Currently everything we see in the Universe with our telescope – across all wavelengths – only constitutes 20% of the mass of the Universe, with the other 80% comprising what is known as “dark matter” (since we cannot see it, but we know it must be there because of its gravitational effect). Dark matter candidates can either be baryonic (which in astronomy means all objects made of ‘normal’ atomic matter), or non-baryonic, or a mix of the two. Non-baryonic dark matter is usually subdivided into two categories – hot dark matter (HDM) and cold dark matter (CDM). HDM requires particles with near-zero mass (like neutrinos) to travel very close to the speed of light (i.e. relativistic particles) to make up the missing mass in the Universe, while CDM requires more massive particles that travel at sub-relativistic speeds. Many astronomers believe that the missing matter in the Universe is likely in the form of some sub-atomic particle that we have not yet detected (but which particle accelerators like the Large Hadron Collider hope to find), which requires an extension to the standard model of particle physics. Weakly Interactive Massive Particles (WIMPs) – a class CDM particles – are one of the leading candidates for dark matter. WIMPs interact via gravity and the weak force, and do not interact with electromagnetism (so they don’t absorb or emit light and hence they cannot be seen) and they don’t interact with strongly with other particles or atomic nuclei. However, when WIMPs interact with each other they can annihilate and produce gamma rays.
Using this knowledge, Markus Ackermann and collaborators used the Large Area Telescope (LAT) on board NASA’s Fermi Gamma-ray Space Telescope to search for gamma rays resulting from WIMP annihilation coming from dwarf satellite galaxies around the Milky Way. Dwarf spheroidal galaxies are ideal for this type of study because they contain large amounts of dark matter, little gas and very low star formation, and no gamma-ray emitting stellar remnants like pulsars and supernova remnants.
By studying 10 dwarf spheroidal satellite galaxies of the Milky Way using 24 months of Fermi-LAT data, Ackermann et al. detected no gamma-ray signals consistent with WIMP annihilations – thus they detected no dark matter signal. Thus for the first time they have been able to constrain the WIMP interaction rates and mass ranges that can not be dark matter. Further studies will include additional Fermi observations of higher energy gamma-rays and newly discovered dwarf galaxies. For more details, see
- Fermi Observations of Dwarf Galaxies Provide New Insights on Dark Matter, NASA Fermi News
- Constraining Dark Matter Models from a Combined Analysis of Milky Way Satellites with the Fermi Large Area Telescope, Ackermann et al. (2011), Phys. Rev. Lett. 107, 241302 [Swinburne login required to access PDF]
(Catarina Ubach & Sarah Maddison)