Astronomers have lengthy been puzzled by two unusual phenomena on the coronary heart of our galaxy. First, the fuel within the central molecular zone (CMZ), a dense and chaotic area close to the Milky Manner’s core, seems to be ionized (that means it’s electrically charged as a result of it has misplaced electrons) at a surprisingly excessive fee.
Second, telescopes have detected a mysterious glow of gamma rays with an power of 511 kilo-electronvolts (keV) (which corresponds to the power of an electron at relaxation).
Curiously, such gamma rays are produced when an electron and its antimatter counterpart—all basic charged particles have antimatter variations of themselves which are 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, printed in Bodily Evaluate Letters, my colleagues and I present that each could possibly be linked to one of the elusive elements within the universe: darkish matter. Particularly, we suggest {that a} new type of darkish matter, much less huge than the categories astronomers sometimes search for, could possibly be the wrongdoer.
Hidden Course of
The CMZ spans virtually 700 mild years and incorporates among the most dense molecular fuel within the galaxy. Through the years, scientists have discovered that this area is unusually ionized, that means the hydrogen molecules there are being break up into charged particles (electrons and nuclei) at a a lot sooner fee than anticipated.
This could possibly be the results of sources resembling cosmic rays and starlight that bombard the fuel. Nevertheless, 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 Nineteen Seventies, however nonetheless has no clearly recognized supply. A number of candidates have been proposed, together with supernovas, huge stars, black holes, and neutron stars. Nevertheless, none totally clarify the sample or depth of the emission.
We requested a easy query: May each phenomena be attributable to the identical hidden course of?
Darkish matter makes up round 85 p.c of the matter within the universe, however it doesn’t emit or take up mild. Whereas its gravitational results are clear, scientists don’t but know what it’s fabricated from.
One risk, usually ignored, is that darkish matter particles could possibly be very mild, with plenty only a few million electronvolts, far lighter than a proton, and nonetheless play a cosmic function. These mild darkish matter candidates are usually known 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 contact with their very own antiparticles within the galactic middle and annihilate one another, producing electrons and positrons.
Within the dense fuel of the CMZ, these low-energy particles would rapidly lose power 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 power domestically, which matches the noticed ionization profile fairly properly.
Utilizing detailed simulations, we discovered that this straightforward 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, resembling its mass and interplay energy, don’t battle with any recognized constraints from the early universe. Darkish matter of this sort seems to be a severe possibility.
The Positron Puzzle
If darkish matter is creating positrons within the CMZ, these particles will ultimately decelerate and ultimately annihilate with electrons within the setting, producing gamma-rays at precisely 511-keV power. This would supply 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 properly. This putting discovering means that the 2 alerts might doubtlessly originate from the identical supply, mild darkish matter.
The precise brightness of the 511-keV line relies on a number of elements, together with how effectively positrons type sure states with electrons and the place precisely they annihilate although. These particulars are nonetheless unsure.
A New Strategy to Check the Invisible
No matter whether or not the 511-keV emission and CMZ ionization share a typical supply, the ionization fee within the CMZ is rising as a priceless new remark to review darkish matter. Particularly, it offers a strategy to check fashions involving mild darkish matter particles, that are troublesome to detect utilizing conventional laboratory experiments.
Transfer observations of the Milky Manner 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 necessary, 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 Manner might supply new clues concerning the basic nature of darkish matter.
Future telescopes with higher decision will be capable to present extra info on the spatial distribution and relationships between the 511-keV line and the CMZ ionization fee. In the meantime, continued observations of the CMZ might assist rule out, or strengthen, the darkish matter clarification.
Both manner, these unusual alerts from the center of the galaxy remind us that the universe continues to be filled with surprises. Typically, wanting inward, to the dynamic, glowing middle of our personal galaxy, reveals probably the most surprising hints of what lies past.
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