Size isn't everything: Unlocking the dark secrets of dwarf galaxies(Forwarded)

Press Office
University of Cambridge
Cambridge, U.K.

Contact:
Laura Morgan
University of Cambridge
, 01223 332300

23 July 2003

Size isn't everything

Unlocking the dark secrets of dwarf galaxies

New research on dwarf spheroidal galaxies by a team of astronomers at the
University of Cambridge promises a real astronomical first: detection, for the
first time, of the true outer limits of a galaxy.

The team is presenting today (23 July 2003) at the 25th General Assembly of the
International Astronomical Union (IAUXXV) in Sydney, Australia. The research
could provide the key to understanding how larger galaxies were formed,
including our own Milky Way galaxy.

The rare dwarf spheroidal galaxies display few stars but contain massive amounts
of 'dark matter' or matter that does not emit radiation that can be observed by
astronomers. The team studied these galaxies in detail using some of the largest
optical telescopes on earth in order to probe their dark secrets. Dwarf
spheroidal galaxies are widely believed to be the building blocks from which
galaxies were formed.

By studying the motion of many stars the scientists have created a picture of
how the mass of the galaxy is arranged. Surprisingly, when the Cambridge team
looked at the stars at the edge of one such galaxy, Draco, they found that the
outer stars were moving so quickly that the galaxy could only stay together if
it contained 100 times more dark matter than the mass of the stars alone. Using
detailed models of the motions of stars in a galaxy containing large quantities
of dark matter, the group was able to demonstrate their observations could only
be understood if the galaxy was surrounded by a large halo of dark matter.

Observations of the Ursa Minor dwarf spheroidal galaxy presented a new
complication in the study. The team found an unexpected clump of slow-moving
stars interpreted as the dead remains of one of the pure star systems, a
globular cluster. The cluster should have been scattered across the galaxy, but
it was still held together. The team realised this was only possible if the dark
matter were arranged in a manner very differently from standard galaxies.

In May 2003, further research into Ursa Minor showed the stars in the very
outermost parts are not moving quickly like the stars at the edge of Draco.
Several theories are being investigated including dark matter from edge of Ursa
Minor has been snatched away from the galaxy by its massive parent, the Milky
Way, allowing some stars to wander gently away from their parent. Or they could
be stars which wandered too close to other stars in the centre of the galaxy and
were slung out to the edge of the galaxy as a result.

Whatever the explanation, the findings promise a real astronomical first:
detection, for the first time, of the true outer limits of a galaxy.

Gerry Gilmore, Professor of Experimental Philosophy at the Institute of
Astronomy at the University of Cambridge, said: "This research, utilising some
of the largest optical telescopes on earth, has provided us with insight to the
makeup of these rare dwarf galaxies. This research helps astronomers better
understand how galaxies were formed, and help take into account dark matter in
all galaxies."

Notes for Editors:

1. The Institute of Astronomy, formed in 1972 as the amalgamation of Cambridge
University Observatory (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 .

2. The General Assembly of the International Astronomical Union (IAU GA2003) is
the major triennial meeting of world astronomers. The IAU is the peak global
body representing astronomers with over 8000 members worldwide. More information
can be found at http://www.astronomy2003.com/ .

For more information, contact:

1. Press and Publications Office, University of Cambridge, Tel: 01223 332300,
email:

2. Dr. Mark Wilkinson/Professor Gerry Gilmore, Institute of Astronomy, Tel: 0771
277 4522; email:

3. Dr. Jan Kleyna, Institute of Astronomy, email:

4. Dr. Wyn Evans, Institute of Astronomy, email: