The center of the Milky Way glows. Yes, there’s a big chonkin ‘black hole, and it’s a very energetic region, but there’s an extra high energy, gamma ray glow, above the activity we know about, and it’s something that still to be explained.
This glow is called the Galactic Center GeV Excess (GCE), and astronomers have been trying to figure it out for years. One much-discussed explanation is that the glow could theoretically be produced by the destruction of dark matter – but new research is a nail in the coffin of that idea.
In a series of extended models that include recent developments in simulating the galactic hump and other sources of gamma-ray emission in the galactic center, a team of astrophysicists has eradicated dark matter destruction as the source of the glow.
This finding, says the team, gives dark matter less space to hide – place stronger restrictions on their properties that may help in future searches.
“For 40 years or so, the leading candidate for dark matter among particle physicists has been a thermal, weakly interacting, and low-scale particle,” said astrophysicist Kevork Abazajian of the University of California Irvine (UCI).
“This result excludes for the first time that candidate to whole-mass particles.”
The GCE was first spotted in a little over ten years, when the Fermi Gamma-ray Space Telescope began exploring the region. Gamma rays are the highest energy electromagnetic waves in the Universe, and they are produced by the heaviest objects, such as millisecond pulsars, neutron stars, colliding neutron stars, black holes and supernovae.
The problem was, when it came time to analyze Fermi’s observations, after all known gamma ray sources had been extracted, we ended up with a gamma ray glow in the heart of the Milky Way that could not be counted.
In space, if you find something that cannot be calculated, it is logical to try to adapt it to other things that cannot be calculated – such as dark matter. This is the name we give to the invisible mass that adds gravity to the Universe.
We can detect dark matter indirectly, because things move differently than they should if only the visible goods had an effect, but we do not know what they really are.
Although we cannot detect dark matter directly, it is possible that it produces radiation that we can see.
If types of dark matter particles Weakly Interacting Massive Particles, like WIMPs, collide with each other – like the collisions in particle accelerators – they would destroy each other, explode in a shower of other particles, including gamma ray photons. Such collisions have been proposed as a potential mechanism that the GCE produces.
However, several studies have found no evidence of WIMP collisions, but this new paper is a step up, the authors say.
“In many models, this particle varies from 10 to 1,000 times the mass of a proton, with more massive particles theoretically less attractive than a particle of dark matter,” said UCI astrophysicist Manoj Kaplinghat.
“In this paper, we eliminate candidates for dark matter over the preferred range, which is a huge improvement in the constraints we place on the possibilities that these are representative of dark matter.”
For more than three years, the team drew a wide range of scenarios for gamma-ray models for the galactic center and its hump – the tightly packed group of stars concentrated around the center. They included all the resources they could get their hands on – star formation, the Fermi bubbles, cosmic ray interactions with molecular gas, and neutron stars and millisecond pulsars.
They found that once they had everything in mind, there was very little room for WIMP destruction. Among all these gamma-ray sources, “there is no significant surplus in the GC that can be attributed to DM destruction,” the researchers write in their paper.
Previous research has shown that the distribution of gamma rays in the galactic center is also inconsistent with the destruction of dark matter. If WIMPs were to blame, the distribution would be smooth – but instead the gamma ray photons are divided into clumps as you would expect to find from point sources, such as stars.
The distribution of stars in the mound, according to the new study, is also inconsistent with the presence of extra dark matter.
That is not to say that the dark matter in our galactic center could not be a hypothetical, massive, weakly interactive type. The team has just managed those of a mass that is often sought after, and then some. The team notes that their findings still strongly suggest an astrophysical origin for the GCE.
“Our study limits the type of part that dark matter can be,” Kaplinghat said. “The multiple lines of evidence for dark matter in the galaxy are robust and unaffected by our work.”
Which means we just have to think a lot further outside the box to find it.
The study was published in Physical assessment D.
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