Dark matter clumps and streams in Milky Way
BY DR EMILY BALDWIN
ASTRONOMY NOW

Posted: August 7, 2008

Using powerful supercomputer simulations, researchers have reason to believe that dense clumps and streams of dark matter lurk in the inner regions of the Milky Way’s galactic halo, in the same neighborhood as our Solar System.

So far, the mysterious dark matter of the Universe has only been inferred through its gravitational effects on stars and galaxies, which is a key assumption of the ‘cold dark matter’ theory, the leading explanation for how the Universe evolved after the Big Bang, as well as the basis for the simulations reported in this week’s issue of the journal Nature.

“Fortunately, for the simulations we do not need to know the mysterious nature of dark matter,” Juerg Diemand, lead author of the paper, tells Astronomy Now. “One can infer the dark matter distribution in the early Universe from observations of tiny temperature fluctuations in the cosmic microwave background. This determines the initial conditions for our simulations. Dark matter interacts with itself and with ordinary matter practically only through gravity, so we ‘simply’ have to solve the gravitational interactions between our simulations particles in an expanding Universe.”

The evolution of structure as simulated for a Milky Way sized-halo over six different redshifts (z), from left to right and top to bottom, 12.8, 12.0, 10.3, 6.8, 3.4 billion years ago, and today. Image: http://www.ucolick.org/~diemand/vl/

Dark matter is thought to account for about 82 percent of the matter in the Universe and as a result, has largely controlled the
evolution of structure in the Universe. The ordinary matter that forms stars and planets has fallen into the ‘gravitational wells’ created by clumps of dark matter, giving rise to galaxies in the centres of dark matter halos. By simulating the gravitational interactions of more than a billion parcels of dark matter over 13.7 billion years, the research team were able to replicate this process. The simulation took a month to run using 3,000 parallel processors and about 1.1 million processor hours on the Jaguar supercomputer at Oak Ridge National Laboratory.

"It simulates the dark matter distribution from near the time of the Big Bang until the present epoch, so practically the entire age of the Universe, and focuses on resolving the halo around a galaxy like the Milky Way," says Diemand. "We see a lot of substructure, even in the inner part of the halo where the Solar System is. Every substructure has its own sub-substructure, and so on. There are lumps on all scales.”

The most massive of the subhalos would likely host dwarf
galaxies such as those observed orbiting the Milky Way. By studying the motions of stars within dwarf galaxies, astronomers can calculate the density of the dark matter in the subhalos and compare that with the densities predicted by the simulation.

"We can make comparisons with the dwarf galaxies and stellar streams associated with the Milky Way. The appearance of these stellar systems is closely linked to the substructure of the dark matter halo," says Diemand.

The wealth of substructure that survives to the present day is shown in this 800kpc square image where over 40,000 subhalos within 402kpc of the centre were resolved. The bottom inset shows the local density and the top inset shows coherent streams of matter formed from material removed from accreted and disrupted subhalos. Image: http://www.ucolick.org/~diemand/vl/

Although the central densities in the simulated dark matter subhalos are consistent with the observations of stellar motions in dwarf galaxies, there remains a discrepancy between the number of dark matter subhalos in the simulation and the number of dwarf galaxies that have been observed around the Milky Way. “Some subhalos may remain dark if, for example, they are not sufficiently massive to support star formation,” says Piero Madau, one of the co-authors on the recent paper.

“We are working on detailed comparisons with the observed stellar halos and satellite galaxies around the Milky Way and other galaxies,” adds Diemand.

Despite the illusive nature of dark matter, some theorists believe that it consists of weakly interacting massive particles (WIMPs), which can annihilate each other, emitting gamma rays when they collide. Such emission could be detected by the recently launched Gamma-ray Large Area Space Telescope (GLAST). "That's what makes this exciting," says Madau. "Some of those clumps are so dense they will emit a lot of gamma rays if there is dark matter annihilation, and it might easily be detected by GLAST."

Diemand comments that for typical WIMPs, anywhere from a handful to a few dozen clear signals should stand out from the gamma-ray background after two years of observations. “That would be a big discovery for GLAST," he says.

GLAST launched successfully in June of this year, and has already detected a dozen powerful gamma-ray bursts, an encouraging sign of good things to come from the mission in the future.