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 at a surprisingly high rate. Second, telescopes have detected a mysterious glow of gamma rays with an energy of 511 kilo-electronvolts (keV). These gamma rays are produced when an electron and its antimatter counterpart, the positron, collide and annihilate in a flash of light.
Despite decades of observation, the causes of both effects have remained unclear. However, a new study published in Physical Review Letters suggests that both phenomena could be linked to one of the most elusive ingredients in the universe: dark matter. In particular, the study proposes that a new form of dark matter, less massive than the types astronomers typically search for, could be responsible for these phenomena.
The CMZ spans almost 700 light years and contains some of the most dense molecular gas in the galaxy. Scientists have found that this region is unusually ionised, meaning that hydrogen molecules are being split into charged particles at a much faster rate than expected. The mysterious 511keV emission, first observed in the 1970s, still has no clearly identified source. Several candidates have been proposed, but none fully explain the pattern or intensity of the emission.
The study suggests that both phenomena could be caused by the same hidden process involving light dark matter particles. These particles, with masses just a few million electronvolts, could interact with their antiparticles in the galactic center and annihilate each other, producing electrons and positrons. In the dense gas of the CMZ, these low-energy particles would efficiently ionise the surrounding hydrogen molecules, matching the observed ionisation profile.
Detailed simulations show that this process, dark matter particles annihilating into electrons and positrons, can naturally explain the ionisation rates observed in the CMZ. The study also suggests that dark matter may be able to replicate some amount of 511keV radiation, providing a direct link between the ionisation and the mysterious glow.
The findings present a new way to test models involving light dark matter particles, which are difficult to detect using traditional laboratory experiments. Future telescopes with better resolution may provide more information on the spatial distribution and relationships between the 511keV line and the CMZ ionisation rate. Continued observations of the CMZ may help strengthen or rule out the dark matter explanation.
In conclusion, these strange signals from the heart of the galaxy remind us that the universe is full of surprises. Exploring the dynamic, glowing center of our own galaxy may reveal unexpected hints of what lies beyond.
