Astronomers are looking for answers to the mysteries of the universe in the depths of the Universe, but some important clues may be hidden very close by – in the center of our galaxy, the Milky Way. Something is clearly happening there that is not fully explained, and this is a direct path to the discovery of the unknown. Researchers from Great Britain, who are ready to help in the search for dark matter, presented their version of what is happening.
The center of the Milky Way, as captured by the Spitzer Space Telescope’s infrared camera. Image Credit: NASA
In the core of the Milky Way — in the so-called Central Molecular Zone (CMZ), 650 to 1,000 light years wide — two phenomena have long been recorded that have not yet been fully explained. First, there is an increased rate of ionization of molecular hydrogen, which is abundant in the center of the galaxy. Second, the entire CMZ region glows in the X-ray range with a radiation energy of 511 keV.
Typically, ionization—the loss of an electron by a hydrogen atom—is explained by supernova explosions, cosmic rays, and supermassive black hole activity. But “the numbers don’t add up”: the region is ionizing inexplicably quickly, as if there were some hidden source there.
As for the X-ray radiation with an energy of 511 keV, this is the rest energy of the electron. Usually, radiation with such energy occurs after the annihilation of an electron and its antiparticle, the positron. As a result, two gamma photons are produced, each with an energy of 511 keV. This line is also uniformly recorded throughout the entire CMZ region. The first and second phenomena cannot be directly linked, but a hypothesis can be put forward that explains both.
Experimentally detecting dark matter in Earth-based laboratories is difficult or impossible, simply because of the fundamental properties of this hypothetical particle, which lacks electromagnetic interaction. But observing traces of such particles in nature, particularly in the center of the Milky Way, could bring us closer to their discovery.
These dark matter particles can interact with their antiparticles. The paper looked at what would happen if these light dark matter particles collided with their antiparticles at the center of a galaxy and annihilated, forming electrons and positrons.
In the dense gas of the CMZ, these low-energy particles would rapidly lose energy and effectively ionize surrounding hydrogen molecules, stripping them of their electrons. Because this region is so dense, the particles cannot spread far. Instead, they release most of their energy locally, which matches the observed ionization profile well. Detailed modeling showed that the proposed mechanism can explain both the high ionization rate and the 511 keV emission lines.
The study found that the predicted profile of dark matter-driven ionization is remarkably smooth across the central region of the Milky Way. This is important because the observed ionization is indeed distributed relatively evenly.
Point sources, such as a black hole at the center of a galaxy or cosmic rays from supernovae, cannot explain this distribution. But a uniformly distributed halo of dark matter can. The results suggest that the center of the Milky Way may provide new clues about the fundamental nature of dark matter.