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.”

NOTES FOR EDITORS:

The Institute of Astronomy was formed in 1972 in an amalgamation of
Cambridge University Observatory (founded in 1823), the Solar Physics
Observatory (1912) and the Institute of Theoretical Astronomy (1967).
It is
a department of the University of Cambridge and is engaged in teaching
and
research in the fields of theoretical and observational astronomy. A
wide
class of theoretical problems are studied, ranging from models of
quasars
and of the evolution of the universe, through theories of the
formation and
evolution of galaxies and stars, X-ray sources and black holes. For
more
information visit www.ast.cam.ac.uk

The 25th General Assembly of the International Astronomical Union (IAU
GA2003) is the major triennial meeting of world astronomers, this year
being
held in Sydney, Australia from July 13-26. The IAU is the peak global
body
representing astronomers with over 8000 members worldwide. More
information
can be found at http://www.astronomy2003.com/