Small galaxy springs "dark matter" surprises

Astronomers from the University of Cambridge, UK, have found for the first time the true outer limits of a galaxy. They have also shown that the dark matter in this galaxy is not distributed in the way conventional theory predicts.

The team – Professor Gerry Gilmore, Dr Mark Wilkinson, Dr Jan Kleyna and Dr Wyn Evans – presents its results today at the 25th General Assembly of the International Astronomical Union in Sydney, Australia. The work could provide the key to understanding how larger galaxies were formed, including our own Milky Way galaxy.

The researchers studied rare “dwarf spheroidal” galaxies. These have few visible stars but contain massive amounts of “dark matter” – a mysterious kind of matter that does not emit its own light or radiation, and therefore cannot be directly observed by astronomers. However, dark matter can be detected by the gravitational pull it exerts on visible objects such as stars.

Astronomers think that dwarf spheroidal galaxies may be the building blocks from which larger, mainstream galaxies were formed.

Some of the dwarf spheroidals – those in our “Local Group” of galaxies – are close enough for astronomers to be able to trace the movements of their individual stars.

A galaxy is held together by the combined gravity of its stars and dark matter. By studying the motion of stars in some of the dwarf spheroidal galaxies, the researchers have created a picture of how the mass of each galaxy is distributed.

In one dwarf spheroidal, found in the constellation Ursa Minor, the team found a clump of slow-moving stars near the galaxy´s centre. They interpreted this clump as the remains of a group of stars known as a globular cluster.

This group of stars flies in the face of the most popular model for how dark matter is distributed in galaxies. The “lambda cold dark matter” model, which explains very well the large-scale structures in the Universe, predicts that dark matter rapidly increases in density towards the centre of a galaxy. If dark matter were distributed in this way in the Ursa Minor dwarf spheroidal galaxy, the star cluster would have been dispersed. The cluster´s existence shows that the dark matter is in fact distributed differently in this galaxy.

Furthermore, additional research into the Ursa Minor dwarf spheroidal has revealed the true edge of that galaxy – the point at which the dark matter stops. In most galaxies the way the stars move indicates that the dark matter extends far beyond the visible starry regions. In the Ursa Minor dwarf spheroidal, however, the stars in its very outermost parts are not moving quickly. This implies that there is little dark matter in the halo surrounding that galaxy.

Perhaps some of the dark matter has been nibbled off at the edges by the nearest massive galaxy (our own Milky Way), allowing some of the stars to slowly wander away. Or maybe the slow-moving stars could be ones that were “flung out” from the centre of the galaxy to its edges. Whatever the explanation, the finding represents the first detection of the true outer limits of a galaxy.

“Simulations of galaxy formation generally predict the existence of many more small galaxies around the Milky Way than are actually observed,” said Gerry Gilmore, Professor of Experimental Philosophy at the Institute of Astronomy at the University of Cambridge. “However, this prediction is based on assumptions about the masses of the galaxies we observe.”

“Our work is aimed at determining how much mass is actually present in the dwarf galaxies around the Milky Way. But until we have a rough idea of where the outer limits of these galaxies lie, we cannot claim to have measured their total mass.”

Contact:

Professor Gerry Gilmore
University of Cambridge, United Kingdom
Email: gil@ast.cam.ac.uk