Dancing Dwarf Galaxies Mystify Astronomers

Geraint Lewis. Artist's impression of small galaxies 'dancing' around a larger one

Large galaxies come accompanied by hordes of smaller ones, sometimes in the process of being destroyed from getting too close. Our models of the universe suggest these smaller galaxies should be randomly distributed, so evidence they aren't could force a radical rethink of how galaxies work, and maybe something even more fundamental.

Dwarf galaxies are usually said to “swarm” around larger ones in a relatively random manner. A competing view holds that smaller galaxies are concentrated in a plane around the larger ones. In Nature, Professor Geirant Lewis of the University of Sydney is proposing something even more surprising – that many small galaxies “dance” in pairs around larger ones.

Lewis is very much in the camp that argues that the fact that most small galaxies seen around the Milky Way and Andromeda lie in a thin plane is no coincidence. However, when using the Sloan Digital Sky Survey to examine the directions of motion of some of the smaller galaxies Lewis’ coauthor Neil Ibata of the Lycee International says, “We were surprised to find that a large proportion of pairs of satellite galaxies have oppositely directed velocities if they are situated on opposite sides of their giant galaxy hosts.”

The study was done on galaxies that are close enough for us to be able to get a good view of the small companions, but looked beyond the Milky Way and Andromeda because, in the authors’ words, “The Local Group may not be a representative environment.”

The phenomena of paired galaxies was visible out to 500,000 light years from the center of the larger galaxy – a huge distance, but closer than large galaxies' most distant stars. The paper adds, “The distribution of galaxies in the larger-scale environment (out to distances of about 6 million light years) is strongly clumped along the axis joining the inner satellite pair.”

"Everywhere we looked we saw this strangely coherent coordinated motion of dwarf galaxies. From this we can extrapolate that these circular planes of dancing dwarfs are universal, seen in about 50% of galaxies," says Lewis. "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."

If these observations turn out to be correct and more than a staggering coincidence, the explanation will require a rewrite of our thinking on some level, but Lewis is not sure how deep we will have to go. The least radical suggestions include unsuspected gas flows from galaxies, but more shocking proposals include a fundamental rethink of dark matter or the workings of gravity.

"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."
 

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