Glow puzzle finds an answer

Astronomers have long puzzled over a mysterious glow of radiation coming from the centre of the Milky Way. One school of thought suggests that the collision and annihilation of dark matter particles are the reason for the radiation. Another school holds that the radiation originates from ancient, ultra-dense stars known as millisecond pulsars. A new study that simulates the galactic centre suggests a resolution to this puzzle: dark matter is responsible for the glow, after all.

The study, published in Physical Review Letters (bit.ly/Matter-Glow) in October, also gives a clue about the nature of the dark matter particles.

Earlier studies have modelled the distribution of dark matter as spherically symmetric. Considering that the Milky Way is flat, the researchers modelled the dark matter distribution as a flattened shape. Furthermore, they consider dark matter to be composed of WIMPs, or weakly interacting massive particles, which can collide with and annihilate each other, emitting radiation in the process. This radiation, according to the researchers, is what is observed as the excess radiation in the galactic centre.

Further evidence from observations is needed to corroborate the result. A clear indication for the alternative explanation invoking millisecond pulsars would be the discovery of enough pulsars to account for the gamma-ray glow. New telescopes with higher resolution are already being built, which could help settle this question. "If these telescopes still fail to find star-like sources and only detect diffuse radiation, it would strengthen the dark matter explanation," says Moorits Mihkel Muru, a postdoctoral fellow at the Leibniz Institute for Astrophysics Potsdam, Germany, and the first author of the paper.

To directly verify this result — detecting dark matter — is extremely challenging, because dark matter doesn't emit or block light, and can only be detected through its gravitational effects on visible matter. Despite decades of searching, no experiment has yet directly detected dark matter particles. "A 'smoking gun' for dark matter would be a signal that matches theoretical predictions precisely, but this requires both more accurate predictions and better telescopes for observations," says Muru. "A measurement of the annihilation signal through the gamma rays would be a big leap forward."