One of the most perplexing problems facing astronomers is the identity of
the mysterious “dark matter” that seems to pervade the Universe.

Evidence suggests that the mass of most galaxies is dominated by dark
matter. As it is dark, it is impossible to detect by conventional
astronomical means, and so its nature and distribution remains unknown.

One of the principal theories for the dark matter in galaxies is that it
comprises small, dim objects known as Massive Compact Halo Objects
(MACHOs). These might be dim stars such as white dwarfs, “failed stars”
known as brown dwarfs, or even black holes.

Some light will be shed on this mystery on Tuesday 3 April at the UK
National Astronomy Meeting in Cambridge, when Dr. Wyn Evans (Oxford
University) will announce the latest results of a search for dark matter
in the neighbouring spiral galaxy of Andromeda.


Dr. Evans is a member of an international team from Oxford, Cambridge,
London and Paris universities who are participating in an ambitious new
survey of the giant Andromeda galaxy.

Since 1999, this galaxy has been monitored four times a night using the
Isaac Newton Telescope in La Palma. By comparing the galaxy night after
night, all sorts of variable sources are discovered — including pulsating
stars, exploding stars and, rarest of all, microlensing events.

Microlensing occurs when light travelling towards Earth is deflected by the
presence of an intervening mass. When a dark object crosses the observer’s
line of sight, it causes characteristic variations in the light from
background stars — just as if a glass lens was being used. This
microlensing enables astronomers to detect dark objects like MACHOs in
other star systems.

Very recently, the survey team has announced the discovery of a short
duration microlensing event towards the Andromeda galaxy. As stars in
Andromeda are (mainly) unresolved into individual points of light, the
event has been detected by following the flux associated with a pixel (a
single picture element), rather than a source star.

“This is the ULTIMATE in scientific detection,” said Dr. Evans, “as not
merely is the lens dark and unseen, but the source is indistinguishable
from the other tens of thousands of stars on the pixel.”

This microlensing event is one of the very first seen in an external
galaxy. It is situated far from the centre of the Andromeda galaxy,
outside the stellar bulge. It also has a very short duration, under two

The interpretation of this event raises a number of interesting
possibilities. First and most exciting, the dark object could be a brown
dwarf (“failed star”) in the outer parts of the Milky Way Galaxy or in the
Andromeda Galaxy. If so, then astronomers have discovered an object that
gives out almost no light and is some 2 million light years away, a
thousand times more distant than any other known brown dwarf.

The other possibility is that the dark object could be a low mass star in
the disk of the Andromeda galaxy.

The search for further microlensing events during this survey of Andromeda
will continue for at least three years in total. The distribution of such
events will then enable astronomers to work out what fraction of the dark
matter halo of the Andromeda galaxy is composed of MACHOS, as well as their
characteristic mass.



Dr. Wyn Evans

Dept. of Theoretical Physics

Keble Rd



Phone: +44 (0)1865-273976


Prof. Bernard Carr

Queen Mary

Mile End Rd


E1 3NS

Phone: +44 (0)20-788-25492


Dr. Paul Hewett

Institute of Astronomy

Madingley Rd



Phone: +44 (0)1223-337507



Dr. Yannick Giraud-Heraud