Is dark matter hiding in plain sight?

An orbiting gamma-ray telescope that has stared into deep space for four years may have picked up signs of dark matter in the Virgo Cluster of Galaxies. The Fermi Large Area Telescope could have spotted an as-yet undetected particle – a dark matter candidate called a neutralino (not a neutrino).

An image of the Virgo Cluster of galaxies by the Hubble Space Telescope. Credit: NASA/ESA

In a study published in January of this year (arXiv:1201.1003v1), Jiaxin Han, Carlos Frenk and others in China, the UK and Germany saw something unusual. Three galaxy clusters—Virgo, Fornax and Coma, showed an excess of high-energy gamma-rays that couldn’t be accounted for from sources such as pulsars, gamma-ray bursts or background emissions – including from within our own Galaxy.

Something else seemed to be emitting radiation; something that continued three degrees out from the clusters (around six times the diameter of the full moon on the sky). However, they couldn’t be sure if the neutralino or some other effect was responsible.

The problem is that dark matter, save for its gravitational effect on radiation and normal matter (the type that makes up stars, galaxies, planets, nebulae, etc.), has never revealed itself. It’s a field of intense study in astrophysics as it accounts for 83-per cent of all the matter in the Universe – forming the nucleation sites of galaxies and creating the large-scale cosmic structure that we see.

This ‘lumpiness’ favours cold dark matter theories as warm dark matter would be too energetic to coalesce. The neutralino, a ‘supersymmetric’ particle is the favoured candidate. It is its own antiparticle so self-annihilations should produce gamma-rays of between 10—10,000Gev.

Now a team from New Zealand think that what they have spotted could indeed be neutralino annihilations, but they also sound a note of caution. Oscar Macías-Ramírez and his team from Canterbury University, Christchurch, used simulations from a study called the Phoenix Project* to model galaxy cluster Dark Matter halos and subhalos. They looked at the previous study by Han, Frank et al which had a strong (but not conclusive) claim to detecting neutralino annihilations. This weakened when cosmic ray contributions were added.

To be thorough, Macías-Ramírez’s team found point sources deep in the data that may have been missed and included them in their study. What they’ve found is that the non-cosmic ray case gives an increased confidence level: the fabled ‘5-sigma’’ result made famous by the recent discovery of the Higgs Boson when the new point sources were not included. But when the new point sources were included, that confidence level fell dramatically. Including the effects of cosmic rays is expected to reduce the significance of the proposed dark matter signal even more. The nature of dark matter (and the existence of the neutralino) it seems, is still a mystery.

What the studies show is that accounting for all possible sources of gamma rays is important in the hunt for the neutralino as it can have a huge impact on results. It’ll be something to temper the efforts of other teams around the world racing to find that first definitive Dark Matter result.

*The Phoenix Project consists of Anders Pinzke, Christoph Pfrommer and Lars Bergström from the University of California Santa Barbara, Heidelberg Institute for Theoretical Studies, the Oskar Klein Centre for Cosmoparticle Physics and Stockholm University respectively.

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