STUDYING ANDROMEDA IN THE DARK
Last Updated on Thursday, 06 May 2010 20:24
Published on Wednesday, 23 February 2005 00:00
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.
MACHOS IN 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 days.
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, Oxford, OX1 3NP
Prof. Bernard Carr, Queen Mary, Mile End Rd, London E1 3NS
Dr. Paul Hewett, Institute of Astronomy, Madingley Rd, Cambridge CB3 0HA
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