As galaxies go, our Milky Way is pretty quiet. Active galaxies have cores that glow brightly, powered by supermassive black holes, and often spit twin jets in opposite directions. The Milky Way’s center shows little activity, but it wasn’t always so peaceful. New evidence of gamma-ray jets suggests that the Milky Way’s central black hole was much more active in the past.
"These faint jets are an after-image of what existed a million years ago," said Meng Su, at the Harvard-Smithsonian Center for Astrophysics (CfA). "They strengthen the case for an active galactic nucleus in the Milky Way’s relatively recent past."
The two jets, were revealed by NASA’s Fermi space telescope. They extend from the galactic center to a distance of 27,000 light-years above and below the galactic plane. They are the first such gamma-ray jets ever found, and the only ones close enough to resolve with Fermi.
The jets may be related to gamma-ray bubbles that Fermi detected in 2010. Those also stretch 27,000 light-years from the center of the Milky Way. However, where the bubbles are perpendicular to the galactic plane, the gamma-ray jets are tilted at an angle of 15 degrees. This may reflect a tilt of the accretion disk surrounding the supermassive black hole.
"The central accretion disk can warp as it spirals in toward the black hole, under the influence of the black hole’s spin," explained co-author Douglas Finkbeiner of the CfA. "The magnetic field embedded in the disk therefore accelerates the jet material along the spin axis of the black hole, which may not be aligned with the Milky Way."
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