Gamma rays — the most energetic waves in the electromagnetic spectrum — are typically produced by the hottest, most extreme objects in the Universe, such as supernovae and pulsars. There is a glow of gamma rays at the center of our Milky Way Galaxy — known as the Galactic center gamma-ray excess (GCE) — with properties that are difficult for astrophysicists to explain given what they know about the distribution of stars and gas in the Galaxy. There are two leading possibilities for what may be producing this radiation: a population of pulsars, or, more enticingly, a cloud of dark matter, colliding with itself to produce a glut of gamma rays. In 2015, a team of researchers from MIT and Princeton University determined that the GCE likely came from sources like pulsars. Now, a new analysis shows that dark matter — and not pulsars — could be responsible for the GCE.
Milky Way’s center produces more high-energy gamma rays than can be explained by known sources. This image shows the Milky Way in visible light and a gamma-ray map from NASA’s Fermi Gamma-ray Space Telescope. Image credit: NASA’s Goddard Space Flight Center / A. Mellinger, Central Michigan University / T. Linden, University of Chicago.
While the Milky Way more or less resembles a flat disk in space, the GCE occupies a more spherical region, extending about 5,000 light-years in every direction from the Galactic center.
In the 2015 study, MIT’s Dr. Tracy Slatyer and colleagues developed a method to determine whether the profile of this spherical region is smooth or ‘grainy.’
The team reasoned that, if pulsars are the source of the GCE, and these pulsars are relatively bright, the gamma rays they emit should inhabit a spherical region that, when imaged, looks grainy, with dark gaps between the bright spots where the pulsars sit. If, however, dark matter is the source of the gamma ray excess, the spherical region should look smooth.
“We saw it was 100% grainy, and so we said, ‘oh, dark matter can’t do that, so it must be something else’,” Dr. Slatyer said.
“My hope was that this would be just the first of many studies of the Galactic center region using similar techniques. But by 2018, the main cross-checks of the method were still the ones we’d done in 2015, which made me pretty nervous that we might have missed something.”
In the new study, Dr. Slatyer and MIT’s Dr. Rebecca Leane repeated the approach from the 2015 study, but instead of feeding into the model with data from NASA’s Fermi Gamma-ray Space Telescope, they essentially drew up a fake map of the sky, including a signal of dark matter, and pulsars that were not associated with the GCE.
They fed this map into the model and found that, despite there being a dark matter signal within the spherical region, the model concluded this region was most likely grainy and therefore dominated by pulsars.
“This was the first clue that our method wasn’t foolproof,” Dr. Slatyer said.
The team then fed the model with Fermi data along with a fake signal of dark matter.
Despite the deliberate plan, their statistical analysis again missed the dark matter signal and returned a grainy, pulsar-like picture.
Even when they turned up the dark matter signal to four times the size of the actual GCE, their method failed to see it.
“By that stage, I was pretty excited, because I knew the implications were very big — it meant that the dark matter explanation was back on the table,” Dr. Leane said.
The scientists are working to better understand the bias in their approach, and hope to tune out this bias in the future.
“If it’s really dark matter, this would be the first evidence of dark matter interacting with visible matter through forces other than gravity,” Dr. Leane said.
“The nature of dark matter is one of the biggest open questions in physics at the moment. Identifying this signal as dark matter may allow us to finally expose the fundamental identity of dark matter. No matter what the excess turns out to be, we will learn something new about the Universe.”
The team’s paper appears in the journal Physical Review Letters.
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Rebecca K. Leane & Tracy R. Slatyer. 2019. Revival of the Dark Matter Hypothesis for the Galactic Center Gamma-Ray Excess. Phys. Rev. Lett 123 (24): 241101; doi: 10.1103/PhysRevLett.123.241101
