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  Current Research Highlight :: December 2003 all highlights

Can we see the dark matter?

Our Milky Way, like most other spiral galaxies, is surrounded by an extended halo of unseen dark matter at least ten times as big and ten times as massive as the part of the galaxy we see. Scientists at the Max-Planck-Institute for Astrophysics have carried out supercomputer simulations of the growth of such a halo in order to understand its expected structure. If the dark matter is made of neutralinos then gamma-rays from their self-annihilations might be detectable with next-generation gamma-ray telescopes. The astrophysicists found that there is a good chance for the next-generation telescope GLAST to detect these gamma-rays and thus to unveil the nature of the dark matter.

Fig. 1: Dark matter density distribution of the simulated Milky Way halo in logarithmic scale. The image shown is 600,000 light years on a side. If future experiments detect annihilation radiation it will likely be from inner Galaxy, a region a few percent of the size of the one shown here.

Movie showing a fly-around and a fly-through of the Milky Way halo: linkPfeilExtern.gif Milky Way Dark Matter Halo, 2.7 MB, DivX-Format

(For Playback under Windows you need the free DivX5-Codec from linkPfeilExtern.gif Under Linux e.g. mplayer or xine can be used.)

Fig. 2: Artist's views of GLAST (upper panel) and VERITAS (lower panel) two next-generation gamma-ray telescopes. (Images courtesy of GLAST and VERITAS collaborations)

Fig. 3: This figure shows the detectability of various supersymmetric models. Each point corresponds to a valid model resulting in a specific mass and corresponding interaction cross-section of the neutralino particle. The lines show the detectability limits for typical observations of GLAST and VERITAS. Points lying above the lines of the telescopes indicate models that could be detected.
The lowest solid line corresponds to an observation of the galactic centre with GLAST, the long dashed line to an observation of a Milky Way satellite galaxy like the Large Magellanic Cloud (LMC). Whereas there are many models that could be detectable with GLAST observations, very few models would produce enough gamma-rays to allow a detection with VERITAS.

In 1933 the swiss astronomer Fritz Zwicky observed the velocities of galaxies in clusters of galaxies and found a surprising result: The mass of the galaxies he saw was far too low to explain their motions within the cluster. He concluded that there had to be additional "dark" matter associated with the galaxy clusters.

Today we know that about 90% of the of the total matter in the Universe is not only dark - i.e. does not emit any light - but in addition it must be made of some mysterious yet unknown kind of particles. It is one of the greatest challenges in cosmology today to identify the nature of this dark matter.

One of the most probable candidates for the dark matter is a particle called neutralino. This particle arises naturally in theories that extend the standard model of particle physics. These supersymmetric theories introduce a new symmetry - supersymmetry - which assigns to each boson a new corresponding supersymmetric fermion particle and vice versa. So far none of these newly predicted particles has been detected. They are supposed to have energies too high to be probed with current particle accelerators.

The neutralinos might, however, self-annihilate when they collide in dense regions of the Universe, producing, among other particles, gamma rays of high energy. The idea is to try to detect this radiation and thus finally find out about the nature of the dark matter particle and its mass. Dark matter annihilation is very sensitive to the density of the dark matter, and so to the detailed structure of the dark halos that surround our own and other galaxies. Our Milky Way is the prime target for detection, especially its centre which is "only" about 26,000 lightyears away.

The MPA group used a large supercomputer at the Max Planck Society's Garching Supercomputer Centre to simulate the assembly of a DM halo very similar to our own with a world-record spatial resolution (Fig 1). For different parameters of the supersymmetric theory they computed the amount of gamma-radiation expected and compared it to the detection limits of two next-generation gamma-ray telescopes - one being a satellite mission (Fig 2. upper panel: The Gamma Ray Large Area Space Telescope GLAST) the other a ground based telescope (Fig 2. lower panel Very Energetic Radiation Imaging Telescope Array System VERITAS).

They found that with a new proposed detection strategy which searches for gamma-rays over a wide area of the sky ten or twenty degrees away from the Galactic Centre, there is a good chance that GLAST will detect annihilation radiation from the inner regions of the Milky Way (Fig 3). We might finally be able to "see" the dark matter and unveil its still mysterious nature.

Felix Stoehr, Simon D. M. White, Volker Springel and Giuseppe Tormen

Further reading:
F. Stoehr, S. D. M. White, V. Springel and G. Tormen
"Dark Matter Annihilation in the Halo of the Milky Way",
MNRAS, volume 345, page 1313

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last modified: 2003-12-11