Milky Way's Dark Halo Revealed
New research by astronomers at the University of Southampton has for the first time measured the shape of the dark halo of our own Galaxy, the Milky Way, revealing its three-dimensional structure. With the "dark matter glasses" provided by a new technique, Drs Robert Olling and Michael Merrifield have shown that the dark matter in the Milky Way is distributed in an approximately spherical halo.
For more than twenty years, astronomers have known that galaxies contain much more mass than can be explained by their visible components such as stars and glowing gas clouds. Studies of the visible light emitted by spiral galaxies like the Milky Way reveal that most of the stars are arranged in a thin disk roughly 10,000 light years across, which defines the plane of the galaxy. Similarly, observations at radio wavelengths imply that such galaxies contains a large amount of hydrogen gas, also distributed in a thin disk that is even larger than the disk of stars. This gas, like the stars, rotates about the centre of the galaxy. By measuring the speed at which it rotates, astronomers can "weigh" the galaxy. Such measurements show that galaxies must contain some unseen massive component in addition to their luminous stars etc., and that this dark matter extends beyond the centre of the galaxy much further than the stellar component. However, not much can be inferred about the properties of the dark matter, simply because it does not give out any light. In particular, little is known about how the dark matter is distributed in galaxies: does it form a flat disk, a spherical halo, or something in-between?
Now, Dr Robert Olling and Dr Michael Merrifield of Southampton University have developed a new technique for measuring the shape of the dark halo based on the thickness of the layer of hydrogen gas in a galaxy. The method uses the fact that the gas layer thickness is dictated by the amount of mass close to the plane of the galaxy: if there is a lot of dark matter close to the plane, then the pull of its gravity will squeeze the gas down into a very thin layer; if, on the other hand, the dark matter is spread in a more diffuse spherical halo, then it will have less influence on the material in the plane of the galaxy, and so the gas layer will be thicker.
In a paper to be presented on Wednesday 9th April 1997 at the UK's 1997 National Astronomy Meeting (hosted by Southampton University), Drs Olling and Merrifield will present the application of this method to observations of the Milky Way. Their results show that our Galaxy contains a fairly thick layer of gas. That means it must be surrounded by an approximately spherical halo of dark matter. This result is somewhat surprising, since the currently favoured theory describing galaxy formation (in which the dark halo is made up of "cold dark matter") predicts that galaxy halos should in general be fairly strongly flattened. Either we live in a galaxy which happens to have an unusually round halo, or the theoretical astrophysicists must think again about the processes by which galaxies like the Milky Way form.
An illustration is available on the following WWW site:
How the Milky Way might appear through "dark matter glasses." The blue points represent stars, the green points show the location of the hydrogen gas layer, and the red points show the extensive, almost spherical, distribution of dark matter. The yellow dot represents the location of the Sun. It is the changing thickness of the gas layer that allows astronomers to measure the shape of the dark matter distribution.