VIEWING THE MILKY WAY THROUGH DARK MATTER GLASSES

For more than twenty years, astronomers have known that galaxies
contain much more mass than can be explained by their visible
components.  However, not much can be inferred about the properties of
this "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?  However, new research has for the first time measured the
shape of dark halo of our own galaxy, the Milky Way, revealing its
three-dimensional structure.  With the "dark Matter glasses" provided
by this new technique, astronomers have shown that the dark matter in
the Milky Way is distributed in an approximately spherical halo.

Studies of the optical light emitted by spiral galaxies like the Milky
Way reveal that most of the stars in such a system are arranged in a
thin disk of material with a diameter of around 10,000 light years,
which defines the plane of the galaxy.  Similarly, observations at
radio wavelengths imply that such a galaxy contains a large amount of
hydrogen gas, also distributed in a thin disk, but reaching out to
even greater radii than the stars.  This gas, like the stars, is found
to rotate about the centre of the galaxy, and by measuring the speed
at which it rotates, astronomers can "weigh" the galaxy.  It is by
such measurements that it is known that galaxies must contain some
unseen massive component in addition to their luminous elements, and
that this dark matter extends much further than the stellar component.

Unfortunately, these measurements provide only a crude measure of the
distribution of dark matter, and do not tell us whether the material
is in an extensive spherical halo or a more flattened distribution
akin to the stellar disk.  However, 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 the 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 at this year's 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, and hence that 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 be generally fairly strongly flattened.  Thus,
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.



FIGURE CAPTION.  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.  It is the
changing thickness of the gas layer that allows astronomers to
measure the shape of the dark matter distribution.