Bright gamma-ray sources spotted at centre of Andromeda

Most gamma rays emanating from the Andromeda galaxy come from its centre, rather than throughout the galaxy, as previously expected. That is the conclusion of astronomers who have used NASA's Fermi Gamma-ray Space Telescope to study Andromeda, which at 2.5 million light-years distance is the nearest major galaxy to Earth. The result is reminiscent of the previous – and unexpected – observation by Fermi that there is an excess of gamma rays coming from the centre of the Milky Way. The astronomers have proposed
several explanations for the Andromeda observation. One is that the gamma rays are produced by the decay of hypothetical dark-matter particles, which are expected to concentrate at galactic centres. Another is that there may be an unexpectedly high concentration of pulsars at the centre. These are spinning neutron stars that emit copious gamma rays. The next step for the team is to look at X-ray and radio emissions, which could help scientists work out if the gamma rays are indeed produced by pulsars. The observations are described in The Astrophysical Journal.