Dark matter, the mysterious substance that makes up a large part of the material universe, is quite elusive. Although experiments on ground and in space have not yet found his tracks, their results are helping scientists to eliminate many of the theoretical possibilities. Earlier this year, three studies have been published using the data of a gamma-ray space telescope NASA Fermi collected over six years, and NASA expands its mission to find dark matter, using an innovative approach. "We were looking for the usual things in the community and did not find anything interesting, so started looking for a completely new way, - says Julie McEnery, a scholar of the" Fermi "at Space Center. NASA Goddard in Greenbelt, Maryland. - Guided by our results, "Fermi" ruled out a lot of candidates and found that dark matter contributes to the formation of only a small part of the gamma-ray background outside our Milky Way galaxy, and identified strong restrictions for dark matter particles in the second largest galaxy in orbit our ". Dark matter neither emits nor absorbs light, interacting with the rest of the universe through gravity, but at her expense accounts for about 80% of matter in the universe. Astronomers observe its manifestations throughout the cosmos: the rotation of galaxies in the distortion of the light passing through the galactic clusters, and in the simulation of the early universe, which is needed in the presence of dark matter to galaxies in general were able to form. The leading candidates for dark matter are presented hypothetical particles of different classes. Scientists believe that the gamma rays, the highest energy form of light, can help detect the presence of certain types of suspected dark matter particles. Previously, "Fermi" looking for gamma-ray signals associated with dark matter, which could talk about it, in the center of our galaxy and in the smaller dwarf galaxies orbiting our own. Although, there was no convincing signals, these results removed candidates from a specific range of masses and interaction rates, further narrowing the range of possible properties of dark matter particles. Among the new studies, the most exotic examines the likelihood that dark matter may consist of hypothetical particles called axions or other particles with similar properties. An intriguing aspect of the axion-like particles is their ability to transform into gamma rays and back in contact with strong magnetic fields. This transformation is, in theory, should leave characteristic traces of such gaps and bands in the spectrum of a bright gamma ray source. Manuel Meyer from Stockholm University led the search of these effects in gamma rays from the NGC 1275 galaxy clusters central Perseus, located 240 million light-years away. High-energy emission from NGC 1275 is believed to be associated with a supermassive black hole at its center. Like all clusters of galaxies, the Perseus cluster is filled with hot gas, permeated by magnetic fields, which could enable the transition from gamma rays to the axion-like particles. That is, some gamma-rays from the NGC 1275 could turn into profusely - and possibly back - towards us. Group Meyer collected surveillance telescope Fermi LAT project and sought the predicted distortions in the gamma-ray signal. Their findings, published April 20 in Physical Review Letters, exclude small range of axion-like particles that could make up dark matter 4%. "Although we do not yet know what the dark matter, our results show that we can verify the axion-like model and apply the most serious restrictions on the date specified for the masses - says Meyer. - It is noteworthy that we achieved sensitivity, which we thought would be possible only during a single laboratory experiment: it is a good indicator for the "Fermi". Another broad class of candidates for dark matter - it WIMPs, weakly interacting massive particles (WIMP). In some versions WIMP collision mutually destroys them or makes intermediate rapidly decaying particles. Both scenarios lead to gamma-rays that can be detected by LAT. Regina Caputo of the University of California at Santa Cruz was looking for these signals in the Small Magellanic Cloud (SMC), which is 200 000 light-years from Earth and is the second largest of the small satellite galaxies of the Milky Way. Some MMOs appeal to search for dark matter is that it is relatively close to us, and its gamma-ray emission from traditional sources, such as forming stars and pulsars, are well known. More importantly, the astronomers have produced high-precision measurements MMO rotation curve, which showed how its rotational speed changes with increasing distance from the center, and how much of it there is dark matter. In a paper published March 22 Physical Review D, Caputo and her colleagues simulated the dark matter content in the MMO and have shown that it is sufficiently large to produce a detectable signal for the two types of WIMPs. «LAT definitely sees MMO gamma rays, but we can explain all the traditional sources - says Caputo. - The absence of signals of dark matter annihilation was statistically significant. " In the third study, researchers led by Mark Ajello from Clemson University in South Carolina and Mattia Di Mauro of the SLAC National Accelerator Laboratory in California decided to go in another direction. Instead of looking at specific astronomical targets, the team took as a basis for the analysis of LAT data collected for 6.5 years, and analyzed the background glow of gamma rays across the sky. The nature of this light, the extragalactic gamma-ray background (EGB), discussed since NASA SAS-2 of the first to measure it in the early 1970s. "Fermi" shows that a large part of the world is born in the relatively rare sources of gamma rays, Blazar, which are fueled by material falling into giant black holes. On blazars account for more than half of the gamma-ray sources, which saw the "Fermi", and they make up an even greater share of the new catalog of high-energy gamma rays, drawn up by the LAT. Some models predict that gamma-rays (EGB) can occur far from the interactions of dark matter particles, like WIMP annihilation or decay. In a detailed analysis of the high-energy gamma rays EGB, published April 14, Physical Review Letters, Ajello and his team have shown that blazars and other discrete sources can explain almost all of this radiation. "There remains very little room for signals from exotic sources of extragalactic gamma-ray background, which in turn means that any contribution of these sources will be relatively small, - says Ajello. - This information can help us to set limits on how often-vipy particles can collide and break up. While all of these recent studies have left empty-handed, the search for dark matter is continuing both in space and on the ground. "Fermi" added to this quest Alpha Magnetic Spectrometer NASA, the particle detector to the International Space Station.
Source: NASA press release
