The dark side of the Milky Way

Galaxy is shown to be spherical, not flat
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The Milky Way galaxy in which we live is not really flat, as people have thought, but roughly spherical, according to new research by British astronomers.

The conventional description is of a disc of stars with two spiral "arms" - with our sun far out in one of them - giving the galaxy a radius of about 10,000 light years. But the new calculations suggest there is a huge sphere of "dark matter" which we cannot see spreading out to a distance of 150,000 light years in a spherical "halo", say Robert Olling and Michael Merrifield, of Southampton University.

The latest findings, to be announced today at the National Astronomical Meeting in Southampton, are illustrated bottom right: the blue points represent stars, the green points are a layer of hydrogen gas along the plane of the galaxy, and the red points are "dark matter".

The reason we haven't seen the true shape of our galaxy before is that dark matter is literally invisible: it does not radiate heat or light, and so cannot be detected by telescope. Current theories suggest it may be planet-sized chunks of matter which never reached the critical mass needed to start emitting light, or even minuscule black holes left over from the early stages of the Big Bang 15 billion years ago.

But astronomers have known for more than 20 years that dark matter exists - and constitutes more than 90 per cent of the mass of the universe - because of its gravitational effects on visible objects such as stars and even galaxies.

"Previous estimates about the amount of dark matter always assumed that it was distributed in a sphere. We have shown that it is," said Dr Olling. "It is a very satisfying result. I have been struggling with these calculations for nine months."

The latest work calculated the presence and arrangement of dark matter around the Milky Way by observing its effects on a layer of hydrogen gas which is dispersed along the plane of the galaxy with the stars.

Dr Olling and Dr Merrifield estimated that the thickness of the hydrogen layer would depend on the arrangement of the dark matter. A lot of dark matter close to the galaxy's plane would pull the layer down, making it thin; but a more diffuse distribution would make the gas layer thicker.

The latest work suggests a huge intergalactic spread of dark matter. However, Dr Olling said a lot of work remained to be done. "The Milky Way is particularly problematic to study, because we're standing in the midst of it, - that makes it difficult to see the overall perspective. It's not like a distant galaxy where you can see it all. One of the problems we have been struggling with is that we don't know how far away we are from the centre of the galaxy, or how fast it's rotating. We know to 10 per cent accuracy, but that error is enough to make a big difference."

Astronomers at the University of Durham reckon galaxies formed from matter in the early universe full of elementary particles known as "cold dark matter". These then create "embryo galaxies" which collide and merge to form the familiar spiral and elliptical galaxy formations. The theory is based on observations of proto-galaxies which formed when the universe was only about 1 billion years old - less than a tenth of its present age.