Date : 25 March 2004                 Ref. PN 04/17 (NAM 14)
Issued by: RAS Press Officers
Peter Bond
Phone: +44 (0)1483-268672       Fax: +44 (0)1483-274047
E-mail: PeterRBond@aol.com  Mobile phone: +44 (0)7711-213486
Dr Jacqueline Mitton
Phone: +44 (0)1223-564914       Fax: +44 (0)1223-572892
E-mail: jmitton@dial.pipex.com  Mobile phone: +44 (0)7770-386133
National Astronomy Meeting Press Room phones (30 March - 2 April only):
+44 (0)1908 659726     +44 (0)1908 659729     +44 (0)1908 659730
RAS Web site: http://www.ras.org.uk
Meeting Web site: http://physics.open.ac.uk/NAM/
An international team of astronomers has used the UK's 2.5-m Isaac Newton
Telescope on La Palma in the Canary Islands to map the Andromeda Galaxy
(otherwise known as M31) and a large area of sky all around it. Their work
over the last few years has created the most detailed image of a large
spiral galaxy that currently exists. Dr Mike Irwin of the University of
Cambridge, one of the team leaders, reports on some of the latest findings
on Wednesday 31 March, when he will tell the RAS National Astronomy Meeting
at the Open University about the first clear evidence that M31 is pulling
one of its bright satellite galaxies apart, and the discovery of 14 
previously unknown globular clusters orbiting far from the centre of M31 
which could have been left behind when Andromeda devoured their parent 
Located around 2.5 million light years away, the Andromeda Galaxy is the
most distant object visible to the naked eye, and is considered to be the
sister galaxy of our own Milky Way. By studying this galactic neighbour,
astronomers hope to understand more about the formation and evolution of
many of the billions of spiral galaxies in the universe, including the
Milky Way.
For their survey, the team have taken 150 individual images with a sensitive
electronic CCD camera, which reveal millions of individual stars. It extends
over an area 100 times greater than all earlier studies combined. The reason
for scanning such a large area is that. around bright galaxies. there is a
tenuous "halo" of stars which are leftovers from the formation of the galaxy
billions of years ago. Studying this "fossil" information reveals evidence
for how the halo, and hence the rest of the galaxy, has built up over cosmic
Traditionally, galaxy halos were thought to be relatively smooth and devoid
of substructure. In fact the new survey shows that Andromeda's halo is the
exact opposite: it has a wealth of structure, indicating that it has ripped
apart smaller galaxies that came too close and that the halo is built up
from their remains. "Given that the disk of Andromeda appears so pristine,
we were shocked to discover that its halo shows so much evidence for a
history of interactions with other galaxies," says Mike Irwin.
At this year's National Astronomy Meeting, the Andromeda team report the
discovery of a large stream of stars which appears to have been pulled out
of one of Andromeda's well-known satellite galaxies, NGC205. The visible
part of the apparent stream extends nearly 50,000 light years from the main
body of this small elliptical galaxy and was previously unknown despite the
fact that NGC 205 has been well-studied.
"This is the first clear indication that one of Andromeda's companion
galaxies is being ripped apart as we watch," commented team member Alan
McConnachie, a doctoral student at Cambridge.
The 14 globular clusters the team has found orbiting far out from M31 may be
evidence of Andromeda's past cannibalism. Globular clusters are ancient
systems of hundreds of thousands of stars, which are seen around many
galaxies, and provide many clues to their evolutionary history. "Since the
most distant of these globular clusters is some 250,000  light years from
the centre of M31, our work shows that M31's halo extends far beyond the
edge of the bright part of the galaxy disk," said Avon Huxor of the
University of Hertfordshire.
"Both these discoveries will greatly aid in understanding the evolution of
these nearby galaxies and should shed light on how our own Galaxy became
what it is today," commented Nial Tanvir, another team member from the
University of Hertfordshire.
Expected to be available from the following web site from 30 March:
Dr Mike Irwin, University of Cambridge
Phone: (+44) (0)1223 337524             E-mail: mike@ast.cam.ac.uk
Dr Nial Tanvir, University of Hertfordshire
Phone:  (+44) (0)1707 286299            E-mail: nrt@star.herts.ac.uk
Home:  (+44) (0)1763 241841
Mobile: (+44) (0)7980 136499
Mr Alan McConnachie, University of Cambridge
Phone:  (+44) (0)1223 766653           E-mail: alan@ast.cam.ac.uk
Dr Avon Huxor, University of Hertfordshire
Phone: (+44) (0)1707 286068             E-mail: ahuxor@star.herts.ac.uk
Other team members include:
Dr Terry Bridges, Queens University, Ontario
Dr Scott Chapman, California Institute of Technology, Pasadena
Dr Annette Ferguson, Max-Planck-Institut fr Astrophysik, Munich
Dr Rodrigo Ibata, Observertoire de Strasbourg
Dr Geraint Lewis, University of Sydney