NASA's Fermi mission expands its search for dark matter

August 12, 2016, NASA's Goddard Space Flight Center
Top: Gamma rays (magenta lines) coming from a bright source like NGC 1275 in the Perseus galaxy cluster should form a particular type of spectrum (right). Bottom: Gamma rays convert into hypothetical axion-like particles (green dashes) and back again when they encounter magnetic fields (gray curves). The resulting gamma-ray spectrum ((lower curve at right) would show unusual steps and gaps not seen in Fermi data, which means a range of these particles cannot make up a portion of dark matter. Credit: SLAC National Accelerator Laboratory/Chris Smith

Dark matter, the mysterious substance that constitutes most of the material universe, remains as elusive as ever. Although experiments on the ground and in space have yet to find a trace of dark matter, the results are helping scientists rule out some of the many theoretical possibilities. Three studies published earlier this year, using six or more years of data from NASA's Fermi Gamma-ray Space Telescope, have broadened the mission's dark matter hunt using some novel approaches.

"We've looked for the usual suspects in the usual places and found no solid signals, so we've started searching in some creative new ways," said Julie McEnery, Fermi project scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "With these results, Fermi has excluded more candidates, has shown that dark matter can contribute to only a small part of the gamma-ray background beyond our galaxy, the Milky Way, and has produced strong limits for dark matter particles in the second-largest galaxy orbiting it."

Dark matter neither emits nor absorbs light, primarily interacts with the rest of the universe through gravity, yet accounts for about 80 percent of the matter in the universe. Astronomers see its effects throughout the cosmos—in the rotation of galaxies, in the distortion of light passing through galaxy clusters, and in simulations of the early universe, which require the presence of dark matter to form galaxies at all.

The leading candidates for dark matter are different classes of . Scientists think , the highest-energy form of light, can help reveal the presence of some of types of proposed dark matter particles. Previously, Fermi has searched for tell-tale gamma-ray signals associated with dark matter in the center of our galaxy and in small dwarf galaxies orbiting our own. Although no convincing signals were found, these results eliminated candidates within a specific range of masses and interaction rates, further limiting the possible characteristics of dark matter particles.

Among the new studies, the most exotic scenario investigated was the possibility that dark matter might consist of hypothetical particles called axions or other particles with similar properties. An intriguing aspect of axion-like particles is their ability to convert into gamma rays and back again when they interact with strong magnetic fields. These conversions would leave behind characteristic traces, like gaps or steps, in the spectrum of a bright gamma-ray source.

The Small Magellanic Cloud (SMC), at center, is the second-largest satellite galaxy orbiting our own. This image superimposes a photograph of the SMC with one half of a model of its dark matter (right of center). Lighter colors indicate greater density and show a strong concentration toward the galaxy's center. Ninety-five percent of the dark matter is contained within a circle tracing the outer edge of the model shown. In six years of data, Fermi finds no indication of gamma rays from the SMC's dark matter. Credit: Dark matter, R. Caputo et al. 2016; background, Axel Mellinger, Central Michigan University

Manuel Meyer at Stockholm University led a study to search for these effects in the gamma rays from NGC 1275, the central galaxy of the Perseus galaxy cluster, located about 240 million light-years away. High-energy emissions from NGC 1275 are thought to be associated with a supermassive black hole at its center. Like all galaxy clusters, the Perseus cluster is filled with hot gas threaded with magnetic fields, which would enable the switch between gamma rays and axion-like particles. This means some of the gamma rays coming from NGC 1275 could convert into axions—and potentially back again—as they make their way to us.

Meyer's team collected observations from Fermi's Large Area Telescope (LAT) and searched for predicted distortions in the gamma-ray signal. The findings, published April 20 in Physical Review Letters, exclude a small range of axion-like particles that could have comprised about 4 percent of dark matter.

"While we don't yet know what dark matter is, our results show we can probe axion-like models and provide the strongest constraints to date for certain masses," Meyer said. "Remarkably, we reached a sensitivity we thought would only be possible in a dedicated laboratory experiment, which is quite a testament to Fermi."

Another broad class of are called Weakly Interacting Massive Particles (WIMPs). In some versions, colliding WIMPs either mutually annihilate or produce an intermediate, quickly decaying particle. Both scenarios result in gamma rays that can be detected by the LAT.

Regina Caputo at the University of California, Santa Cruz, sought these signals from the Small Magellanic Cloud (SMC), which is located about 200,000 light-years away and is the second-largest of the small satellite galaxies orbiting the Milky Way. Part of the SMC's appeal for a dark matter search is that it lies comparatively close to us and its gamma-ray emission from conventional sources, like star formation and pulsars, is well understood. Most importantly, astronomers have high-precision measurements of the SMC's rotation curve, which shows how its rotational speed changes with distance from its center and indicates how much dark matter is present. In a paper published in Physical Review D on March 22, Caputo and her colleagues modeled the dark matter content of the SMC, showing it possessed enough to produce detectable signals for two WIMP types.

This animation switches between two images of the gamma-ray sky as seen by Fermi's Large Area Telescope (LAT), one using the first three months of LAT data, the other showing a cumulative exposure of seven years. The blue color, representing the fewest gamma rays, includes the extragalactic gamma-ray background. Blazars make up most of the bright sources shown (colored red to white). With increasing exposure, Fermi reveals more of them. A new study shows blazars are almost completely responsible for the background glow. Credit: NASA/DOE/Fermi LAT Collaboration
"The LAT definitely sees gamma rays from the SMC, but we can explain them all through conventional sources," Caputo said. "No signal from dark matter annihilation was found to be statistically significant."

In the third study, researchers led by Marco Ajello at Clemson University in South Carolina and Mattia Di Mauro at SLAC National Accelerator Laboratory in California took the search in a different direction. Instead of looking at specific astronomical targets, the team used more than 6.5 years of LAT data to analyze the background glow of gamma rays seen all over the sky.

The nature of this light, called the extragalactic gamma-ray background (EGB) has been debated since it was first measured by NASA's Small Astronomy Satellite 2 in the early 1970s. Fermi has shown that much of this light arises from unresolved gamma-ray sources, particularly galaxies called blazars, which are powered by material falling toward gigantic black holes. Blazars constitute more than half of the total gamma-ray sources seen by Fermi, and they make up an even greater share in a new LAT catalog of the highest-energy gamma rays.

Some models predict that EGB gamma rays could arise from distant interactions of , such as the annihilation or decay of WIMPs. In a detailed analysis of high-energy EGB gamma rays, published April 14 in Physical Review Letters, Ajello and his team show that blazars and other discrete sources can account for nearly all of this emission.

"There is very little room left for signals from exotic sources in the extragalactic gamma-ray background, which in turn means that any contribution from these sources must be quite small," Ajello said. "This information may help us place limits on how often WIMP particles collide or decay."

Although these latest studies have come up empty-handed, the quest to find continues both in space and in ground-based experiments. Fermi is joined in its search by NASA's Alpha Magnetic Spectrometer, a particle detector on the International Space Station.

Explore further: Dark matter does not contain certain axion-like particles

More information: M. Ajello et al, Search for Spectral Irregularities due to Photon–Axionlike-Particle Oscillations with the Fermi Large Area Telescope, Physical Review Letters (2016). DOI: 10.1103/PhysRevLett.116.161101

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1 / 5 (7) Aug 12, 2016
There is evidence of the dark matter every time a double slit experiment is performed, it's what waves.
1.5 / 5 (8) Aug 12, 2016
Dark matter, the mysterious substance that constitutes most of the material universe, remains as elusive as ever.

"We've looked for the usual suspects in the usual places and found no solid signals, so we've started searching in some creative new ways," said Julie McEnery

...............this is just so easy, why even take the bother to Comment on where zany Zwicky's ideas have taken this field of pseudo-science.
4 / 5 (8) Aug 12, 2016
Zwicky was a pioneer in the field. But nowadays, when anyone can check the existence of dark matter for themselves in the CB spectra that Planck collected, it is robust knowledge. [ https://galileosp...niverse/ ]

The outstanding question is dark matter properties, besides being particulate, massive and little interacting with EM as its name suggests. That is what Fermi has made inroads in by rejecting some of the possibilities.
1 / 5 (6) Aug 12, 2016
Zwicky was a pioneer in the field. But nowadays, when anyone can check the existence of dark matter for themselves in the CB spectra that Planck collected, it is robust knowledge.

Oh please torb, stop doing the twist. For over 80 years you Zwickyites have been making promises to us that DM discovery is just over the next horizon, and everytime we get there what do we see? Yeah, the usual UNMISSING 100% VM. Get a job as an Astrologer, they have better odds at getting prognostications right.
1.7 / 5 (6) Aug 12, 2016
I prefer astrologers over nasty frauds like you.
They are not vindictive little narcissists in general.

..........compared to 80 years of Zwicky's fraud? I think he died before I was even born, but no doubt you knew him, you're probably a relative of his.
Captain Stumpy
4.2 / 5 (5) Aug 13, 2016
80 years of Zwicky's fraud?
so you published a peer reviewed journal study demonstrating that not only did he defraud the scientific community but you've demonstrated everything he published is wrong because you have evidence????


where is the link to that study & why aren't you holding a nobel?
[satirical sarcasm and hyperbole]


you're making a claim that is directly contradictory of the evidence which is typically called a "false claim" ( http://www.auburn...ion.html ) but is colloquially known as a "baldfaced intentional lie"

evidence or STFU troll girl!
Captain Stumpy
4.2 / 5 (5) Aug 13, 2016
You do not get a Nobel prize for proving something or someone wrong
and there is no prize for Vindictive Narcissism.
yeah, i know that but the idiot trolling benji doesn't...

besides, i did put the following after my comment:
[satirical sarcasm and hyperbole]
it is so that literate folk know that i'm being satirical, sarcastic and hyperbolic ...


what benji is (and all the other trolls are) thinking is this:
if she actually proves something that refutes a longstanding validated claim in science, then tons of teenage groupies, cash, fame, job-offers for doing nothing but looking pretty and an instant Nobel prize will be waiting for them when they step out the house...

of course, that is like winning the lotto every week without playing the actual lotto

... and considering we've proven she isn't literate in math, science, engineering, nukes or astrophysics/astronomy (the lexicon OR the reality) or science in general...

1.7 / 5 (6) Aug 13, 2016
80 years of Zwicky's fraud?

so you published a peer reviewed journal study demonstrating that not only did he defraud the scientific community but you've demonstrated everything he published is wrong because you have evidence?

...........his TIRED LIGHT THEORY followed up soon thereafter with his DARK MATTER theory of Cosmic Fairy Dust. What manner of Astro-physics aficionado are you that you've never heard of these frauds?

Aug 13, 2016
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Aug 13, 2016
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