Gamma Rays From Milky Way’s Past Show Historically Active Black Hole
May 30, 2012

Gamma Rays From Milky Way’s Past Show Historically Active Black Hole

Lee Rannals for

Gamma-ray beams seen in the Milky Way's central black hole suggest that the galaxy's center was much more active in the past, according to new research.

Harvard University astrophysicists used an image taken by NASA's Fermi space telescope to reveal gamma-rays from the Milky Way millions of years ago.

"These faint jets are a ghost or after-image of what existed a million years ago," Meng Su, an astronomer at the Harvard-Smithsonian Center for Astrophysics (CfA), and lead author of a new paper in the Astrophysical Journal, said in a prepared statement.

"They strengthen the case for an active galactic nucleus in the Milky Way's relatively recent past," he added.

The two beams extend from the galactic center to a distance of 27,000 light years above and below the galactic plane.  The jets are the first such gamma-ray jets ever found, and the only ones close enough to be seen by Fermi.

The beams may be related to mysterious gamma-ray bubbles that Fermi picked-up on back in 2010.  Those bubbles stretched 27,000 light-years from the center of the Milky Way.

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

The jets were produced when plasma squirted out from the galactic center, after a corkscrew-like magnetic field kept it tightly focused.  The gamma-ray bubbles were created by a "wind" of hot matter blowing out from the black hole's accretion disk.

The jets and bubbles are both powered by inverse Compton scattering, according to the researchers.  During this process, electrons moving near the speed of light collide with low-energy light.  This collision increases the energy of the photons into the gamma-ray part of the electromagnetic spectrum.

"These jets probably flickered on and off as the supermassive black hole alternately gulped and sipped material," said Finkbeiner.

He said that a molecular cloud weighing about 10,000 times as much as the Sun would be required in order to make this influx of matter for the galactic core fire up again.

"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," he in the press release.