Milky Way's Gamma-Ray Bubbles

Su et al, 2010; Fermi-LAT, DOE, NASA

Larger and jumbo gamma-ray images.

Enormous gamma-ray bubbles emanating
like bi-polar jets from the center of the
galaxy, both above and below, have been
detected (more).

Bi-Polar Gamma-Ray "Fermi" Bubbles

On November 9, 2010, astronomers using the Fermi Gamma-ray Space Telescope (FGST, formerly named GLAST) announced the discovery of enormous bubbles of gamma-ray emission emanating above and below the Milky Way's galactic center and extending well beyond the central bulge, based on two years of observational data. Spanning about 25,000 light-years above and below the galactic disk and more than half of the visible sky from Constellation Virgo to Constellation Grus, hints of the bubbles were also found in previous scans of the galaxy in radio, microwave, and X-ray images. They may be millions of years old and related to previous outbursts of activity by the Milky Way's black holes (NASA feature and briefing materials; Astronomy Picture of the Day; Su et al, 2010; and Dennis Overbye, New York Times, November 9, 2010).

Unknown artist, GSFC, NASA

Larger and jumbo illustrations.

The bubbles extend for 25,000
light-years, from the galactic
center through the Milky Way's
central bulge (more).

The energetic gamma-rays in the bubbles are produced when super fast ("relativistic") electrons collide with less energetic photons (NASA feature; and and briefing materials; and Su et al, 2010). These gamma-ray emissions more energetic than the more diffuse gamma-ray "fog" or "haze" of high-energy electrons detected within the inner galaxy, but particularly towards the galactic center. It coincided with the position of an enormous "microwave haze" in the radio spectrum discovered previously by NASA’s Wilkinson Microwave Anisotropy Probe (WMAP), which some astronomers were speculating as possibly caused by dark matter (Dennis Overbye, New York Times, October 30, 2009; and Dobler et al, 2009). Conflicting with the dark matter hypothesis, the bubbles themselves appear to have well-defined edges, and their structural shape and emissions suggest that they were formed by a large and relatively fast burst of energy.

Unknown artist, GSFC, NASA

Larger and jumbo illustrations.

The energetic gamma-rays in the bubbles
are produced when super fast electrons
collide with less energetic photons (more).

The astronomers speculate that past accretion of matter into the supermassive black hole at the Milky Way's galactic center that creates bi-polar galactic jets and/or a burst of star birth that created galactic winds may have created the bubbles. Bi-polar jets have been detected in many other galaxies, where such fast particle jets are powered by matter falling toward a central black hole. Although the Milky Way's black hole does not appear to emit such jets or winds today, astronomers presume that such jets existed in the past, and so its gamma-ray bubbles may have formed from huge gas outflows result from a burst of star formation, perhaps the one that produced many massive star clusters in the Milky Way's center several million years ago (NASA feature and briefing materials).

Fermi-LAT, DOE, NASA

Larger and jumbo gamma-ray images.

The Fermi Gamma-ray Space Telescope
has been developing a gamma-ray
map of the Milky Way Galaxy over
the past two years (more).

As over 100,000 stars are located a light year of the Milky Way's supermassive central black hole, one team of astronomers has calculated that one of these stars could be captured by the black hole and torn apart by its gravity every 30,000 years or so. The gamma-ray bubbles would be formed as only around half of the star's mass would be accreted by the black hole, while the other half is blown out by the hole as bi-polar jets that collide at high speed into gas in the galactic halo to produce the observed gamma rays. At least one astronomer, however, believes that the destruction of individual stars by the central black hole would probably not be sufficient to produce the sharp edges detected around the Fermi bubbles, and that more mass (i.e., a huge cloud of gas or a star cluster) would be needed in a rarer event that occurs only once every one to 10 million years (Cheng et al, 2011; and Staff, New Scientist, March 19, 2011).

Other Information
- More information on galactic jets.