New study suggests galactic bulge emissions not due to dark matter

Mysterious signal comes from very old stars at centre of our galaxy
Credit: Australian National University

A team of researchers from the U.S., New Zealand, Australia
and Germany has found evidence suggesting that a type of star
formation near the center of the Milky Way is responsible for
large gamma ray emissions, not dark matter. In their paper
published in the journal Nature Astronomy, the group
describes their study of the stars in the formation, what
they found, and what it could mean for dark matter theory.

Over the past several years, a consensus of sorts has emerged
among astrophysicists to explain the large from the center of the Milky
Way—they are likely due to particles (WIMPs) bumping into each
other or with regular matter, it was theorized. But in this new
effort, the researchers report evidence of another source,
casting doubt on dark matter as the likely cause of the

The researchers have been studying data from the Fermi
Gamma-Ray Space Telescope, which has been in orbit for the past
decade. They were able to see that the gamma rays actually
mirrored the distribution of stars near the center of the
galaxy—they were formed in the shape of an X, not a sphere as
would be expected if it were caused by dark matter
interactions. In building a model to recreate what they had
found, the team discovered that a more likely explanation was a
collection of millisecond pulsars (rapidly spinning neutron
stars)—their combined emissions appeared to have merged to
create the signal that was originally attributed to dark
matter—the new source may be less exciting, they note, but at
least it is explainable in reasonably concrete terms.

Mysterious signal comes from very old stars at centre of our galaxy
Credit: Australian National University

The researchers note that while their findings offer the most
likely explanation for the gamma ray signals, which constitutes
progress in understanding our own galaxy, they also put a bit
of a damper on enthusiasm for dark matter—the case for it goes
back to things like light bending in odd ways, or the strange
behavior observed in some galaxies, which do not offer much to
go on.

Explore further:

Does dark matter annihilate quicker in the Milky Way?

More information: Oscar Macias et al. Galactic bulge
preferred over dark matter for the Galactic centre gamma-ray
excess, Nature Astronomy (2018). DOI: 10.1038/s41550-018-0414-3

An anomalous gamma-ray excess emission has been found in the
Fermi Large Area Telescope data covering the centre of the
Galaxy. Several theories have been proposed for this ‘Galactic
centre excess’. They include self-annihilation of dark-matter
particles4, an unresolved population of millisecond pulsars, an
unresolved population of young pulsars, or a series of burst
events. Here, we report on an analysis that exploits
hydrodynamical modelling to register the position of
interstellar gas associated with diffuse Galactic gamma-ray
emission. We find evidence that the Galactic centre excess
gamma rays are statistically better described by the stellar
over-density in the Galactic bulge and the nuclear stellar
bulge, rather than a spherical excess. Given its non-spherical
nature, we argue that the Galactic centre excess is not a
dark-matter phenomenon but rather associated with the stellar
population of the Galactic bulge and the nuclear bulge.

Press release

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