Milky Way's Gamma-ray Bubbles Shaped by Dark Matter?

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Dark matter, the elusive stuff that makes up a substantial portion of all the mass in the universe, is largely a mystery to astronomers. They've tried finding it and creating it, but so far have unearthed no conclusive proof as to what exactly it is, though most theories state that we interact with it through gravity.

But Christoph Weniger, of the Max Planck Institute for Physics in Munich, has a different theory to explain new possible evidence for dark matter. By carrying out statistical analysis of publicly available data from NASA’s Fermi Gamma-ray Space Telescope, he's found a gamma-ray line across the sky that he says is a clear signature of dark matter.

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Astrophysicists generally think that supermassive black holes, like the one at the center of the Milky Way, release jets that interact with surrounding dark matter. This interaction is thought to be the source of high-energy gamma rays that satellites like Fermi can detect. What satellites can see are the photons produced when these jets interact with dark matter.

Weniger looked for signs of such an interaction in about three and a half years' worth of gamma-ray observations carried out by the Fermi satellite's Large Area Telescope.

To increase his chances of success, he only considered data from those regions of the Milky Way that should generate the highest ratios of dark-matter photons to photons from background sources. He was looking specifically for a peak in energy, a sign that a photon was produced by the collision between and annihilation of two particles; the photon left over should have the same mass as one dark-matter particle. This energy would theoretically appear as a very narrow peak, a line in gamma-ray spectra, distinct from the broad energy distribution seen across the visible universe.

That’s just what he found — a line in the gamma-ray spectrum.

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But he’s quick to admit it’s a provisional result. His data points come from about 50 photons, and he’ll need a lot more to prove conclusively that his line is related to dark matter. It’s possible the line he observed is from a known, though no less mysterious, astronomical phenomenon: the pair of enormous gamma ray-emitting bubbles extending outward from the plane of the Milky Way.

In December 2010, scientists working with the Fermi telescope found two giant lobes extending from the black hole at the center of our galaxy.

Twenty-five thousand light-years high, each bubble spans more than half of the visible sky, reaching from the constellation Virgo to the constellation Grus, and may be relatively young at just a million or so years old.

The bubbles are a recent find, normally masked by the fog of gamma rays that appears throughout the sky as a result of particles moving near the speed of light interacting with light and interstellar gas in the Milky Way. Scientists only found the bubbles by manipulating the data from the telescope to draw out the striking feature.

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The manipulated images show the bubbles have well-defined edges, suggesting they were formed as a result of a large and relatively rapid energy release — the source of which is still unknown. Interestingly, the energy cutoff of the bubbles corresponds to the gamma-ray line Weniger found, the one he’s associating with a dark-matter signature.

It’s possible the bubbles and the line have the same origin. Or, dark matter might be the cause of the bubbles' defined endpoint.

Whether or not the two observations turn out to be linked — which of course hinges on conclusive proof of Weniger’s gamma-ray line — both are very cool and part of the fascinating and mystery nature of our corner of the universe.

Image credit: NASA-Goddard

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