New Scientist | Space | 27 Aug 2010
Multi-GeV photons from electron–dark-matter scattering near active galactic nuclei - ED Bloom, JD WellsTWO of the darkest things in the universe may be making light - or at least, radiation. When jets spat out by a supermassive black hole at the centre of a galaxy collide with dark matter, they could produce gamma rays detectable from Earth - possible evidence of the elusive dark stuff.
Jets of particles are propelled away from black holes at near the speed of light. Akin to a cosmic belch, they are thought to be connected with matter falling into the black hole. Stefano Profumo of the University of California, Santa Cruz, and his colleagues calculated how electrons in one of these jets would interact with any surrounding dark matter.
They looked specifically at the types of dark matter particles predicted by two major theories: one is supersymmetry, which proposes that each ordinary particle has a superpartner, and the other assumes that the universe is hiding a fourth spatial dimension.
They found that rather than simply ricocheting off one another, some of the electrons and dark matter particles could fuse together, transforming into a single, supersymmetric or extra-dimensional version of the electron. This particle would be heavy, and much of the electron's kinetic energy would be dumped into making the new particle. As a result, the particle would be almost standing still.
If the particle were then to decay into an electron and a ground-state dark matter particle, the electron would release gamma rays. Unlike a particle travelling fast, like those in the jets, the slow-moving particle would emit rays that could travel in any direction. This could potentially make them easier to distinguish from the flood of photons in the jet, says collaborator Mikhail Gorshteyn of Indiana University in Bloomington.
The idea that particles from a black hole could interact with dark matter to produce gamma rays has been proposed before, but a previous study suggested the rays would be too faint to see from Earth (Physical Review D 57 (1998), DOI: 10.1103/PhysRevD.57.1299).
However, Profumo's team found that within a narrow range of electron energies, nearly all electrons colliding with dark matter will convert into the supersymmetric or extra-dimensional version. This "resonance" effect would produce gamma rays that could be seen in detectors near Earth, such as NASA's Fermi Space Telescope, says Gorshteyn.
The team calculate that the effect could account for the frequencies of gamma rays measured by Fermi coming from the black hole at the centre of the galaxy Centaurus A (arXiv:1008.2230). However, the frequency spectrum of gamma rays from another galaxy, Messier 87, does not match their predictions.
- Physical Review D 57(2) (15 Jan 1998) DOI: 10.1103/PhysRevD.57.1299
- arXiv.org > astro-ph > arXiv:1008.2230 > 12 Aug 2010