Astronomers have long been puzzled by two strange phenomena at the center of our galaxy. First, the gas within the central molecular zone (CMZ), a dense and chaotic region near the Milky Way’s core, appears to be ionized (meaning it’s electrically charged since 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 which are near similar, but with opposite charge—the positron, collide and annihilate in a flash of sunshine.
The causes of each effects have remained unclear, despite a long time of remark. But in a brand new study, published in Physical Review Letters, my colleagues and I show that each might be linked to one of the vital elusive ingredients within the universe: dark matter. Specifically, we propose that a brand new type of dark matter, less massive than the categories astronomers typically search for, might be the offender.
Hidden Process
The CMZ spans almost 700 light years and comprises a few of the most dense molecular gas within the galaxy. Over time, scientists have found that this region is unusually ionized, meaning the hydrogen molecules there are being split into charged particles (electrons and nuclei) at a much faster rate than expected.
This might be the results of sources akin to cosmic rays and starlight that bombard the gas. Nonetheless, these alone don’t appear to have the opportunity to account for the observed levels.
The opposite mystery, the 511-keV emission, was first observed within the Seventies, but still has no clearly identified source. Several candidates have been proposed, including supernovas, massive stars, black holes, and neutron stars. Nonetheless, none fully explain the pattern or intensity of the emission.
We asked an easy query: Could each phenomena be brought on by the identical hidden process?
Dark matter makes up around 85 percent of the matter within the universe, nevertheless it doesn’t emit or absorb light. While its gravitational effects are clear, scientists don’t yet know what it’s manufactured from.
One possibility, often ignored, is that dark matter particles might be very light, with masses just 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 occur if these light dark matter particles are available in contact with their very own antiparticles within the galactic center and annihilate one another, producing electrons and positrons.
Within the dense gas of the CMZ, these low-energy particles would quickly lose energy and ionize the encompassing hydrogen molecules very efficiently by knocking off their electrons. Since the region is so dense, the particles wouldn’t travel far. As a substitute, they’d deposit most of their energy locally, which matches the observed ionization profile quite well.
Using detailed simulations, we found that this straightforward process, dark matter particles annihilating into electrons and positrons, can naturally explain the ionization rates observed within the CMZ.
Even higher, the required properties of the dark matter, akin to its mass and interaction strength, don’t conflict with any known constraints from the early universe. Dark matter of this type appears to be a serious option.
The Positron Puzzle
If dark matter is creating positrons within the CMZ, those particles will eventually decelerate and eventually annihilate with electrons within the environment, producing gamma-rays at exactly 511-keV energy. This could provide a direct link between the ionization and the mysterious glow.
We found that while dark matter can explain the ionization, it can also have the opportunity to duplicate some amount of 511-keV radiation as well. This striking finding suggests that the 2 signals may potentially originate from the identical source, light dark matter.
The precise brightness of the 511-keV line depends upon several aspects, including how efficiently positrons form certain states with electrons and where exactly they annihilate though. These details are still uncertain.
A Recent Method to Test the Invisible
No matter whether the 511-keV emission and CMZ ionization share a typical source, the ionization rate within the CMZ is emerging as a worthwhile recent remark to check dark matter. Specifically, it provides a solution to test models involving light dark matter particles, that are difficult to detect using traditional laboratory experiments.
Move observations of the Milky Way could help test theories of dark matter. ESO/Y. Beletsky, CC BY-SA
In our study, we showed that the expected ionization profile from dark matter is remarkably flat across the CMZ. This is very important, since the observed ionization is indeed spread relatively evenly.
Point sources akin to the black hole at the middle of the galaxy or cosmic ray sources like supernovas (exploding stars) cannot easily explain this. But a easily distributed dark matter halo can.
Our findings suggest that the middle of the Milky Way may offer recent clues in regards to the fundamental nature of dark matter.
Future telescopes with higher resolution will have the opportunity to supply more information on the spatial distribution and relationships between the 511-keV line and the CMZ ionization rate. Meanwhile, continued observations of the CMZ may help rule out, or strengthen, the dark matter explanation.
Either way, these strange signals from the center of the galaxy remind us that the universe remains to be stuffed with surprises. Sometimes, looking inward, to the dynamic, glowing center of our own galaxy, reveals probably the most unexpected hints of what lies beyond.
