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| Photo: David A. Aguilar (CfA) |
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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