Astronomers Yet To Come Up With A Definitive Dark Matter Explanation

The exact brightness of the 511keV line depends on several factors like how efficiently positrons form bound states with electrons and where exactly they annihilate though. These details are still uncertain

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 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 feel that a new form of dark matter, less massive than the types astronomers typically look for, could be the culprit.

Hidden process:

The CMZ spans almost 700 light years and contains some of the denser 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 lighter 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.

In our work, we studied what 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. In the dense gas of the CMZ, these low-energy particles would quickly lose energy and ionise the surrounding hydrogen molecules very efficiently by knocking off their electrons. Because the region is so dense, the particles would not travel far.

Instead, they would deposit most of their energy locally, which matches the observed ionisation profile quite well. Using detailed simulations, we found that this simple process, dark matter particles annihilating into electrons and positrons, can naturally explain the ionisation rates observed in the CMZ.

Even better, the required properties of the dark matter, such as its mass and interaction strength, do not conflict with any known constraints from the early universe. Dark matter of this kind appears to be a serious option.

The positron puzzle:

If dark matter is creating positrons in the CMZ, those particles will eventually slow down and eventually annihilate with electrons in the environment, producing gamma-rays at exactly 511keV energy. This would provide a direct link between the ionisation and the mysterious glow. We found that while dark matter can explain the ionisation, it may also be able to replicate some amount of 511keV radiation as well.

This striking finding suggests that the two signals may potentially originate from the same source, light dark matter. The exact brightness of the 511keV line depends on several factors like how efficiently positrons form bound states with electrons and where exactly they annihilate though. These details are still uncertain.

A new way to test the invisible:

Regardless of whether the 511keV emission and the CMZ ionisation share a common source, the ionisation rate in the CMZ is emerging as a valuable new observation to study dark matter. In particular, it provides a way to test models involving light dark matter particles, which are difficult to detect using traditional laboratory experiments. In our study, we showed that the predicted ionisation profile from dark matter is remarkably flat across the CMZ. This is important, because the observed ionisation is indeed spread relatively evenly. Point sources such as the black hole at the centre of the galaxy or cosmic ray sources like supernovas (exploding stars) cannot easily explain this. But a smoothly distributed dark matter halo can. Our findings suggest that the centre of the Milky Way may offer new clues about the fundamental nature of dark matter. Future telescopes with better resolution will be able to provide more information on the spatial distribution and relationships between the 511 keV line and the CMZ ionisation rate.

Meanwhile, continued observations of the CMZ may help rule out, or strengthen, the dark matter explanation.

Either way, these strange signals from the heart of the galaxy remind us that the universe is still full of surprises. Sometimes, looking inward to the dynamic, glowing centre of our own galaxy reveals the most unexpected hints of what lies beyond.

(The writer is Postdoctoral Research Fellow in Department of Physics, King's College London)