John P. Millis, PhD for redOrbit.com — Your Universe Online
Scientists continue the search to identify the nature of dark matter that seems to pervade the Universe. Since by its very nature it does not interact well — or perhaps at all — with electromagnetic radiation, direct detection of this mysterious form of matter has proven elusive.
Despite this obstacle, astronomers have become convinced of its existence because of how it can be ℠seen´ to interact with normal, luminous matter. To begin closing in on an understanding of this mysterious substance, scientists from England, Taiwan and Japan have used the Prime Focus Camera (Suprime-Cam) aboard the Subaru Telescope to examine the distribution of dark matter in fifty galaxy clusters — the largest structures in the Universe.
According to Graham Smith from the University of Birmingham, England, and co-leader of the research team, “A galaxy cluster is like a huge city viewed from above during the night. Each bright city light is a galaxy, and the dark areas between the lights that appear to be empty during the night are actually full of dark matter. You can think of the dark matter in a galaxy cluster as being the infrastructure within which the galaxies live.”
Since there are various models of dark matter — typically known as Hot, Warm, and Cold — there are different predictions as to how the dark matter would be distributed throughout the cluster. Hot dark matter models, for instance, would expect the particles to be rather evenly distributed since their high-energy means that they would move too fast to really clump together gravitationally. Conversely, cold dark matter would be slower moving with more massive particles. As a result the dark matter would more easily clump together.
For this study, the team used a phenomenon known as gravitational lensing to determine where the dark matter was distributed. Since most dark matter theories contend that the particles only interact with other matter via the gravitational force, there is no obvious way to measure the dark matter directly. Instead, scientists use telescopes to search for distortions of distant galaxies caused by the gravitational warping — or lensing — of the light from these objects.
Lead author Nobuhiro Okabe, from the Academia Sinica, Taiwan, explains, “The Subaru Telescope is a fantastic instrument for gravitational lensing measurements. It allows us to measure very precisely how the dark matter in galaxy clusters distorts light from distant galaxies and gauge tiny changes in the appearance of a huge number of faint galaxies.”
For each observation, the researchers mapped the distribution of dark matter to create a concentration parameter, which is a measure of a galaxy cluster´s average density. Comparison of the dark matter maps of all 50 galaxies shows ranging values for the concentration parameter, but what was most telling was how the concentration parameter converged when the researchers used a technique known as data stacking.
Instead of analyzing an individual source — in this case a single galaxy cluster — a group of similar source types are analyzed together, as if the data were all from a single object. The down side is that the individual source information is lost, but on the other hand, general trends about the data which may be too subtle to characterize for an individual source will be revealed in the stacked analysis.
What the team found was that the density of the dark matter was greater near the center of the galaxy clusters, on average, and gradually becomes more diffuse moving out towards the cluster´s edge. This is consistent with a cold dark matter model, which, along with other evidence, has been the leading candidate theory for some years. The problem is that there is no candidate particle that has been identified by experimental particle physicists that can fit the cold dark matter mold.
The initial indication is that cold dark matter models continue to lead the way, yet we still lack a good candidate particle. So, where to go from here? “We don’t stop here,” noted Smith. “For example, we can improve our work by measuring dark matter density on even smaller scales, right in the center of these galaxy clusters. Additional measurements on smaller scales will help us to learn more about dark matter in the future.”