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Durham team discover a cosmic flow of galaxies across one billion light years of the universe

January 29, 1999

They have discovered that the distribution of dark matter is far less smooth than has been predicted by theorists, and that the Milky Way is among a flow of galaxies moving together in the direction of the constellation Vela in the southern hemisphere.

The group's findings are reported in tomorrow's edition of Science magazine and will be published in the 20 February issue of The Astrophysical Journal Letters.

These are the first results from the SMAC (Streaming Motions of Abell Clusters) survey, by Dr Russell Smith, who worked on the project for his PhD thesis in the Department of Physics at Durham, with his supervisors Dr John Lucey and Prof Roger Davies, and with Dr Michael Hudson (University of Victoria) and Dr David Schlegel (Princeton University), both of whom worked in Durham on the project.

The team observed galaxies in 56 galaxy clusters spanning a volume some 1.2 billion light years in diameter. By measuring the motions, rather than the positions of the galaxies, the group investigated the distribution of matter, seen and unseen, in and around this huge region of space.

The motions of galaxies are mainly caused by the expansion of the Universe. However, because the Universe is not perfectly smooth, neither is the expansion. Astronomers have known for some time that our galaxy, the Milky Way, and our nearest neighbour, Andromeda, are moving at 600 kilometres per second with respect to the distant cosmic microwave background. The findings of the SMAC collaboration show that galaxies far beyond our local neighbourhood are also moving in a similar direction with a similar speed.

In 1985 astronomers (including Prof Davies) discovered that the nearby galaxies had a large motion outflowing towards the southern constellation of Centaurus and argued that this was caused by the gravitational pull of a large structure dubbed the 'Great Attractor'. The SMAC survey indicates that this may not be the complete explanation.

Dr Smith said:
"Our survey goes far beyond the proposed location of the Great Attractor and we still see outward motion of galaxies beyond it. So what is doing the pulling? We suspect that no single object can be identified as responsible for this flow. Probably several regions contribute to the gravitational effect on the local neighbourhood".

Indeed, the two most prominent concentrations of galaxy clusters, called 'superclusters', lie in the southern sky, close to the direction of the observed flow.

The team's survey may also provide clues about dark matter, one of the great mysteries of the Universe.

Dr Hudson said:
"The speed of the galaxy flow and the scale over which it extends indicate that the Universe is far less homogenous than we had supposed. 90 per cent of the Universe is composed of dark matter and the nature of dark matter is unknown. Popular theories about dark matter predict that the Universe should be very smooth on these very large scales - so smooth that flows like this should be extremely rare".

Dr Lucey said:
"For the SMAC survey we gathered data from telescopes in the Canary Islands, Chile and Australia. Our initial goal was to map how the Great Attractor distorts the nearby expansion of the Universe. The size and extent of the galaxy flow measured by SMAC is very remarkable."

Professor Davies said:
"The SMAC report is the latest in a series of (sometimes contradictory) research into the motion of galaxies in the expanding Universe. Our team, and other groups, will be busy in coming months, trying to resolve and interpret these discrepancies."

For example, these latest findings are in good agreement with a recent survey of galaxy clusters to be reported by Jeffrey Willick of Stanford University. However, an investigation based at Cornell University in New York found no large flow of galaxies at these distances.

The SMAC team used the Isaac Newton and Jacobus Kapetyn telescopes on the Canary Islands, the Anglo-Australian Telescope in Australia, and the Cerro-Tololo Inter-American Observatory 0.9m telescope in Chile.


Durham, University of