Cosmic Flow of Galaxies Seen Across a Billion Light Years

The Universe is lumpier than current theories suggest and the Milky Way is part of a massive flow of galaxies moving together at 2.2 million kilometres per hour toward the constellation of Vela in the southern hemisphere, according to astronomers at the University of Victoria, the University of Durham, England and Princeton University.
The group's findings will be reported in tomorrow's edition of Science magazine and published in the Feb. 20 issue of The Astrophysical Journal Letters.
These are the first results from the SMAC (Streaming Motions of Abell Clusters) survey, led by Dr. Michael Hudson of UVic and Dr. Russell Smith of Durham, with Dr. John Lucey and Prof. Roger Davies (Durham) and Dr. David Schlegel (Princeton).
It's known that our galaxy and our nearest neighbour, Andromeda, move at 600 kilometres per second with respect to the distant cosmic microwave background. But the findings of the SMAC collaboration suggest that galaxies far beyond our local neighbourhood are also moving in a similar direction and speed.
The SMAC team observed 699 galaxies in 56 galaxy clusters spanning a volume of 1.2 billion light years in diameter. Measuring motions rather than positions of galaxies, the group investigated the distribution of matter (seen and unseen) in and around this vast region of space.
So what is doing the pulling? The study means that the Great Attractor--the region of space once thought to be pulling all galaxies toward it--may not be the final explanation. "Our survey goes beyond the proposed location of the Great Attractor and we still see outward motion of galaxies beyond it," says Hudson.
"We suspect that no single object can be identified as responsible for the flow," says Smith. "Probably several regions contribute to the pull on the local region." The two most prominent concentrations of galaxy clusters are "superclusters" 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.
"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," says Hudson. "At least 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."
The SMAC team employed telescopes in the Canary Islands, Australia and Chile. The study is the latest in sometimes contradictory research into the motion of galaxies. The team, and other groups, is attempting to resolve and interpret these discrepancies ahead of an international conference on cosmic flows in Victoria in July.

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