team – the stars within the satellites are moving much faster than predicted by Newton’s law of gravity. This would imply that the galaxies are held together by a preponderance of dark matter. So what’s the way out of the dilemma?

‘Changing the laws of motion’

“This can only be remedied by changing the laws of motion when the accelerations are very weak,” says Kroupa. “How this can be achieved has to do with the nature of gravity, where we have an incomplete understanding.”

This is where alternative gravitational theories such as MOND might come into play. By modifying Newton’s law of gravity for weak gravitational fields, dark matter is not needed to explain these mysterious observations.

Other cosmologists are cautious. “MOND is one way out of the satellite problem but it has problems of its own,” says Ken Freeman at the Australian National University in Canberra. “It works quite well for individual galaxies but it does not work for large clusters of galaxies, which are also very rich in dark matter in Newtonian theory.”

The research will be published in The Astrophysical Journal and Monthly Notices of the Royal Astronomical Society.