Patterns of
a type of high-energy radiation called gamma rays throughout the Milky Way
aren't the signature of mysterious dark matter as had previously been
suggested, a new study shows.
Over the
past five years, gamma-ray measurements from the European satellite INTEGRAL
have perplexed astronomers because the distribution of the gamma rays across
different parts of the Milky Way didn't match what astronomers expected to see.
Some
astronomers suggested that various forms of dark
matter — the intangible stuff thought to make up about 98 percent of all
matter in the universe — might be the source of these gamma rays, which would
be further proof the existence of the mysterious matter.
But one
team of astrophysicists has shown that this explanation is unlikely, and that
instead the pattern of gamma rays can be explained by the way so-called
"anti-matter positrons" from the radioactive decay of elements
propagate through the galaxy.
"There
is no great mystery," said Richard Lingenfelter of the University of California San Diego and one of the co-authors of the new study. "The
observed distribution of gamma rays is in fact quite consistent with the
standard picture.
Positron
annihilation
Positrons
are subatomic particles that are the antimatter
counterpart of everyday electrons, and our galaxy — and others — are filled
with them.
When an
electron and a positron encounter one another in space, the two particles are
annihilated and their energy is released as gamma rays.
"These
positrons are born at nearly the speed of light, and travel thousands of
light-years before they slow down enough in dense clouds of gas to have a
chance of joining with an electron to annihilate in a dance of death,"
said study team member James Higdon of Claremont Colleges in California.
The
positrons are born in supernova explosions, and travel from there to all parts
of the galaxy, including the galactic center, the spiral arms and even the
outer reaches of the Milky Way, known as the galactic halo.
The team
calculated that most of the gamma rays should be concentrated in the inner
regions of the galaxy — the same pattern that showed up in the satellite data.
"The
observed distribution of gamma rays is consistent with the standard picture
where the source of positrons is the radioactive decay of isotopes of nickel,
titanium and aluminum produced in supernova explosions of stars more massive
than the sun," said Richard Rothschild, also of UCSD.
If dark
matter decays or annihilations were behind the gamma rays, the positrons would
instead be annihilated very close to their supernova birthplaces.
"We
clearly demonstrated this was not the case, and that the distribution of the
gamma rays observed by the gamma ray satellite was not a detection or
indication of a 'dark matter signal,'" Lingenfelter said.
The
research, funded by NASA, is detailed in two papers, one in the June 10 issue
of the Astrophysical Journal, and the other in the July 10 issue of the
journal Physical Review Letters.
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