Astronomers have lengthy been puzzled by two unusual phenomena on the coronary heart of our galaxy. First, the gasoline within the central molecular zone (CMZ), a dense and chaotic area close to the Milky Approach’s core, seems to be ionized (which means it’s electrically charged as a result of it has misplaced electrons) at a surprisingly excessive charge.
Second, telescopes have detected a mysterious glow of gamma rays with an vitality of 511 kilo-electronvolts (keV) (which corresponds to the vitality of an electron at relaxation).
Curiously, such gamma rays are produced when an electron and its antimatter counterpart—all elementary charged particles have antimatter variations of themselves which can be close to an identical, however with reverse cost—the positron, collide and annihilate in a flash of sunshine.
The causes of each results have remained unclear, regardless of a long time of remark. However in a brand new research, revealed in Bodily Evaluate Letters, my colleagues and I present that each might be linked to one of the elusive components within the universe: darkish matter. Specifically, we suggest {that a} new type of darkish matter, much less large than the categories astronomers sometimes search for, might be the perpetrator.
Hidden Course of
The CMZ spans nearly 700 mild years and comprises among the most dense molecular gasoline within the galaxy. Over time, scientists have discovered that this area is unusually ionized, which means the hydrogen molecules there are being cut up into charged particles (electrons and nuclei) at a a lot sooner charge than anticipated.
This might be the results of sources akin to cosmic rays and starlight that bombard the gasoline. Nonetheless, these alone don’t appear to have the ability to account for the noticed ranges.
The opposite thriller, the 511-keV emission, was first noticed within the Seventies, however nonetheless has no clearly recognized supply. A number of candidates have been proposed, together with supernovas, large stars, black holes, and neutron stars. Nonetheless, none absolutely clarify the sample or depth of the emission.
We requested a easy query: Might each phenomena be attributable to the identical hidden course of?
Darkish matter makes up round 85 p.c of the matter within the universe, but it surely doesn’t emit or soak up mild. Whereas its gravitational results are clear, scientists don’t but know what it’s fabricated from.
One chance, typically ignored, is that darkish matter particles might be very mild, with lots only a few million electronvolts, far lighter than a proton, and nonetheless play a cosmic function. These mild darkish matter candidates are typically referred to as sub-GeV (giga electronvolts) darkish matter particles.
Such darkish matter particles might work together with their antiparticles. In our work, we studied what would occur if these mild darkish matter particles are available in contact with their very own antiparticles within the galactic middle and annihilate one another, producing electrons and positrons.
Within the dense gasoline of the CMZ, these low-energy particles would shortly lose vitality and ionize the encompassing hydrogen molecules very effectively by knocking off their electrons. As a result of the area is so dense, the particles wouldn’t journey far. As an alternative, they’d deposit most of their vitality domestically, which matches the noticed ionization profile fairly nicely.
Utilizing detailed simulations, we discovered that this easy course of, darkish matter particles annihilating into electrons and positrons, can naturally clarify the ionization charges noticed within the CMZ.
Even higher, the required properties of the darkish matter, akin to its mass and interplay energy, don’t battle with any identified constraints from the early universe. Darkish matter of this sort seems to be a critical possibility.
The Positron Puzzle
If darkish matter is creating positrons within the CMZ, these particles will finally decelerate and finally annihilate with electrons within the surroundings, producing gamma-rays at precisely 511-keV vitality. This would offer a direct hyperlink between the ionization and the mysterious glow.
We discovered that whereas darkish matter can clarify the ionization, it could additionally be capable to replicate some quantity of 511-keV radiation as nicely. This putting discovering means that the 2 indicators might probably originate from the identical supply, mild darkish matter.
The precise brightness of the 511-keV line depends upon a number of elements, together with how effectively positrons kind certain states with electrons and the place precisely they annihilate although. These particulars are nonetheless unsure.
A New Technique to Check the Invisible
No matter whether or not the 511-keV emission and CMZ ionization share a standard supply, the ionization charge within the CMZ is rising as a precious new remark to check darkish matter. Specifically, it gives a technique to check fashions involving mild darkish matter particles, that are troublesome to detect utilizing conventional laboratory experiments.
Transfer observations of the Milky Approach might assist check theories of darkish matter. ESO/Y. Beletsky, CC BY-SA
In our research, we confirmed that the expected ionization profile from darkish matter is remarkably flat throughout the CMZ. That is essential, as a result of the noticed ionization is certainly unfold comparatively evenly.
Level sources such because the black gap on the middle of the galaxy or cosmic ray sources like supernovas (exploding stars) can not simply clarify this. However a easily distributed darkish matter halo can.
Our findings counsel that the middle of the Milky Approach might supply new clues concerning the elementary nature of darkish matter.
Future telescopes with higher decision will be capable to present extra data on the spatial distribution and relationships between the 511-keV line and the CMZ ionization charge. In the meantime, continued observations of the CMZ might assist rule out, or strengthen, the darkish matter rationalization.
Both approach, these unusual indicators from the guts of the galaxy remind us that the universe continues to be stuffed with surprises. Typically, wanting inward, to the dynamic, glowing middle of our personal galaxy, reveals essentially the most sudden hints of what lies past.
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