Astronomers at the University of Groningen Kapteyn Institute have succeeded for the first time in calculating the weight distribution of dark matter on an extremely small scale, based on observations. In an article in Nature this week they describe how a depiction of a galaxy was analysed using a gravitational lens. ‘This analysis provides a unique way to test theories on dark matter’, says research leader Prof. Léon Koopmans.
Based on the orbits of stars and galaxies, astronomers assume that there is a great deal more matter in the universe than is visible. What this invisible – dark – matter consists of is as yet unknown, but it is suspected to be elementary particles. To learn more about the nature of dark matter, research is being conducted into its distribution across the universe.
Although dark matter does not emit light, it can be observed indirectly. This is not only due to stars’ orbits, but also due to the fact that dark matter has the property of being able to act as a lens.The physical principle this is based on, however, is not the same as that of a normal optical lens made from glass. Mass causes gravity and Einstein’s general theory of relativity states that gravity equals the space-time curvature. A light beam travelling through the curved space also appears to be curved and thus acts as a lens. Since gravity is the reason behind this phenomenon, it is known as a gravitational lens.
The principle that the gravitational lens is based on means that the curvature of the light beam can be related to the mass of the dark matter causing it. Any irregularities in the image can be used to calculate irregularities in the distribution of the dark matter’s mass.
Astronomers, including the group led by Prof. Koopmans, have discovered many gravitational lenses in the past few years shaped like what are known as Einstein rings, and which provide an image of a galaxy lying behind the lens. In recent years, Koopmans and his team have developed a powerful analysis method enabling them to use such images to make detailed statements about the distribution of the dark matter.
In the Nature article they analyse an Einstein ring using an image made in June 2010 with the Near Infrared Camera of the W.M. Keck telescope on Hawaii. ‘Theories on dark matter predict that the mass distribution has a certain structure’, says Koopmans. ‘We have managed to analyse an image for the first time and the results fit the theory very well. But a single study is of course far from conclusive – when we’ve managed a dozen or so, we’ll be able to make real claims. If we find any deviations in future studies, this will certainly have consequences for the theory.’
Gravitational detection of a low-mass dark satellite galaxy at cosmological distance. S. Vegetti, D.J. Lagattuta, J.P. McKean, M.W. Auger, C.D. Fassnacht & L.V.E. Koopmans. Nature, 19 January 2012, doi:10.1038/nature10669
For more information: Prof. L.V.E. Koopmans
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