‘The Empty-Sky Gamma-Ray Mystery’ -Evidence of Dark Matter?


Milky Way Gamma-Ray Sky


“Something like the diffuse glow of gamma ray background that permeates our Milky Way Galaxy could conceivably be evidence for dark matter,” researcher Mark Krumholz, a theoretical and computational astrophysicist at  Australian National University, told The Daily Galaxy.

Astrophysicists previously speculated that accretion disks around supermassive black holes, extremely active regions of star formation both within our Milky Way and in neighboring galaxies, or even annihilation of dark matter particles could be ultimately responsible for the sea of gamma-ray radiation stretching between the stars. Researchers from the Australian National University (ANU) have confirmed that star-forming galaxies are responsible for creating a significant majority of our galaxy’s gamma-ray background.

The image above shows the gamma-ray entire sky based on five years of data from the LAT instrument on NASA’s Fermi Gamma-ray Space Telescope. The bright band of diffuse glow along the map’s center marks the central plane of our Milky Way Galaxy. (NASA / DOE / Fermi LAT Collaboration).

Appear in Patches of Seemingly “Empty Sky”

Lead author Dr. Matt Roth, from the ANU Research School of Astronomy and Astrophysics, said until now it has been unclear what created gamma-rays—one of the most energetic forms of light in the Universe—that appear in patches of seemingly “empty sky.” These supposedly empty regions may actually contain very diffuse gas, which can emit gamma rays if they absorb energetic particles like cosmic rays that were originally generated elsewhere within the Milky Way or even in nearby galaxies. Alternatively, it was speculated that these empty patches of sky are truly devoid of normal matter, suggesting instead that dark matter permeates this empty space and therefore the gamma rays derive from the collision and annihilation of elusive dark matter particles. 

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A Significant Milestone

“It’s a significant milestone to finally discover the origins of this gamma-ray emission, solving a mystery of the Universe astronomers have been trying to decipher since the 1960s,” Dr. Roth said.

“There are two obvious sources that produce large amounts of gamma-rays seen in the Universe. One when gas falls into the supermassive black holes which are found at the centers of all galaxies—called an active galactic nucleus (AGN)—and the other associated with star formation in the disks of galaxies.” The third possibility, which is more exotic and still hypothetical, is that dark matter annihilation produced the gamma rays.

Generated by Star-forming Galaxies

“We modeled the gamma-ray emission from all the galaxies in the Universe and compared our results with the predictions for other sources and found that it is star-forming galaxies that produce the majority of this diffuse gamma-ray radiation and not the AGN process.”

ANU researchers were able to pinpoint what created these mysterious gamma-rays after obtaining a better understanding of how cosmic rays—particles that travel at speeds very close to the speed of light—move through the gas between the stars. Cosmic rays are important because they create large amounts of gamma-ray emission in star-forming galaxies when they collide with the interstellar gas.

Hubble and Fermi Gamma-Ray space Telescope Data

Data from NASA’s Hubble Space Telescope and Fermi Gamma-Ray Space Telescope was a key resource used to uncover the unknown origins of the gamma-rays. Researchers analyzed information about many galaxies such as their star-formation rates, total masses, physical size and distances from Earth.

“Our model can also be used to make predictions for radio emission—the electromagnetic radiation that has a frequency similar to a car radio—from star-forming galaxies, which could help researchers understand more about the internal structure of galaxies,” Dr. Roth said.

“We are currently looking at producing maps of the gamma-ray sky that can be used to inform upcoming gamma-ray observations from next-generation telescopes. This includes the Cherenkov Telescope Array, which Australia is involved in.”

“This new technology will hopefully allow us to observe many more star-forming galaxies in gamma-rays than we can detect with current gamma-ray telescopes.”

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Implications for Dark Matter

The discovery could offer clues to help astronomers solve other mysteries of the Universe, such as what kind of particles make up dark matter — one of the holy grails of astrophysics. The discovery that the gamma-ray background is dominated by star formation suggests that the annihilation of theoretical dark matter particles can only marginally contribute to the overall glow of gamma rays. Other methods and experiments will be needed to detect the still mysterious dark matter, either directly or indirectly.  

Cosmological Gamma-ray Background Could be Evidence for Dark Matter

In an email to The Daily Galaxy,  co-author Krumholz wrote : “one of the ways that people look for dark matter is by searching for gamma-ray signals of unknown origin, since, if dark matter decays or interacts, the end result is very likely to be gamma-rays. 

“However,” Krumholtz explains, “before jumping to the conclusion that a signal comes from dark matter, it is important to rule out alternative explanations. Occam’s Razor says we should try to explain things in terms of the physics we already know, before we start to think about new physics. Here we have shown that physics we know — cosmic rays accelerated by supernovae interacting with galaxies’ interstellar gas —is sufficient to explain the cosmological gamma-ray background, without appealing to new physics.

“We can also turn this argument around,” Krumholz continued, “explaining that a particular signal comes from a source other than dark matter sets limits on how bright any dark matter contribution could be. We have shown that the entire cosmological gamma-ray background can be explained purely in terms of known physics, so, at least when averaged over the whole sky, any gamma-rays that come from dark matter would have to be much dimmer than the ones produced by cosmic rays. A constraint like this can rule out some candidate theories for dark matter, and can suggest observational strategies to test other theories.”

Source: Matt A. Roth et al, The diffuse γ-ray background is dominated by star-forming galaxies, Nature (2021). DOI: 10.1038/s41586-021-03802-x Matt A. Roth et al, The diffuse γ-ray background is dominated by star-forming galaxies, Nature (2021). DOI: 10.1038/s41586-021-

The Daily Galaxy, Maxwell Moe, astrophysicist, NASA Einstein Fellow, University of Arizona via Mark KrumholzNature and ANU



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