London: Astronomers have long been puzzled by two strange phenomena at the heart of our galaxy. First, the gas in the central molecular zone (CMZ), a dense and chaotic region near the Milky Way’s core, appears to be ionised (meaning it is electrically charged because it has lost electrons) at a surprisingly high rate.
Second, telescopes have detected a mysterious glow of gamma rays with an energy of 511 kilo-electronvolts (keV) (which corresponds to the energy of an electron at rest).
Interestingly, such gamma rays are produced when an electron and its antimatter counterpart (all fundamental charged particles have antimatter versions of themselves that are near identical, but with opposite charge), the positron, collide and annihilate in a flash of light. The causes of both effects have remained unclear, despite decades of observation. But in a new study, published in Physical Review Letters, we show that both could be linked to one of the most elusive ingredients in the universe: dark matter. In particular, we propose that a new form of dark matter, less massive than the types astronomers typically look for, could be the culprit.
The CMZ spans almost 700 light years and contains some of the most dense molecular gas in the galaxy. Over the years, scientists have found that this region is unusually ionised, meaning the hydrogen molecules there are being split into charged particles (electrons and nuclei) at a much faster rate than expected.
This could be the result of sources such as cosmic rays and star light that bombard the gas. However, these alone don’t seem to be able to account for the observed levels.
The other mystery, the 511keV emission, was first observed in the 1970s, but still has no clearly identified source. Several candidates have been proposed, including supernovas, massive stars, black holes and neutron stars. However, none fully explain the pattern or intensity of the emission.
We asked a simple question: could both phenomena be caused by the same hidden process? Dark matter makes up around 85 per cent of the matter in the universe, but it does not emit or absorb light. While its gravitational effects are clear, scientists do not yet know what it is made of.
One possibility, often overlooked, is that dark matter particles could be very light, with masses just a few million electronvolts, far lighter than a proton, and still play a cosmic role. These light dark matter candidates are generally called sub-GeV (giga electronvolts) dark matter particles. Such dark matter particles may interact with their antiparticles.
We studeied that would happen if these light dark matter particles come in contact with their own antiparticles in the galactic centre and annihilate each other, producing electrons and positrons.