Read more: 鈥�Mysteries of the Milky Way鈥�
Take 10 billion tonnes of antimatter and 10 billion tonnes of matter, and stir. Our galaxy mixes up one of these explosive cocktails every second, resulting in a warm inner glow of gamma radiation. Each photon created in this process carries an energy exactly equivalent to the annihilated mass of an electron and its antimatter counterpart, a positron. But what could be pumping out so many positrons?
Most galactic radiation 鈥� including visible light, ultraviolet, infrared, X-rays, radio waves and gamma rays of other energies 鈥� comes predominantly from the Milky Way鈥檚 flat disc, where brilliant, short-lived new stars are formed. But the annihilation gamma rays come mainly from the galaxy鈥檚 much smaller, bulging centre. 鈥淚t is a unique case,鈥� says of the Institute of Astrophysics in Paris, France.
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There are some odd suggestions for this odd origin. The positrons could be created by decaying dark matter or blobs of exotic quantum matter called Q-balls; or they could be spat out by microscopic black holes or a tangle of cosmic strings, snags in the structure of space-time. Or it might be something more familiar. Supernova explosions in the Milky Way鈥檚 disc create radioactive isotopes that emit positrons as they decay, and neutron stars and black holes can make antimatter when they feast on material from a sibling star. Some of these objects also inhabit the galactic bulge, although only enough to account for a fraction of the gamma-ray emission seen there.
The picture changes if the galaxy鈥檚 magnetic field can funnel positrons from the disc into the central bulge. That depends on the field鈥檚 basic shape, something we could learn from observations of how radio waves from distant sources are polarised. It also depends on whether positrons can travel tens of thousands of light years before being annihilated. That will be much harder to work out, because it depends on the small-scale details of magnetic fields and interstellar gas beyond the power of our telescopes.
If positrons can travel so far, another possibility opens up. 鈥淭hey could come from an event that happened long ago in the central black hole,鈥� says Prantzos. Starved of fuel, our galaxy鈥檚 black hole is currently quiet (see 鈥淢ilky Way mysteries: Five oddities of our galaxy鈥�). Millions of years ago, though, it might have flared up, pumping out positrons that have since pushed out through the central bulge, creating a spherical halo of annihilation. Without a more detailed picture of the gamma-ray emissions, for now the truth remains out there.