Where will new Fermi telescope find dark matter?

Some believe the best chance of a detection lies in nearby dwarf galaxies, since they should contain dense nuggets of dark matter that could be relatively easy to pinpoint.

But a new study argues that a diffuse dark matter 'halo' surrounding the Milky Way offers an even better shot at glimpsing the mysterious stuff.

"I would bet on it," says lead author Volker Springel of the Max Planck Institute for Astrophysics in Garching, Germany. "And I'd be willing to risk a bit of money as well."

No one knows what dark matter is, since its presence is inferred only by its gravitational pull on normal matter. But researchers believe it is made up of particles that annihilate when they collide with each other. This produces gamma-ray photons that could be detected by telescopes such as Fermi (formerly called GLAST), which launched in June.

The question is, where is the best place to observe these photons?

Some say it is the mammoth cloud, or halo, of dark matter that surrounds the Milky Way. Such giant halos are thought to have coalesced from clumps of dark matter that formed soon after the big bang.

But small clumps of dark matter - weighing as little as the Earth - should still exist. They are expected to cluster in the halos of low-mass dwarf galaxies, a number of which orbit the Milky Way.
Diffuse glow

To find out where Fermi is likely to spot a dark matter signal, Springer and colleagues ran detailed computer simulations of dark matter in a Milky Way-like galaxy. They found that the best chance of a detection came from the diffuse halo itself, and in particular its central region, within the orbit of the Sun (scroll down for images).

The diffuse halo does produce the highest number of gamma rays from dark matter annihilation, says Jürg Diemand of the University of California, Santa Cruz, who was not involved in the study. "That's simply because we are only [26,000 light years] away from the centre of the massive main halo, while dwarf galaxies have much smaller halos and are further away," Diemand told New Scientist.

But he disagrees with the team's conclusion that the halo offers the best chance of a dark matter detection. That's because ordinary matter can also produce a diffuse glow of gamma rays - for example, when gas is hit by charged particles called cosmic rays - and researchers do not yet know how to tease apart the two different signals.

"Current models of diffuse gamma-ray signals from ordinary matter have uncertainties in the spatial and energy distribution which are much larger than the expected diffuse dark matter signal," he says.

It would be easier, he says, to distinguish between the two types of signal in dwarf galaxies. Unlike the diffuse gamma-ray glow expected from the main halo, dwarf galaxies would appear like single "point sources" of radiation to Fermi.

So astronomers could observe the sky around them at a variety of wavelengths to see if other, ordinary sources - such as gas clouds - could be responsible for the gamma rays.

"In principle, it's possible to make sure there's not enough gas along that line of sight to give you such a point source," Diemand says, adding that a dark matter signal might be seen by Fermi within its first year of operation.

But team member Carlos Frenk of Durham University in the UK says it should be possible to distinguish between dark matter and normal, 'foreground' matter in the halo, as well.

"The foreground should not be a show-stopper," Frenk told New Scientist.

"Fermi is able to measure the intensity of gamma rays at different energies," he says. "The dark matter signal is expected to have a very different 'energy spectrum' from the foregrounds and this will greatly ease the task of subtracting the unwanted foreground."

"The key question that remains is how intense the dark matter radiation will be in the first place," he continues. "And this depends on the largely unknown properties of the elementary particles that we are looking for."

Journal reference: Nature (vol 456, p 73)