NASA Detects "Powerful Signals From The Center Of The Milky Way And Andromeda Galaxy" -- May Be Evidence Of The Mysterious Dark Side Of The Universe
NASA's Fermi Gamma-ray Space Telescope has found a signal at the center of the neighboring Andromeda galaxy that could indicate the presence of the mysterious dark side of the universes known as dark matter. The signal is similar to one seen by Fermi at the center of our own Milky Way galaxy.
Surprisingly,
the latest Fermi data shows the gamma rays in Andromeda -- also known as M31 --
are confined to the galaxy's center instead of spread throughout. To explain
this unusual distribution, scientists are proposing that the emission may come
from several undetermined sources. One of them could be dark matter, an unknown
substance that makes up most of the universe.
The image of
the Milky Way above, shown in visible light, superimposes a gamma-ray map of
the galactic center from NASA's Fermi. Raw data transitions to a view with all
known sources removed, revealing a gamma-ray excess hinting at the presence of
dark matter.
The Milky
Way's galactic center teems with gamma-ray sources, from interacting binary
systems and isolated pulsars to supernova remnants and particles colliding with
interstellar gas.
It's also where astronomers expect to find the galaxy's highest density of dark matter, which only affects normal matter and radiation through its gravity. Large amounts of dark matter attract normal matter, forming a foundation upon which visible structures, like galaxies, are built.
It's also where astronomers expect to find the galaxy's highest density of dark matter, which only affects normal matter and radiation through its gravity. Large amounts of dark matter attract normal matter, forming a foundation upon which visible structures, like galaxies, are built.
No one knows
the true nature of dark matter, but WIMPs, or Weakly Interacting Massive
Particles, represent a leading class of candidates. Theorists have envisioned a
wide range of WIMP types, some of which may either mutually annihilate or
produce an intermediate, quickly decaying particle when they collide. Both of
these pathways end with the production of gamma rays -- the most energetic form
of light -- at energies within the detection range of Fermi's Large Area
Telescope (LAT).
When
astronomers carefully subtract all known gamma-ray sources from LAT
observations of the galactic center, a patch of leftover emission remains. This
excess appears most prominent at energies between 1 and 3 billion electron
volts (GeV) -- roughly a billion times greater than that of visible light -- and
extends outward at least 5,000 light-years from the galactic center.
Hooper and
his colleagues conclude that annihilations of dark matter particles with a mass
between 31 and 40 GeV provide a remarkable fit for the excess based on its
gamma-ray spectrum, its symmetry around the galactic center, and its overall
brightness. Writing in a paper submitted to the journal Physical Review D, the
researchers say that these features are difficult to reconcile with other
explanations proposed so far, although they note that plausible alternatives
not requiring dark matter may yet materialize.
"Dark
matter in this mass range can be probed by direct detection and by the Large
Hadron Collider (LHC), so if this is dark matter, we're already learning about
its interactions from the lack of detection so far," said co-author Tracy
Slatyer, a theoretical physicist at MIT in Cambridge, Mass. "This is a
very exciting signal, and while the case is not yet closed, in the future we
might well look back and say this was where we saw dark matter annihilation for
the first time."
The
researchers caution that it will take multiple sightings – in other
astronomical objects, the LHC or in some of the direct-detection experiments
now being conducted around the world -- to validate their dark matter
interpretation.
A Fermi 2015
study was an example of innovative techniques applied to Fermi data by the
science community, said Peter Michelson, a professor of physics at Stanford
University in California and the LAT principal investigator. "The Fermi
LAT Collaboration continues to examine the extraordinarily complex central
region of the galaxy, but until this study is complete we can neither confirm
nor refute this interesting analysis."
While the great amount of dark matter expected at the galactic center should produce a strong signal, competition from many other gamma-ray sources complicates any case for a detection. But turning the problem on its head provides another way to attack it. Instead of looking at the largest nearby collection of dark matter, look where the signal has fewer challenges.
Dwarf
galaxies orbiting the Milky Way lack other types of gamma-ray emitters and
contain large amounts of dark matter for their size – in fact, they're the most
dark-matter-dominated sources known. But there's a tradeoff. Because they lie
much farther away and contain much less total dark matter than the center of
the Milky Way, dwarf galaxies produce a much weaker signal and require many
years of observations to establish a secure detection.
For the past
few years, the LAT team has been searching dwarf galaxies for hints of dark
matter. The published results of these studies have set stringent limits on the
mass ranges and interaction rates for many proposed WIMPs, even eliminating
some models.
There's
about a one-in-12 chance that what they saw in 2015 in the dwarf galaxies is
not even a signal at all, but a fluctuation in the gamma-ray background,
explained Elliott Bloom, a member of the LAT Collaboration at the Kavli
Institute for Particle Astrophysics and Cosmology, jointly located at the SLAC
National Accelerator Laboratory and Stanford University. If it's real, the
signal should grow stronger as Fermi acquires additional years of observations
and as wide-field astronomical surveys discover new dwarfs. "If we
ultimately see a significant signal," he added, "it could be a very
strong confirmation of the dark matter signal claimed in the galactic
center."
Via NASA Goddard; A. Mellinger, CMU; T. Linden, Univ. of Chicago
Comments
Post a Comment