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 Method’s core, seems to be ionised (which 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 elementary charged particles have antimatter variations of themselves which are 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 many years of commentary. However in a brand new research, revealed in Bodily Evaluation Letters, we present that each may very well be linked to one of the vital elusive components within the universe: darkish matter. Particularly, we suggest {that a} new type of darkish matter, much less huge than the kinds astronomers sometimes search for, may very well be the offender.
Hidden course of
The CMZ spans nearly 700 gentle years and incorporates among the most dense molecular fuel within the galaxy. Over time, scientists have discovered that this area is unusually ionised, which means the hydrogen molecules there are being break up into charged particles (electrons and nuclei) at a a lot sooner fee than anticipated.
This may very well be the results of sources corresponding to cosmic rays and star gentle that bombard the fuel. Nonetheless, these alone don’t appear to have the ability to account for the noticed ranges.
The opposite thriller, the 511keV 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. Nonetheless, none absolutely 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% of the matter within the universe, but it surely doesn’t emit or soak up gentle. Whereas its gravitational results are clear, scientists don’t but know what it’s manufactured from.
One risk, typically ignored, is that darkish matter particles may very well be very gentle, with plenty only a few million electronvolts, far lighter than a proton, and nonetheless play a cosmic position. These gentle 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 gentle darkish matter particles are available contact with their very own antiparticles within the galactic centre and annihilate one another, producing electrons and positrons.
Within the dense fuel of the CMZ, these low-energy particles would shortly lose power and ionise 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 a substitute, they’d deposit most of their power regionally, which matches the noticed ionisation profile fairly effectively.
Utilizing detailed simulations, we discovered that this straightforward course of, darkish matter particles annihilating into electrons and positrons, can naturally clarify the ionisation charges noticed within the CMZ.
Even higher, the required properties of the darkish matter, corresponding to its mass and interplay power, don’t battle with any recognized constraints from the early universe. Darkish matter of this sort 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 setting, producing gamma-rays at precisely 511keV power. This would offer a direct hyperlink between the ionisation and the mysterious glow.
We discovered that whereas darkish matter can clarify the ionisation, it could additionally be capable of replicate some quantity of 511keV radiation as effectively. This putting discovering means that the 2 alerts might doubtlessly originate from the identical supply, gentle darkish matter.
The precise brightness of the 511keV 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 brand new approach to take a look at the invisible
No matter whether or not the 511keV emission and the CMZ ionisation share a typical supply, the ionisation fee within the CMZ is rising as a worthwhile new commentary to review darkish matter. Particularly, it offers a approach to take a look at fashions involving gentle darkish matter particles, that are troublesome to detect utilizing conventional laboratory experiments.
ESO/Y. Beletsky, CC BY-SA
In our research, we confirmed that the expected ionisation profile from darkish matter is remarkably flat throughout the CMZ. That is essential, as a result of the noticed ionisation is certainly unfold comparatively evenly.
Level sources such because the black gap on the centre of the galaxy or cosmic ray sources like supernovas (exploding stars) can’t simply clarify this. However a easily distributed darkish matter halo can.
Our findings recommend that the centre of the Milky Method might provide new clues in regards to the elementary nature of darkish matter.
Future telescopes with higher decision will be capable of present extra info on the spatial distribution and relationships between the 511 keV line and the CMZ ionisation fee. 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 guts of the galaxy remind us that the universe continues to be stuffed with surprises. Generally, trying inward, to the dynamic, glowing centre of our personal galaxy, reveals essentially the most sudden hints of what lies past.
