Milky Way's Gamma-Ray Bubbles and Jets
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Su et al, 2010;
Larger and jumbo gamma-ray images.
Enormous gamma-ray bubbles and jets
emanating like bi-polar jets from
the center of the galaxy, both above
and below, have been detected (more).
On January 3, 2013, astronomers working with the CSIROs 64-m Parkes radio telescope announced the detection of two giant bi-polar, supersonic outflows of charged particles, which emerge from the center of our Galaxy as linearly polarized, radio lobes (with "three ridge-like substructures") and stretch more than halfway across the sky. The flows extend for roughly 50,000 light-years in both directions outwards from the galactic plane at the core. Seen from Earth, the two outflows extend about two-thirds across the sky from horizon to horizon. These outflows correspond to a "haze" of microwave emission previously spotted by the WMAP and Planck space telescopes and regions of gamma-ray emission detected with NASA's Fermi space telescope in 2010, which were called "Fermi Bubbles." Instead being created by quasar-like outbursts from the supermassive black hole at the Galactic Core, the radio observations appear to confirm that the outflows are driven by many generations of new stars forming and exploding as supernovae in the Galactic Core over the last hundred million years, along with their magnetic fields (CSIRO news release; and Carretti et al, 2013).
Eli Bressert, S-PASS, CSIRO
Larger and jumbo composite images.
The outflows of charged particles
stretch for roughly 50,000 light-years,
from the galactic center through the
Milky Way's central bulge (more).
Bi-Polar Gamma-Ray Jets
On May 29, 2012, astronomers using the Fermi Gamma-ray Space Telescope (FGST, formerly named GLAST) announced the detection of giant gamma-ray jets emanating from the galactic core above and below the Milky Way's galactic center and extending some 27,000 light-years through the central bulge. The astronomers believe that the faint jets are a ghostly "after-image" of much brighter beams produced by a much more active, galactic nucleus around a million years ago. These jets likely were produced when plasma squirted out from the supermassive black hole at the galactic center after it consumed a molecular cloud and stars totalling roughly around 10,000 Solar-masses, and they followed "a corkscrew-like magnetic field that kept it tightly focused." The astronomers also hypothesized that the previously discovered, gamma-ray bubbles which also extend from the galactice core likely were "created by a "wind" of hot matter blowing outward from the black hole's accretion disk" and so extending through a much greater volume of space than did the very narrow jets. Both the gamma-ray jets and bubbles were probably powered by inverse Compton scattering, where electrons accelerated to near the speed of light collided with low-energy light (e.g., radio or infrared photons), which boosted their energy into the gamma-ray part of the electromagnetic spectrum. Although the bubbles are perpendicular to the galactic plane, the jets are tilted at an angle of 15 degrees, which may reflect a tilt of the accretion disk surrounding the supermassive black hole as its accretion disk can warp as it spirals in towards the hole (CfA press release; and Su and Finkbeiner, May 26, 2012).
Bi-Polar Gamma-Ray 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 27,000 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).
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.
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).
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 get 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).
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