Satellite galaxies knocking Newton

BY KULVINDER SINGH CHADHA

ASTRONOMY NOW

Posted: 20 April, 2009

The behaviour of the Milky Way's satellite galaxies suggest that Newton's gravitation doesn't apply to them, so say a team of scientists from Germany, Austria and Australia. If this is the case then is a modification of gravity needed, like the one introduced by Einstein in his General Theory of Relativity? This will be the question that the scientists from the University of Vienna, the University of Bonn and the Australian National University will be asking in Hatfield, Hertfordshire at the 2009 Joint European National Astronomy Meeting.

The Milky Way's eleven brightest dwarf satellite galaxies rotate in the same plane as our Galaxy. The formation process that led to this suggests that their dynamics can only be explained with a modification of Newtonian gravitation. Image: NASA/JPL-Caltech/R Hurt (SSC).

The team looked at small dwarf galaxies orbiting the Milky Way, some of which are exceedingly small and faint; containing only a few thousand stars. Standard cosmological models predict that there should be hundreds of such faint dwarfs around large galaxies like our own, but only a few dozen have been found around the Milky Way. What's more, when the scientists looked closely, they found the galaxies weren't where they should be. "There is something odd about their distribution," says Professor Pavel Kroupa of Bonn's Argelander Institute for Astronomy. "They should be uniformly arranged around the Milky Way but that is not what we've found."

They found that eleven of the brightest dwarf galaxies lie in the same plane as, and orbit around our Galaxy, much like the planets around the Sun. Kroupa and his team believe that the only explanation for this is if the satellite dwarfs were created from collisions between young galaxies long ago. But this introduces a major problem, as former colleague Dr Manuel Metz (now at the German Centre of Air and Space) explains. "Calculations suggest that the dwarf galaxies cannot contain any dark matter if they were created in this way, which contradicts all other evidence." Dark matter is invoked in galaxy dynamics to explain the observed motions. Without it, the dwarf galaxies would fly themselves apart from their fast motions, as there wouldn't be enough mass holding them together. The suggestion that dark matter couldn't coalesce due to the way the dwarfs formed implies only one thing to the scientists, as Metz explains: "The only solution is to reject Newton's theory."