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A new hypothesis about dark matter could offer answers to some of our galaxy's biggest puzzles. Researchers suggest that an alternative form of this invisible substance could explain the Milky Way's unusual distribution of stars and gravitational patterns.
Scientists have observed two intriguing phenomena at the heart of the Milky Way. One is the surprisingly high rate of ionization of gas in the so-called central molecular zone (CMZ), a dense and chaotic region close to the galactic nucleus.
The other is a persistent glow of gamma rays with an energy of 511 kiloelectronvolts (keV), identified by telescopes since the 1970s.
Now, a new study published in the journal Physical Review Letters proposes that both effects may have a common cause: a light and previously little-considered form of dark matter.
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Overcharged gas
The MCZ spans approximately 700 light-years, composed of some of the densest molecular gases in the galaxy.
One fact caught the attention of astronomers: the gas present there is strongly ionized. This means that the hydrogen molecules are being broken in charged particles (electrons and nuclei) at a much higher speed than expected.
This type of ionization could be caused by cosmic rays or radiation from stars, but these known sources do not explain the levels recorded in the ZMC. The observed intensity is above what these traditional mechanisms would be capable of producing.
Gamma rays without clear source
The second mystery is the 511 keV glow. This specific radiation is emitted when an electron encounters its antiparticle, the positron. When they collide, the two annihilate each other, releasing this characteristic energy in the form of light.
The origin of these positrons, however, is still not understood. Explanations such as supernovae, black holes and neutron stars have been suggested. None, to date, have been able to fully justify the pattern or intensity of the observed brightness.
The question that led to the hypothesis
With this data in hand, the researchers decided to investigate a new possibility: what if both phenomena, ionization and gamma rays, are linked to the same hidden process?
The proposal involves an unlikely candidate, but one that is beginning to gain traction: a light form of dark matter.
This type of particle, lighter than a proton, has been little studied to date. But it may represent a significant fraction of the invisible matter that makes up 85% of the universe.
These particles, called sub-GeV (less than one gigaelectronvolt) dark matter, could annihilate with their own antiparticles, generating electrons and positrons in the process.
Energy released in the right place
The team of scientists simulated how these particles would behave in the MCZ. Because it is an extremely dense region, the energy released by these annihilations would be quickly absorbed by the surrounding gas. This local effect would cause the hydrogen molecules to be ionized very efficiently — exactly what is observed in the MCZ.
Simulations have shown that this hypothesis works well. The ionization profile predicted by theory matches the actual behavior of the gas observed by astronomers.
Furthermore, the parameters used in the model, such as the mass and interaction strength of dark matter, do not violate any of the known constraints of the physics of the early universe.
The link with gamma rays
The same annihilation that generates electrons also produces positrons. These, in turn, can slow down and encounter electrons in the environment. When this happens, gamma rays with an energy of 511 keV are emitted.
The hypothesis therefore unites the two phenomena into a single explanation: light dark matter and its interactions in the galactic center.
The amount of brightness generated depends on several factors, such as how efficiently positrons combine with electrons and the exact location where these annihilations occur. These details are not yet fully defined, but evidence suggests that there is a possible connection between the two signals.
New tool to study the invisible
Even though the link with gamma rays still needs more evidence, the researchers highlight that the ionization rate in the ZMC can be used as a new tool to investigate dark matter.
Because these particles are so light, laboratory experiments on Earth have difficulty detecting them. But observing the behavior of gas at the center of the galaxy could be an effective way to identify their presence.
The simulations also indicate that the distribution of ionization caused by dark matter would be relatively uniform across the MCZ. This is consistent with what telescopes are observing.
Sources such as the central black hole or supernova explosions usually produce localized ionizations, not so widespread. A halo of dark matter distributed around the galactic center, on the other hand, fits this profile better.
What the future may reveal
The study suggests that the center of the Milky Way is a promising region for revealing the nature of dark matter. With more advanced telescopes in the future, it will be possible to map both the ionization rate and the emission of gamma rays more precisely.
These new observations could confirm or discard the hypothesis that both phenomena are caused by the same source.
In the meantime, the ZMC continues to be watched closely. Each new measurement could help to decipher one of the greatest mysteries of modern physics.
Even without definitive answers, the work reinforces a simple and powerful idea: the universe still holds many secrets. And sometimes, all we need to do is look within, into the heart of our own galaxy, to find clues about what lies beyond.
