Dark Matter Structures in the Milky Way Resolved

Cosmologists from the US and Switzerland were able to resolve dark matter structures in the Milky Way. Using a computer simulation, the team was able to detect dark matter sub-halos and streams contained within the dark matter halo engulfing our galaxy. Many of these dark matter clumps and streams lie in the solar system vicinity. Better understanding of these structures may shed some light on the enigmatic dark matter.
 Artist's impression of the Fermi Gamma-ray Space Telescope (formerly named the Gamma-ray Large Area Space Telescope, or GLAST) in orbit. The Fermi may detect gamma ray emissions from WIMP interactions.
Artist’s impression of the Fermi
Gamma-ray Space Telescope
(formerly named the Gamma-ray
Large Area Space Telescope,
or GLAST) in orbit. The Fermi may
detect gamma ray emissions from
WIMP interactions (credit: NASA
/ Goddard Space Flight Center)

The accepted theory regarding the formation of structures in the universe stipulates that initial fluctuations in the density of the earlier universe have grown over time resulting in the setup we see today. It is the gravitational interaction of dark matter, a mysterious substance comprising about a quarter of the matter in the universe, which affected these fluctuations, creating clumps of dark matter.

 
According to this CDM (Cold Dark Matter) model, small structures merged and created bigger ones. In particular, every galaxy is nested in a dark matter halo resulting from such merges. Computer simulations have revealed that these halos are not smooth and uniform, but have inner sub-halos. The new research used a simulation to resolve this sub-structure in the Milky Way.
 
The simulation was done on the Jaguar supercomputer at Oak Ridge National Laboratory and tracked a system with over a billion dark matter particles from shortly after the Big Bang to today. “This is the best resolved calculation of the Milky Way’s halo ever carried out, with a mass resolution five to sixty times better than the previous largest computations,” explains Piero Madau from the University of California in Santa Cruz, one of the cosmologists who collaborated in this research. “Previously, the inner regions of the halo came out smooth but now we have enough detail to see dense clumps of dark matter.”
 
Over 40,000 sub-halos were resolved, with densities that agree with observations. Some of these sub-halos were themselves not uniform, but had a sub-structure of their own. In addition to the sub-halos, dark matter streams were detected. These streams formed from material secreted from disturbed sub-halos. Hundreds of the sub-halos and a number of streams were located near the solar system.
 
A disk galaxy surrounded by its dark matter halo (Credit: John Kormendy,  University of Texas) 
A disk galaxy surrounded by its dark
matter halo (Credit: John Kormendy,
University of Texas)

Since dark matter particles interact only gravitationally, they are hard to detect. However, according to one theory, as these particles called WIMPs (weakly interacting massive particles) interact; they can annihilate each other and in the process emit gamma rays. These emissions can be detected by the Gamma-ray Large Area Space Telescope (GLAST), launched by NASA in June. Piero Madau commented: “That’s what makes this exciting; 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, added: “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.”

TFOT reported on research revealing that the dark matter density in the solar system is higher than the average density in the galaxy’s dark matter halo. TFOT also covered the launch of GLAST, a NASA telescope that should help improve our understanding in a variety of subjects, from our solar system to dark matter, and even the most fundamental laws of physics.
 
Further information on the new research, which was published in Nature magazine, can be found in the Arxiv website (PDF).