Over a decade ago, researchers discovered a lot more gamma-rays coming from the center of the Milky Way than they thought possible. At the time, dark matter was considered one of the culprits for the galactic center excess (GCE), but a new study provides the strongest evidence yet that this is not the case.
The study, published in Physical Review D, provides constraints on GCE produced by what we know of dark matter. This is an important caveat as dark matter remains hypothetical and everything we think we know about it comes from experiments that have told us what dark matter is not, rather than what it is.
When the GCE was discovered, astronomers considered many other possible sources, such as star formation, cosmic rays being deflected, and neutron stars. Although a single cause has not been found, over the years the case for dark matter has become more complex to justify, even though some glimmer of hope remained. This new study suggests we should forget about that possibility completely.
“We looked at all of the different modeling that goes on in the Galactic Center, including molecular gas, stellar emissions, and high-energy electrons that scatter low-energy photons,” co-author Dr Oscar Macias, from the Kavli Institute for the Physics and Mathematics of the Universe at the University of Tokyo, said in a statement. “We took over three years to pull all of these new, better models together and examine the emissions, finding that there is little room left for dark matter.”
“If you peer at the Galactic Center, you see that the stars are distributed in a boxy way,” co-author Manoj Kaplinghat, a professor from the University of California Irvine (UCI), added. “There’s a disk of stars, and right in the center, there’s a bulge that’s about 10 degrees on the sky, and it’s actually a very specific shape – sort of an asymmetric box – and this shape leaves very little room for additional dark matter.”
This doesn’t mean that dark matter doesn’t exist, the researchers stress, but due to particle interactions, it can’t produce such a distinct gamma-rays signal.
“There are a lot of alternative dark matter candidates out there,” added co-author Kevork Abazajian, a professor from UCI. “The search is going to be more like a fishing expedition where you don’t already know where the fish are.”