By turning their gaze to small satellite galaxies where the total mass is most dominated by dark matter, astrophysicists using data from the Fermi Gamma-ray Space Telescope have achieved the tightest constraints on the properties of dark matter particles to date.
Optical image of the Fornax satellite galaxy, one of the dwarf galaxies used in the Fermi-LAT analysis that looked for the gamma-ray signature of dark matter annihilation coming from satellite galaxies of our Milky Way.
Where should one look for particle dark matter? If you ask the man on the street, he might say in the signatures of the collisions in particle accelerators such as at CERN. Another person might say in dark matter direct detection experiments deep underground. On the other hand, data from the Fermi Gamma-ray Space Telescope has recently shown that very interesting targets may be small galaxies tens of thousands of light-years away.
Our Milky Way Galaxy has a couple of dozen nearby small 'dwarf' satellite galaxies, which have each very low mass by astronomical standards, less than one thousandth of the mass of the Milky Way itself. The observations of the motions of the stars in these satellite galaxies have revealed that they have hundreds to thousands times more mass in invisible dark matter than visible stars, making them the most dark matter dominated objects thus far seen in the Universe, right here in our own backyard.
One of the most exciting science goals of the Fermi mission involves using Fermi's Large Area Telescope (LAT), which was built at and is operated from SLAC, to search for gamma-ray photons resulting from dark matter particles annihilating into other subatomic particles and eventually into gamma rays. Since these small galaxies have no conventional 'astrophysical' sources of high energy gamma rays, such as large numbers of supernova explosions and lots of interstellar gas, any gamma-ray emission that is observed must arise from dark matter. In a recent study the Fermi-LAT collaboration, including KIPAC postdoctoral researcher Louis Strigari and Graduate Student Alex Drlica-Wagner, and led by Johann Cohen-Tanugi of the Universite Montpilier 2 in France and formerly of KIPAC, have combed through data for ten of these objects and searched for the gamma rays that would be a signal of dark matter annihilation.
By combining the emission from these ten galaxies, the Fermi-LAT team was able to place the most stringent limits on dark matter to date, ruling out dark matter particle of mass less than about 30 GeV at cosmologically plausible annihilation cross sections. With this technique, the Fermi-LAT is the only current instrument that is able to study dark matter in the all-important mass regime predicted by cosmological theory. It is expected with more data in the next several years that Fermi-LAT observations will be able to access an even larger part of the mass and annihilation cross section parameter space of common particle dark matter models.
This work is described in a paper submitted to Physical Review Letters and available from astro-ph at arXiv1108.3546.
Tidbit Author: Louie Strigari and Jack Singal