Clusters of galaxies are the most massive structures in the universe. Most of the mass in these clusters is considered to be dark matter. The Fermi LAT monitors these clusters for a gamma-ray signal from dark matter annihilation. No such signal has been found yet, but the non-observation starts to constrain a wide range of proposed dark matter models.
Clusters of galaxies are the most massive structures in the universe. We can infer that most of the mass in these clusters is dark matter, both from observations of the motion of the individual galaxies as well as gravitational lensing effects. This feature makes the closest of these clusters prime targets to look for gamma-ray emission from the annihilation or decay products of dark matter particles.
Their huge mass compensates or in some cases even overcompensates for their considerably larger distance compared to dark matter dominated dwarf galaxies in the vicinity of the Milky Way (which are also studied by the LAT), resulting in a better sensitivity to dark matter signals for a range of models. The value of this search is additionally underlined by its complimentary character to the observations of dwarf galaxies. Potential dark matter signals have to be compatible for both classes of objects but rely on different model assumptions.
The Fermi LAT galaxy cluster monitoring program with significant contributions from KIPAC researchers Keith Bechtol and Stefan Funk has not found gamma-ray emission from any of the monitored galaxy clusters in the first 11 month of Fermi LAT operations. However, the non-observation of a signal from the Fornax cluster allows to constrain a large range of dark matter models predicting a stable particle based on the theory of Supersymmetry. In addition, models predicting dark matter annihilating/decaying dominantly into leptons can be constrained quite severely, even with conservative assumptions on the dark matter substructure present in the galaxy clusters. Such models have obtained a large recent interest, as they are favored to explain the Fermi LAT cosmic-ray electron spectrum as well as the PAMELA electron/positron fraction without
violating constraints from the measurements of anti-protons in the cosmic rays.
More statistics on gamma-rays - already collected by the LAT and continuing to grow - will continue to improve these constraints in the future or ultimately lead to the detection of a dark matter signal from galaxy clusters.