03 May 2017 News

Milky Way dark matter still as elusive as ever

A pulsar with beams of radiation that mimic a lighthouse beam. Image: Mark Garlick/SPL
A pulsar with beams of radiation that mimic a lighthouse beam. Image: Mark Garlick/SPL

Dark matter, the mysterious substance that makes up 85 percent of all matter, is set to remain mysterious a while longer, as researchers discover that a perplexing glow at the centre of the Milky Way is most likely caused by pulsars and not dark matter as previously suggested.

The mysterious glow at the heart of our galaxy is made up of gamma-ray emission and it had been thought that dark matter was linked with the signal. Now, an international team of astrophysicists, including researchers from the Department of Energy's SLAC National Accelerator Laboratory have assigned the glow to a more ordinary perpetrator instead.

"Our study shows that we don't need dark matter to understand the gamma-ray emissions of our galaxy," said Mattia Di Mauro from the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC. "Instead, we have identified a population of pulsars in the region around the galactic centre, which sheds new light on the formation history of the Milky Way."

Dark matter has been making scientists scratch their head in puzzlement since it was first inferred to exist. Although yet unproved, dark matter is believed to be composed of yet-to-be-discovered particles that almost never interact with regular matter, however researchers know that dark matter exists because it bends light from distant galaxies and affects how galaxies rotate.

One potential way to spot dark matter could be through the emission of radiation when the particles either decay or collide and destroy each other and it has been widely theorised that these processes would produce gamma rays when it occurred. And with more gamma rays coming from the galactic centre than expected, this region has been heavily scrutinised for signals of the dark stuff.

Now however, using the Large Area Telescope (LAT) on NASA's Fermi Gamma-ray Space Telescope, a team of scientists, including researchers from the Department of Energy's SLAC National Accelerator Laboratory, have found that the gamma-ray glow is not as smooth as it should be, if the material they were searching for was dark matter.

"Two recent studies by teams in the U.S. and the Netherlands have shown that the gamma-ray excess at the galactic centre is speckled, not smooth as we would expect for a dark matter signal," said Eric Charles, a Kavli Institute for Particle Astrophysics and Cosmology (KIPAC) researcher, who contributed to the new analysis. "Those results suggest the speckles may be due to point sources that we can't see as individual sources with the LAT because the density of gamma-ray sources is very high and the diffuse glow is brightest at the galactic centre.”

Instead the signal has been assigned to a population of pulsars – incredibly dense, rapidly spinning cores of collapsed stars that emit light predominantly in gamma-rays and radio waves.

"Considering that about 70 percent of all point sources in the Milky Way are pulsars, they were the most likely candidates," said Mattia Di Mauro from KIPAC and who led the analysis for the Fermi LAT Collaboration. "But we used one of their physical properties to come to our conclusion. Pulsars have very distinct spectra - that is, their emissions vary in a specific way with the energy of the gamma rays they emit. Using the shape of these spectra, we were able to model the glow of the galactic centre correctly with a population of about 1,000 pulsars and without introducing processes that involve dark matter particles."

While it might seem disappointing that dark matter still remains elusive as ever, the new study demonstrates that the speckled gamma-ray signal is consistent with pulsars, thus helping to narrow the search on how this enigmatic substance might be identified in future observations.

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