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Invisible clumps in the galaxy

New simulations find clumps of dark matter in the solar system’s neighborhood

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11:57am, August 6, 2008
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Clumps of invisible “dark matter” lurk in the same galactic neighborhood as the solar system, a powerful new computer simulation shows. The finding, reported in the Aug. 7 Nature, could help scientists determine what the unseen material is made of.

Surrounding every galaxy is a halo of mysterious dark matter that can only be detected through its gravitational tug on stars and galaxies. This invisible halo is more spherical and much larger than the visible galaxy it encapsulates. Past computer simulations suggested that relatively dense concentrations of dark matter would form in gravitationally bound “subhalos” within the galactic halo. But in those simulations, subhalos did not show up in the inner regions of a galaxy.

“We were surprised by how many of these dark matter clumps survived in the central region of a galaxy in this new simulation,” says study coauthor Michael Kuhlen, a theoretical cosmologist at the Institute for Advanced Study in Princeton, N.J. Gravitational forces are much stronger in the inner galactic region, so it had been unclear how many, if any, subhalos could survive there.

During the simulation, hundreds of the clumps appeared to form near the solar circle, which sits about 26,000 light-years from the galactic center. The edge of the stellar disk of the Milky Way is about 50,000 light-years from the center of the galaxy. Tens of thousands of clumps appeared in the simulation when looking at the galaxy and its invisible halo which extended all the way out to roughly 1 million light-years from the galactic center.

The simulation, which is the largest published so far, is based on the leading explanation for how the universe evolved after the Big Bang. According to this explanation, gravity first acted more strongly on tiny regions with higher-than-average numbers of dark matter particles. That then led to the growth of the first dark matter clumps, which kept glomming together into larger and larger clumps, eventually pulling in enough normal matter to form galaxies. This scenario assumes the dark matter particles are relatively slow moving and thus designated as “cold.”

Led by University of California, Santa Cruz theoretical cosmologist Jürg Diemand, the team simulated the gravitational interaction of more than a billion of those early, cold, dark matter particles and followed their growth and distribution in a galaxy similar to the Milky Way. The simulation started just after the Big Bang and ran for a hypothetical 13.7 billion years, stopping at modern times. The researchers note that the simulation does not model any forms of normal matter such as stars or planets.

Still, the clumps of dark matter in the simulation have densities that are remarkably similar to densities that a University of California, Irvine research group found when simulating the formation of the Milky Way and its satellite dwarf galaxies, says James Bullock, the astrophysicist who leads the UC-Irvine group and was not involved in the new study.

“This is a remarkable success of the particular model simulated and adds strong support to the idea that the dark matter is made up of particles that are ‘cold.’ There are a number of planned experiments aimed at detecting the dark matter that are betting on it being cold, so this is generally good news for the community,” Bullock says.

Simulating dark matter clumps in this inner galactic region might help scientists directly detect interactions of dark matter particles and figure out what makes them up, says study coauthor Piero Madau, a theoretical cosmologist at UC-Santa Cruz.

Scientists theorize that cold dark matter consists of weakly interacting massive particles, or WIMPs. When they collide, WIMPS can annihilate each other and emit gamma rays. The natural place to look for these gamma rays is at the galactic center since the density of dark matter clumps in a galaxy is greatest there, Kuhlen says. That is where researchers might first look for those gamma rays using the recently launched GLAST, NASA’s Gamma-ray Large Area Space Telescope. But, Kuhlen notes, gamma rays from other inner galactic sources, such as supernova remnants, might make the positive identification of dark matter gamma rays impossible.

“The clumpiness of dark matter in our new simulation points to the possibility that we may observe annihilation going on in subhalos that lie away from the galactic center, possibly in regions closer to the distance where the sun sits in the galaxy,” he says. He and his colleagues report predictions for the signals that GLAST could detect from those annihilations in a paper that will appear in the Sept. 10 Astrophysical Journal.

And, Madau notes, larger simulations that might help constrain the nature of dark matter even more are already in the works.

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