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 Method’s core, seems to be ionized (which means it’s electrically charged as a result of it has misplaced electrons) at a surprisingly excessive price.
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).
Apparently, such gamma rays are produced when an electron and its antimatter counterpart—all elementary charged particles have antimatter variations of themselves which might be close to similar, 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 commentary. However in a brand new examine, printed in Bodily Evaluate Letters, my colleagues and I present that each might be linked to probably the most elusive elements within the universe: darkish matter. Particularly, we suggest {that a} new type of darkish matter, much less large than the kinds astronomers usually search for, might be the offender.
Hidden Course of
The CMZ spans virtually 700 gentle years and comprises among the most dense molecular gasoline within the galaxy. Through the years, scientists have discovered that this area is unusually ionized, which means the hydrogen molecules there are being break up into charged particles (electrons and nuclei) at a a lot quicker price than anticipated.
This might be the results of sources similar 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 totally 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, nevertheless it doesn’t emit or take up gentle. Whereas its gravitational results are clear, scientists don’t but know what it’s manufactured from.
One risk, typically missed, is that darkish matter particles might be very gentle, with lots just some million electronvolts, far lighter than a proton, and nonetheless play a cosmic position. These gentle darkish matter candidates are usually 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 gentle darkish matter particles are available in contact with their very own antiparticles within the galactic heart 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 regionally, which matches the noticed ionization profile fairly effectively.
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, similar to its mass and interplay power, don’t battle with any recognized constraints from the early universe. Darkish matter of this type seems to be a severe choice.
The Positron Puzzle
If darkish matter is creating positrons within the CMZ, these particles will ultimately decelerate and ultimately 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 have the ability to replicate some quantity of 511-keV radiation as effectively. This placing discovering means that the 2 alerts might probably originate from the identical supply, gentle darkish matter.
The precise brightness of the 511-keV line is determined by a number of elements, together with how effectively positrons kind sure states with electrons and the place precisely they annihilate although. These particulars are nonetheless unsure.
A New Option to Check the Invisible
No matter whether or not the 511-keV emission and CMZ ionization share a typical supply, the ionization price within the CMZ is rising as a worthwhile new commentary to review darkish matter. Particularly, it offers a option to take a look at fashions involving gentle darkish matter particles, that are tough to detect utilizing conventional laboratory experiments.
Transfer observations of the Milky Method may assist take a look at theories of darkish matter. ESO/Y. Beletsky, CC BY-SA
In our examine, we confirmed that the expected ionization profile from darkish matter is remarkably flat throughout the CMZ. That is vital, as a result of the noticed ionization is certainly unfold comparatively evenly.
Level sources such because the black gap on the heart 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 Method might provide new clues concerning the elementary nature of darkish matter.
Future telescopes with higher decision will have the ability to present extra data on the spatial distribution and relationships between the 511-keV line and the CMZ ionization price. In the meantime, continued observations of the CMZ might assist rule out, or strengthen, the darkish matter clarification.
Both approach, these unusual alerts from the center of the galaxy remind us that the universe remains to be stuffed with surprises. Typically, trying inward, to the dynamic, glowing heart of our personal galaxy, reveals probably the most sudden hints of what lies past.
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