Mysterious dance of dwarfs may force a cosmic rethink

THE DISCOVERY THAT many small galaxies throughout the universe do not ‘swarm’ around larger ones as bees do but ‘dance’ in orderly orbits is a challenge to our understanding of how the universe formed and evolved.

The finding, by an international team of astronomers, including Professor Geraint Lewis of the University of Sydney, was published in the prestigious science journal Nature today.

“Early in 2013 we announced our startling discovery that half of the dwarf galaxies surrounding the Andromeda Galaxy are orbiting it in an immense plane” said Professor Lewis. “This plane is more than a million light years in diameter, but is very thin, with a width of only 300,000 light years.”

The universe contains billions of galaxies. Some, such as the Milky Way, are immense, containing hundreds of billions of stars. Most galaxies, however, are dwarfs, much smaller and with only a few billion stars.

Many of the larger galaxies have dwarf galaxies circling around them. Astronomers call them satellite galaxies.

Result contradicts standard understandings

For decades astronomers have used computer models to predict how these dwarf galaxies should orbit the large galaxies, and they’d always found that the dwarfs should be scattered randomly.

“Our Andromeda discovery did not agree with expectations, and we felt compelled to explore if it was true of other galaxies throughout the universe,” said Professor Lewis.

Using the Sloan Digital Sky Survey, a remarkable resource of colour images and 3-D maps covering more than a third of the sky, the researchers dissected the properties of thousands of nearby galaxies.

An artist's impression of the orbit of dwarf galaxies about a large galaxy

An artist’s impression of the orbit of dwarf galaxies about a large galaxy. Credit Geraint Lewis. The Hubble Image Archive was used as a source of the galaxies used in this illustration.

They were surprised to find that a large proportion of pairs of satellite galaxies are travelling in opposite directions if they are on opposite sides of larger galaxy hosts, said lead author Neil Ibata of the Lycée International in Strasbourg, France. And each of the dwarfs seemed to orbiting in the same plane, or angle, around the parent galaxy.

“Everywhere we looked we saw this strangely coherent co-ordinated motion of dwarf galaxies,” said Professor Lewis. From this the astronomers have extrapolated that this phenomenon is widespread in the universe, and seen in about 50 percent of galaxies.

“This is a big problem that contradicts our standard cosmological models. It challenges our understanding of how the universe works including the nature of dark matter,” said Professor Lewis.

Keeping an open mind

The researchers think the explanation might lie in some currently unknown physical process that governs how gas flows in the universe, although, as yet, there is no obvious mechanism that can guide dwarf galaxies into narrow planes.

Some experts, however, have made more radical suggestions, including bending and twisting the laws of gravity and motion.

“Throwing out seemingly established laws of physics is unpalatable,” said Professor Lewis, “but if our observations of nature are pointing us in this direction, we have to keep an open mind. That’s what science is all about.”

Adapted from information issued by the University of Sydney.

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