University of California-Santa Cruz

Contact:
Tim Stephens, (831) 459-2495

August 7, 2008

Study shows clumps and streams of dark matter in inner regions of the Milky
Way

Findings suggest GLAST mission could detect evidence of dark matter
particles

Using one of the most powerful supercomputers in the world to simulate the
halo of dark matter that envelopes our galaxy, researchers found dense
clumps and streams of the mysterious stuff lurking in the inner regions of
the halo, in the same neighborhood as our solar system.

"In previous simulations, this region came out smooth, but now we have
enough detail to see clumps of dark matter," said Piero Madau, professor of
astronomy and astrophysics at the University of California, Santa Cruz.

The results, reported in the August 7 issue of the journal Nature, may help
scientists figure out what the dark matter is. So far, it has been detected
only through its gravitational effects on stars and galaxies. According to
one theory, however, dark matter consists of weakly interacting massive
particles (WIMPs), which can annihilate each other and emit gamma rays when
they collide. Gamma rays from dark matter annihilation could be detected by
the recently launched Gamma-ray Large Area Space Telescope (GLAST), which
UCSC physicists helped build.

"That's what makes this exciting," Madau said. "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."

Juerg Diemand, a postdoctoral fellow at UCSC and first author of the Nature
paper, said the simulation is based on the assumptions of "cold dark
matter"
theory, the leading explanation for how the universe evolved after the Big
Bang. In a separate paper that has been accepted for publication in the
Astrophysical Journal, the researchers used their findings to make specific
predictions about the gamma-ray signals that would be detectable by GLAST.
The lead author of this paper is Michael Kuhlen, a former UCSC graduate
student now at the Institute for Advanced Study in Princeton, N.J.

"There are several candidate particles for cold dark matter, and our
predictions for GLAST depend on the assumed particle type and its
properties," Diemand said. "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."

Although the nature of dark matter remains a mystery, it appears to account
for about 82 percent of the matter in the universe. As a result, the
evolution of structure in the universe has been driven by the gravitational
interactions of dark matter. 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 centers of dark matter halos.

According to the cold dark matter theory of cosmological evolution, gravity
acted initially on slight density fluctuations present shortly after the Big
Bang to pull together the first clumps of dark matter. These grew into
larger and larger clumps through the hierarchical merging of smaller
progenitors.

This is the process that Diemand and Madau's team simulated on the Jaguar
supercomputer at Oak Ridge National Laboratory. The simulation took about
one month to run and followed the gravitational interactions of more than a
billion parcels of dark matter over 13.7 billion years. Running on up to
3,000 processors in parallel, the computations used about 1.1 million
processor-hours.

"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,"
Diemand said. "We see a lot of substructure, even in the inner part of the
halo where the solar system is."

The simulation revealed numerous subhalos and streams of dark matter within
the halo of the Milky Way, and more substructure appears within each
subhalo, Madau said. "Every substructure has its own sub-substructure, and
so on. There are lumps on all scales," he said.

The most massive of the subhalos would be likely to 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," Diemand said.

The central densities in the simulated dark matter subhalos are consistent
with the observations of stellar motions in dwarf galaxies, he said. But
there remains a discrepancy between the number of dark matter subhaloes 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, Madau said.

In addition to Diemand and Madau, the coauthors of the Nature paper include
Michael Kuhlen of the Institute for Advanced Study; Marcel Zemp, a
postdoctoral fellow at UCSC, who developed a time-stepping algorithm that
made the simulation remarkably accurate; and Ben Moore, Doug Potter, and
Joachim Stadel at the University of Zurich. This research was supported by
the U.S. Department of Energy, NASA, and the Swiss National Science
Foundation.

Note to reporters: You may contact Madau at (831) 459-3839, and Diemand at
(831) 459-2526. Movies and images of the new simulation can be found at via
lactea project,
     http://www.ucolick.org/~diemand/vl/

IMAGE CAPTION:
[http://www.ucsc.edu/news_events/img/2008/08/darkmatter300.jpg (68KB)]
In this image of local dark matter densities in the inner regions of the
Milky Way galaxy, lines indicate the directions in which particles are
moving. (Credit: M. Zemp)
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