Detection of gamma rays from a newly discovered dwarf galaxy may point to dark matter

March 10, 2015 by Kevin Stacey, Brown University
Scientists at Brown, Carnegie Mellon, and Cambridge universities have detected gamma ray emissions from the direction of the galaxy Reticulum 2. Bright areas indicate a strong gamma ray signal coming from the direction of the galaxy, according to the researchers’ search algorithm. Credit: NASA/DOE/Fermi-LAT Collaboration/Geringer-Sameth & Walker/Carnegie Mellon University/Koushiappas/Brown University

A newly discovered dwarf galaxy orbiting our own Milky Way has offered up a surprise—it appears to be radiating gamma rays, according to an analysis by physicists at Carnegie Mellon, Brown, and Cambridge universities. The exact source of this high-energy light is uncertain at this point, but it just might be a signal of dark matter lurking at the galaxy's center.

"Something in the direction of this dwarf galaxy is emitting ," said Alex Geringer-Sameth, a postdoctoral research associate in CMU's Department of Physics and the paper's lead author. "There's no conventional reason this galaxy should be giving off gamma rays, so it's potentially a signal for ."

The galaxy, named Reticulum 2, was discovered within the last few weeks in the data of the Dark Energy Survey, an experiment that maps the southern sky to understand the accelerated expansion of the universe. At approximately 98,000 light-years from Earth, Reticulum 2 is one of the nearest dwarf galaxies yet detected. Using publicly available data from NASA's Fermi Gamma-ray Space Telescope, CMU's Geringer-Sameth and Matthew Walker and Brown's Savvas Koushiappas have shown gamma rays coming from the direction of the galaxy in excess of what would be expected from normal background.

"In the search for dark matter, gamma rays from a dwarf galaxy have long been considered a very strong signature," said Koushiappas, assistant professor of physics at Brown. "It seems like we may now be detecting such a thing for the first time."

The researchers have submitted their analysis to the journal Physical Review Letters and posted it on arXiv. They caution that while these preliminary results are exciting, there is more work to be done to confirm a dark-matter origin.

Gamma rays and dark matter

No one knows exactly what dark matter is, but it is thought to account for around 80 percent of the matter in the universe. Scientists know that dark matter exists because it exerts gravitational effects on , which explains the observed rotation of galaxies and galaxy clusters as well as fluctuations in the cosmic microwave background.

"The gravitational detection of dark matter tells you very little about the particle behavior of the dark matter," said Matthew Walker, assistant professor of physics and a member of CMU's McWilliams Center for Cosmology. "But now we may have a non-gravitational detection that shows dark matter behaving like a particle, which is a holy grail of sorts."

A leading theory suggests that dark matter particles are WIMPs—Weakly Interacting Massive Particles. When pairs of WIMPs meet, they annihilate one another, giving off . If that's true, then there should be a lot of gamma rays emanating from places where WIMPs are thought to be plentiful, like the dense centers of galaxies. The trouble is, the high-energy rays also originate from many other sources, including black holes and pulsars, which makes it difficult to untangle a dark matter signal from the background noise.

That's why dwarf galaxies are important in the hunt for the . Dwarfs are thought to lack other gamma-ray-producing sources, so a gamma ray flux from a dwarf galaxy would make a very strong case for dark matter.

"They're basically very clean and quiet systems," Koushiappas said.

Scientists have been looking at them for signs of gamma rays for the last several years using NASA's Fermi Gamma-ray Space Telescope. There's never been a convincing signal until now.

Over the last few years Geringer-Sameth, Koushiappas, and Walker have been developing an analysis technique that searches for weak signals in the gamma ray data that could be due to . With the discovery of Reticulum 2, Geringer-Sameth turned his attention to that part of the sky. He looked at all of the gamma rays coming from the direction of the dwarf galaxy as well as gamma rays coming from adjacent areas of the sky to provide a background level.

"There did seem to be an excess of gamma rays, above what you would expect from normal background processes, coming from the direction of this galaxy," Geringer-Sameth said. "Given the way that we think we understand how gamma rays are generated in this region of the sky, it doesn't seem that those processes can explain this signal."

Further study of this 's attributes could reveal hidden sources that may be emitting gamma rays, but the researchers are cautiously optimistic.

"The fact that there are gamma rays and also a clump of dark matter in the same direction makes it quite interesting," Walker said.

Explore further: Bubbles from the galactic center: A key to understanding dark matter and our galaxy's past?

More information: Evidence for Gamma-ray Emission from the Newly Discovered Dwarf Galaxy Reticulum 2, arxiv.org/abs/1503.02320

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13 comments

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AGreatWhopper
2.6 / 5 (15) Mar 10, 2015
. "There's no conventional reason this galaxy should be giving off gamma rays, so it's potentially a signal for dark matter."

And we have this century's winner for most logical fallacies in one statement.
Captain Stumpy
3.8 / 5 (13) Mar 10, 2015
And we have this century's winner for most logical fallacies in one statement.
@whopper
perhaps you misread... try again
read this part
so it's potentially a signal


yyz
4.2 / 5 (10) Mar 10, 2015
Having only skimmed the paper, I find their motivations for studying this newly discovered dwarf galaxy compelling. Reticulum 2(Ret 2) at a distance of 30 kpc is closer to the Milky Way than any other dwarf galaxy, aside from Segue 1 (23 kpc) and the Sagittarius Dwarf Galaxy (24 kpc). Ret 2 is also ~3 times more luminous than Segue 1, potentially pointing to a larger DM content.

However (and this goes to the need for confirmation), a second independent study of these newly discovered dwarf galaxies using Fermi-LAT data found only a small, insignificant excess of gamma rays from Ret 2 (aka DES J03335.6-5403): http://arxiv.org/...03.02632

For the time being, I'm just happily concentrating on the discovery of these new dwarf galaxies:

http://arxiv.org/...03.02584

http://arxiv.org/...03.02079
Anakin
2.3 / 5 (3) Mar 10, 2015
Having only skimmed the paper, I find their motivations for studying this newly discovered dwarf galaxy compelling. Reticulum 2(Ret 2) at a distance of 30 kpc is closer to the Milky Way than any other dwarf galaxy, aside from Segue 1 (23 kpc) and the Sagittarius Dwarf Galaxy (24 kpc). Ret 2 is also ~3 times more luminous than Segue 1, potentially pointing to a larger DM content.

However (and this goes to the need for confirmation), a second independent study of these newly discovered dwarf galaxies using Fermi-LAT data found only a small, insignificant excess of gamma rays from Ret 2 (aka DES J03335.6-5403): http://arxiv.org/...03.02632


I stopped reading when the name Reticulum 2 came in view.
It was to close to the ridicule spell in Hairy Pothead
Benni
1.6 / 5 (7) Mar 11, 2015
"Scientists know that dark matter exists because it exerts gravitational effects on visible matter, which explains the observed rotation of galaxies "

Correction: The DM gravitational effect is only needed to explain "observed rotational effects" of Spiral galaxies which rotate in excess of 200 km/s. The majority of galaxies in the universe are Elliptical galaxies which typically rotate at 2 km/s & do not need the funny farm science explanation of DM to explain what keeps them together, Newtonian gravity works just fine.

Most of the mass of the universe is located inside Elliptical galaxies many of which are 50 times the size of the Milky Way.

Ellipticals are the sources of 99.99999999% of gravitational lensing due to the high gravitational fields associated with these galaxy types, this as compared to the observationally lack of lensing typically associated with Spiral types due to their much lower gravity fields as observed by lack of lensing by these galaxy types.

antialias_physorg
4.4 / 5 (7) Mar 11, 2015
Not if dark matter is in fact antimatter.

Antimatter interacts with light. From it you get the same absorption lines as 'normal' matter. Also antimatter gas heats the same as normal matter which would make large antimatter concentrations luminous.This is not observed. Dark matter isn't antimatter.
Tuxford
2 / 5 (4) Mar 11, 2015
If it is a galaxy, then it almost always have a extra-massive core star generating most of the new matter growing the galaxy, even for smaller dwarf's. And thus, there is no need to look for fabled dark sources for the production of gamma rays. Think conventional explanation first, instead of fantasy first. Otherwise, go work for Disney, where fantasy is valued.
SpiffyKavu
5 / 5 (1) Mar 11, 2015
Anti-matter doesn't necessarily interact with light (anti-neutrino, as an example). In most models, dark matter would be its own anti-particle, so if a DM particle interacts with another DM particle, then they are transformed into something else -- that something else is typically NOT light, but some other "standard" particle (like a bottom quark), which then can interact with light. DM does not directly interact with light, only very indirectly in these models.

Additionally, if DM = anti-matter, then we'd have to explain the factor of ~5 discrepancy between matter and DM. A small asymmetry between particle--anti-particle creation may generally result in a much larger difference, as annihilations generally proceed until nothing else could annihilate, leaving almost zero anti-matter and a bunch of regular matter.
PhysicsAndrea
4.5 / 5 (2) Mar 11, 2015
What about the paper from the Large Area Telescope and Dark Energy Survey Collaborations that did the follow up analysis with the more sensitive "Pass 8" data mentioned in Geringer-Sameth and do not see a statistically significance signal from this object? arXiv 1503.02632

This result came out on the same day as the Geringer-Sameth paper
SpiffyKavu
1 / 5 (1) Mar 11, 2015
I do really like the matter--anti-matter bubble thing.

Empty portions of the universe are not really empty, and the steady-state configuration of an anti-gravity medium would be uniform density (expanding). There would always be a background annihilation rate, since the gravitational force between individual particles is negligible. Is this the source of the isotropic gamma-ray emission? The spectrum does not really allow for proton--anti-proton or electron-positron, etc. collisions alone.

How would it affect the gravitational lensing effect? A uniform density medium would not affect it, except perhaps via an expansion of space. But we see an excess of lensing clumped by galaxy clusters. And from structure formation simulations. So I like the idea of bubbles, as it creates for a simulatable scenario which may say something about the matter--anti-matter asymmetry, but I don't think it can contribute to the dark matter effect.
SpiffyKavu
1 / 5 (1) Mar 11, 2015
PhysicsAndrea: that is why this article describes ... dubious results. I remain skeptical. Sure, this ultra-faint dwarf (UFD) may be an interesting source of DM, but the Fermi team has been looking (as you pointed out with the paper), and failed to be convinced.

Additionally, the energy spectrum of this Reticulum 2 UFD does NOT match the spectrum of the gamma-ray signal coming from the center of our Galaxy. Therefore, they cannot both be interpreted with the same DM model. I like simplicity, and the explanation is that one result is incorrect, incorrectly interpreted, or neither result will hold up to a DM hypothesis.

Just like the 120 GeV line in the Fermi data which disappeared ...
IMP-9
4 / 5 (4) Mar 11, 2015
The DM gravitational effect is only needed to explain "observed rotational effects" of Spiral galaxies which rotate in excess of 200 km/s. The majority of galaxies in the universe are Elliptical galaxies which typically rotate at 2 km/s


No. Firstly dark matter is supported not only by that but by strong and weak lensing, cluster dynamics, x-ray cluster measurements and cosmology. Elipticals too require dark matter. Yes you can find ellipticals which have no bulk rotation but most (according to KMOS 3D) are rotationally supported as opposed to dispersion supported. The overall bulk rotation will be small but the velocity dispersion is what you need to look at in dispersion supported cases. Bulk velocity profile is not the same as the orbital velocity. Many elliptical have rotation velocities of hundreds of kilometers per second and like spirals, show flat rotation curves.
pepe2907
5 / 5 (1) Mar 14, 2015
reset says:
If 2 particles of DM annihilate to produce gamma rays they would have to have a traceable energy level prior to annihilation...


Yes reset, you are right, they should.
But the thing is - you are not there to check for any possible form of energy, or more precisely - any known to man form of energy.
And because you are not there you need something to carry information for existence of that energy from there to you. And the most popular, most used by far, actually almost exclusive way is that information to be carried by electromagnetic waves.
But the thing with DM is - it's DM because it does not participate in electromagnetic interaction, it does not receive electromagnetic energy, it does not radiate it. It's why it's called Dark.
Just the same way as electrons for example don't participate in strong interaction. Do you have any problems with accepting the fact that electrons don't participate in strong int.?

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