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Fermi finds giant bubbles
in Milky Way

Posted: 11 November 2010

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A previously unseen structure spanning across 50,000 light years and possibly millions of years old has been discovered in the heart of our Galaxy.

The structure spans more than half the visible sky from the constellation of Virgo to Grus. Hints of the bubbles' edges were first observed in X-rays (blue) by ROSAT. The gamma rays mapped by Fermi (magenta) extend much farther from the Galaxy's plane. Image: NASA's Goddard Space Flight Center.

Although the source of the bubble is not confirmed, scientists speculate it could be the remnant of an eruption from the supermassive black hole that resides in the centre of our Milky Way Galaxy. “What we see are two gamma-ray-emitting bubbles that extend 25,000 light years north and south of the galactic centre,” describes Doug Finkbeiner of the Harvard-Smithsonian Center for Astrophysics.

Normally, the fog of gamma rays that permeates the sky (resulting from collisions of particles moving at nearly the speed of light with interstellar gas) would prevent detection of the bubble, but thanks to the sensitivity of Fermi's Large Area Telescope (LAT), the team were able to isolate this fog to reveal the giant bubbles.

The gamma-ray structure was uncovered by processing Fermi all-sky data at energies from 1 to 10 billion electron volts. Image: NASA/DOE/Fermi LAT/D. Finkbeiner et al.

The bubble emissions are much more energetic than the gamma-ray fog seen elsewhere in the Milky Way, and appear to have defined edges, suggesting it formed as a result of a large and rapid energy release. One possible culprit includes a particle jet from the supermassive black hole in the Galaxy's centre, a phenomena observed in other galaxies, too. But while there is no evidence for such a jet being active today, the bubble could represent an ancient jet. An alternative theory is that the bubbles were blown out from gas outflow during a burst of star formation, another process also seen in other galaxies.

“Whatever the energy source behind these huge bubbles may be, it is connected to many deep questions in astrophysics,” says David Spergel, a scientist at Princeton University in New Jersey.

The bubbles' presence had already been hinted at by Germany's Roentgen Satellite (ROSAT), which found subtle evidence for bubble 'edges' close to the galactic centre, and NASA's Wilkinson Microwave Anisotropy Probe (WMAP), which detected an excess of radio signals at the locations of the gamma-ray bubbles. The Fermi data provides the clearest picture yet of their presence.