Dark matter is more important in massive galaxies

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. 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.

New research by Ignacio Ferreras (University College London), Prasenjit Saha (Queen Mary, University of London), and Liliya Williams (University of Minnesota) has mapped the mass distribution in 18 galaxies up to ten billion light-years from us. Dr Ferreras and his colleagues found a surprising trend between the proportion of dark matter and the mass of the galaxy, so that the biggest galaxies are dominated by dark matter.

Dr Ferreras and his colleagues took advantage of a rare astronomical phenomenon known as 'gravitational lensing'. The 18 galaxies they studied serendipitously lie in front of quasars, which are bright sources of light but even further away. The gravity of the galaxy and the dark matter distorts the quasar light, causing each of the quasars 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. By analysing the starlight from the galaxies using stellar evolution theory, it is possible to measure the stellar mass. Combining these ideas on 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 of these new findings but not all. After the Big Bang, gas will tend to fall towards the centres of dark-matter halos and there ignite to form stars and thus a galaxy. But why is the dark-matter fraction higher in more massive galaxies? And is the formation process already complete in these galaxies from ten billion years ago? These are some of the puzzles that need to be explained by a future theory of galaxy formation.

The technical paper by Dr Ferreras and his colleagues appeared in the April 10, 2005 issue of the Astrophysical Journal Letters. A link to the paper can be found here.


Two galaxies, both roughly 8 billion light-years away. The false colour infrared images of the galaxies and the doubly-imaged quasars behind them come from the Hubble Space Telescope (courtesy of the CASTLeS collaboration). The graphs with red and blue bands show the mass distribution: blue for the mass in stars, red for the dark matter. Note how the lighter galaxy (catalogued as B1520+530) is mainly stars, whereas the heavier galaxy (B1104-181) is dominated by a dark halo. Our Milky Way galaxy is intermediate between these examples.


To illustrate gravitational lensing, we show the old UCL observatory as we would see it if there was a dark galaxy hiding in the quadrangle. Note the small and distorted second image of the window. Scale this up to universe-size and replace the window with quasars and you have gravitational lensing!

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