Most of the matter in the Universe is not the ordinary kind made up of protons, neutrons, and electrons, but an elusive "dark matter" detectable only from its gravity. Like a tenuous gas, dark matter is all around us - it goes through us all the time without us noticing - but tends to collect in large quantities around galaxies and clusters of galaxies and makes up about one-sixth of the mass of the Universe.
In his talk at the Royal Astronomical Society National Astronomy Meeting in Preston, Dr Ignacio Ferreras of King’s College London presented maps of the distribution of "ordinary" and dark matter in nine galaxies out to a distance of five billion light-years from the Sun.
Dr Ferreras worked with Dr Prasenjit Saha (University of Zurich, Switzerland) and Professor Scott Burles (Massachusetts Institute of Technology, USA) to take advantage of a rare astronomical phenomenon known as 'gravitational lensing'. The galaxies they studied serendipitously lie in front of quasars, which are bright sources of light at even greater distances. The gravity of the nearer galaxy and dark matter distorts the quasar light, causing the quasar to be seen as two or four images. The placement of these mirage images, studied using new theoretical techniques in gravitational lensing, makes it possible to measure the total mass and effectively gives scientists a telescope for dark matter!
By analysing the starlight from the galaxies using stellar evolution theory, it is possible to measure the mass of the stars they contain. Combining these ideas with archival data from the Hubble Space Telescope, Dr Ferreras and his colleagues were able to make dark-matter maps.
Current theories of galaxy formation can explain some but not all of these new findings. After the Big Bang, gas should have fallen towards the centres of dark-matter halos, there igniting to form the stars that go on to make up a galaxy. But why is there a higher proportion of dark matter in more massive galaxies? And had these galaxies already finished forming five billion years ago? These questions will only be answered by future theories of galaxy formation.
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