Ghostly Gamma-Ray Beams Suggest Milky Way's Center Used to be Much More Active

Posted on May 29, 2012

The Milky Way is fairly quiet compared to active galaxies have cores that glow brightly and are powered by supermassive black holes. The Milky Way's center shows little activity. But it wasn't always so peaceful. Researchers at the Harvard-Smithsonian Center for Astrophysics (CfA) have discovered new evidence of ghostly gamma-ray beams that suggest the Milky Way's central black hole was much more active in the past.

CfA astronomer Meng Su, lead author of a new paper in the Astrophysical Journal, says, "These faint jets are a ghost or after-image of what existed a million years ago. They strengthen the case for an active galactic nucleus in the Milky Way's relatively recent past"

The two beams, or 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 newfound jets may be related to mysterious gamma-ray bubbles that Fermi detected in 2010, which also stretch 27,000 light-years from the center of the Milky Way.

The gamma-ray jets are pink in the image above. The previously known gamma-ray bubbles are shown in purple. The bubbles are perpendicular to the galactic plane, but the gamma-ray jets are tilted at an angle of 15 degrees. The astronomers say this may reflect a tilt of the accretion disk surrounding the supermassive black hole.

Co-author Douglas Finkbeiner of the CfA says, "The central accretion disk can warp as it spirals in toward the black hole, under the influence of the black hole's spin. 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."

Finkbeiner says it would take a tremendous influx of matter for the Milky Way's galactic core to fire up again. He says, "Shoving 10,000 suns into the black hole at once would do the trick. Black holes are messy eaters, so some of that material would spew out and power the jets."

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