November 20, 2006

Contact: Tim Stephens 1-831-459-2495
stephens@ucsc.edu

Supercomputer study shows Milky Way's halo of dark matter in unprecedented
detail

For Immediate Release

SANTA CRUZ, CA--Researchers at the University of California, Santa Cruz,
have used NASA's most powerful supercomputer to run the largest simulation
to date of the formation and evolution of the dark matter halo that
envelopes the Milky Way galaxy. Their results show substructures within
the halo in unprecedented detail, providing a valuable tool for
understanding the evolutionary history of our galaxy.

Every galaxy is surrounded by a halo of mysterious dark matter that can
only be detected indirectly by observing its gravitational effects. The
invisible halo is much larger and more spherical than the luminous galaxy
at its center. Recent computer simulations have shown that the halo is
surprisingly clumpy, with relatively dense concentrations of dark matter
in gravitationally bound 'subhalos' within the halo. The new study, which
has been accepted for publication in the Astrophysical Journal, shows much
more extensive substructure than any previous study.

"We find almost 10,000 subhalos, about one order of magnitude more than in
any past simulations, and some of our subhalos exhibit 'subsubstructure.'
This was expected theoretically, but we have shown it for the first time
in a numerical simulation," said Piero Madau, professor of astronomy and
astrophysics at UCSC and a coauthor of the paper.

Juerg Diemand, a Hubble postdoctoral fellow at UCSC and first author of
the paper, said the new results exacerbate what is known as the "missing
satellite problem." The problem is that the clumpiness of the normal
matter in and around our galaxy--in the form of dwarf satellite
galaxies--does not match the clumpiness of the dark matter seen in the
simulation.

"Astronomers keep discovering new dwarf galaxies, but there are still only
about 15 or so, compared to about 120 dark matter subhalos of comparable
size in our simulation. So which ones host the dwarf galaxies, and why?"
Diemand said.

Theoretical models in which star formation is restricted to certain types
of dark matter halos--sufficiently massive or early-forming ones--may help
to resolve the discrepancy, Madau said.

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 "normal" matter that forms
gas and stars has fallen into the "gravitational wells" created by clumps
of dark matter, giving rise to galaxies in the centers of dark matter
halos.

Initially, gravity acted 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 the UCSC researchers simulated on
the Columbia supercomputer at the NASA Ames Research Center, one of the
fastest computers in the world. The simulation took a couple of months to
complete, running on 300 to 400 processors at a time for 320,000
"cpu-hours," Diemand said.

Coauthor Michael Kuhlen, who began working on the project as a graduate
student at UCSC and is now at the Institute for Advanced Study in
Princeton, said the researchers set the initial conditions based on the
most recent results from the Wilkinson Microwave Anisotropy Probe (WMAP)
experiment. Released in March, the new WMAP results provide the most
detailed picture ever of the infant universe.

The simulation starts at about 50 million years after the Big Bang and
calculates the interactions of 234 million particles of dark matter over
13.7 billion years of cosmological time to produce a halo on the same
scale as the Milky Way's. The clumps within the halo are the remnants of
mergers in which the cores of smaller halos survived as gravitationally
bound subhalos orbiting within the larger host system.

The simulation produced five massive subhalos (each more than 30 million
times the mass of the Sun) and many smaller ones within the inner 10
percent of the host halo. Yet only one known dwarf galaxy (Sagittarius) is
that close to the center of the Milky Way, Diemand said.

"There are big clumps of dark matter in the same region where the disk of
the Milky Way would be. So even in the local neighborhood of our solar
system, the distribution of dark matter may be more complicated than we
have assumed," he said.

Astronomers may be able to detect clumps of dark matter within the Milky
Way's halo with future gamma-ray telescopes, but only if the dark matter
consists of the types of particles that would give rise to gamma-ray
emissions. Certain dark matter candidates--such as the neutralino, a
theoretical particle predicted by supersymmetry theory--could annihilate
(that is, be mutually destroyed) in collisions, generating new particles
and emitting gamma rays.

"Existing gamma-ray telescopes have not detected dark matter annihilation,
but upcoming experiments will be more sensitive, so there is some hope
that individual subhalos may produce an observable signature," Kuhlen
said.

In particular, astronomers look forward to interesting results from the
Gamma Ray Large Area Space Telescope (GLAST), scheduled for launch in
2007, he said.

The simulation also provides a useful tool for observational astronomers
studying the oldest stars in our galaxy by providing a link between
current observations and earlier phases of galaxy formation, Diemand said.

"The first small galaxies formed very early, about 500 million years after
the Big Bang, and there are still today stars in our galaxy that formed at
this early time, like a fossil record of early star formation. Our
simulation can provide the context for where those old stars came from and
how they ended up in dwarf galaxies and in certain orbits in the stellar
halo today," Diemand said.

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Notes to reporters: You may contact Diemand at (831) 459-2526 or
diemand@ucolick.org, Kuhlen at (609) 734-8183 or mqk@ias.edu, and Madau at
pmadau@ucolick.org.

Images and movies of the simulation are available on the web at

http://www.ucolick.org/~diemand/vl/

This release is available electronically at the following web site:

http://press.ucsc.edu