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Last Updated: Thursday, 7 April, 2005, 20:42 GMT 21:42 UK
Galactic pancake mystery solved
By Paul Rincon
BBC News science reporter, in Birmingham

Galaxy simulation (Adrian Jenkins)
Simulations show how galaxies evolve under dark matter influence
Astronomers have figured out why a series of small galaxies surrounding the Milky Way are distributed around it in the shape of a pancake.

Theorists believed that the eleven dwarf galaxy companions should have a diffuse, spherical arrangement.

But a University of Durham team used a supercomputer to show how the galaxies could take the pancake form without challenging cosmological theory.

The results were presented at the UK National Astronomy Meeting.

According to cosmological theory, soon after the Big Bang, cold dark matter formed the first large structures in the Universe, which then collapsed under their own weight to form vast halos.

The gravitational pull of these halos sucked in normal matter, providing a focus for the formation of galaxies.

Galaxies are made up of small fragments that merge together bit by bit.

Milky Way impression (Nasa)
The Milky Way is a spiral galaxy
This process should leave a tightly bound galaxy at the core, surrounded by a diffuse sphere of smaller satellite galaxies.

So why the Milky Way's satellites are arranged on a flat circle has been a puzzle to astronomers ever since Donald Lynden Bell of Cambridge University published details of the phenomenon in 1982.

Professor Pavel Kroupa of the University of Bonn has pointed out that the current cosmological model should predict an isotropic - or diffuse - arrangement of satellite galaxies around the Milky Way. The observed distribution is of course a disc (or pancake).

"We argued that there is a perfectly plausible explanation for the observed distribution in terms of well-accepted and standard galaxy interactions," Professor Kroupa told the BBC News website.

"The satellites are probably objects that formed out of interacting galaxies, in a sense like the Earth's Moon."

But, he added, current cosmological theory also predicts a huge number of satellite galaxies, which runs contrary to observations.

Cosmic problem

Now, Noam Libeskind and Carlos Frenk, from the University of Durham, have used the Cosmology Machine supercomputer, based at the city's institution, to simulate and track the formation of a galaxy from its building blocks.

"We studied the distribution of satellite galaxies and looked for flatness," explained Libeskind.

Six simulations carried out on the machine not only came up with the correct number of satellites but also showed the same pancake arrangement seen in the Milky Way.

"It explains a cosmic problem," Professor Frenk told the BBC News website. "If you sat down and thought about how the galaxy formed, you would never come up with the pancake.

"It's a major triumph for the cold dark matter model [of galaxy formation]."

Noam Libeskind now plans to conduct further simulations to investigate how common the formation of these cosmic pancakes really is.

The Royal Astronomical Society's National Astronomy Meeting is being held at the University of Birmingham.

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31 Jul 01 |  Science/Nature

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