NASA's latest satellite observatory, the Fermi telescope, has now been mapping the sky for a couple of months using gamma-rays, a form of radiation which is even more energetic than X-rays. Within a year or two there is a chance that it may detect a glow from the mysterious dark matter whose gravitational effects astronomers first detected more than three quarters of a century ago, but which has so far remained stubbornly invisible to all our telescopes even though it apparently accounts for 85% of all the mass in the Universe. Most cosmologists believe that this dark matter is a new kind of elementary particle yet to be detected on Earth (though the Large Hadron Collider might provide evidence for it once its magnets are fixed). Under the right conditions these particles may produce enough gamma radiation for Fermi to see them.
But where is the best place to look for this gamma-ray signature of dark matter? To answer this question, an international team of astrophysicists from Germany, the UK, Canada and the Netherlands (the "Virgo consortium") have used some of the largest of all European supercomputers to simulate the formation of the dark matter structure that surrounds a galaxy like our own Milky Way. These "dark matter halos" contain over a trillion times the mass of our Sun and are the basic units of cosmic structure.
The simulations by the Virgo team show how the Milky Way's halo grew through a series of violent collisions and mergers from millions of much smaller clumps that emerged from the Big Bang. Most of these were disrupted during the formation process, but some survive, the largest harboring familiar satellites of the Milky Way such as the Large and Small Magellanic Clouds or the Sagittarius dwarf galaxy. Other clumps were too small to make any stars, but are still predicted to lurk in our Galaxy's halo, so far undetected by any telescope.
Gamma rays are produced in regions of high dark matter density when the particles collide and annihilate in a puff of radiation. Many cosmologists have argued that the Milky Way's satellites are the ideal place for Fermi to search for gamma-rays since their centres should be very dense. The Virgo team's simulations demonstrate that this is not so. Their careful calculations show that by far the most easily detectable signal should come from regions of the Milky Way well inside the Sun's position, but well away from the centre itself. Looking right at the centre would be a poor strategy for Fermi because of the danger of confusing the signal with gamma rays coming from other sources, such as the remnants of supernovae or the gas clouds where stars form. Instead, the Virgo team recommend looking 10-30 degrees off-centre, where they predict that the dark matter should glow in gamma-rays in a smoothly varying and characteristic pattern.
If Fermi does detect the predicted emission from the Milky Way's smooth inner halo, then it may, if we are lucky, also see gamma-rays from small (and otherwise invisible) clumps of dark matter which happen to lie particularly close to the Sun. These clumps will be substantially fainter than the main halo, but may still be detectable. The known satellites may also be visible in gamma-rays, although their greater distance will make them even harder to detect.
For further information please contact:
Professor Amina Helmi, Kapteyn Institute, University of Groningen
The simulations were carried out at three of the largest supercomputers in Europe:
The Leibniz-Rechenzentrum Munchen (LRZ) supercomputer where the main simulation was performed The Cosmology Machine at the Institute for Computational Cosmology, University of Durham
The Stella Supercomputer of the LOFAR project at the University of Gronningen
Images of the simulated dark matter halo and animations of its formation may be found at: http://www.mpa-garching.mpg.de/aquarius/
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