Another Dark Matter Search Comes Up Empty
Yet another search for the astrophysical signature of the cosmos’ unseen, exotic dark matter has come up empty.
In a paper published in the journal Nature Astronomy, a team from the University of Amsterdam/GRAPPA and France’s Laboratoire d’Annecy-le-Vieux de Physique Théorique note that diffuse gamma-ray emissions from our Milky Way galaxy’s center are much more likely to be the product of normal astrophysics than dark matter self-annihilation.
For almost a decade, astronomers have been puzzling over diffuse, high-energy gamma-ray emission first detected by the Large Area Telescope (LAT) onboard NASA’s Fermi Gamma-ray Space Telescope. Because it originates from our galaxy’s center, the University of Amsterdam reports that it was initially hoped that it might be long-sought-after indirect evidence that dark matter particles do, in fact, exist and decay.
Referred to as the galactic center excess (GCE), the University of Amsterdam says this excess’ electromagnetic spectrum peaks at energies of a few Giga electron Volts (GeV), which puts it in the very high-energy range of x-rays and gamma rays.
As the paper’s authors note, this emission’s morphology had heretofore been thought to have been more spherical than rectangular. But the team used a novel data analysis and image reconstruction technique dubbed SkyFACT. This enabled the researchers to peruse some eight years of this emission and combine image reconstruction with template fitting (or astronomical pattern matching).
In this case, the question was, does the emission look more spherical or more rectangular? These new results point to the latter, which is in direct conflict with the emission being a byproduct of dark matter particles.
The hope was that this “extra” emission could be from dark matter particles annihilating with each other and cascading into normal particles, resulting in gamma rays, Stacy McGaugh, an astronomer at Case Western Reserve University in Cleveland, who was not an author on this paper, told me. But he says the spectrum of the excess is indistinguishable from the spectrum produced by a new-found population of thousands of rapidly spinning neutron stars, known as millisecond pulsars.
McGaugh says this new paper shows that the spatial distribution of the excess radiation is distributed like the stars in the galaxy’s boxy bulge and the galaxy’s nuclear center and not like a putative, more round dark matter galactic halo. The logical inference, he says, is that if the excess gamma-ray emission is shaped like the stars and emits in proportion to the stars, it is probably something to do with the stars. Hence stellar remnants, like pulsars, are more likely to be the cause of this emission than dark matter, says McGaugh.
“ This is another blow for so-called indirect dark matter detection ,” said McGaugh.
Even so, proponents of dark matter theory contend that 85% of the cosmos is made up of unseen (or non-baryonic) matter that only weakly interacts with gravity.
“There are still many possible signatures of cold dark matter models that have not been looked for in detail,” Christoph Weniger, an astrophysicist at the University of Amsterdam and one of the paper’s authors, told me. “It’s a tough problem and we really just got started.”
Weniger and colleagues hope that follow-up radio observations with South Africa’s mid- and high-frequency MeerKAT radio array and/or the planned Square Kilometer Array (SKA) will confirm their paper’s assertions.