Giant Gamma-ray Bubbles from Fermi-LAT: AGN Activity or Bipolar Galactic
  Wind?
  Data from the Fermi-LAT reveal two large gamma-ray bubbles, extending 50
degrees above and below the Galactic center, with a width of about 40 degrees
in longitude. The gamma-ray emission associated with these bubbles has a
significantly harder spectrum (dN/dE ~ E^-2) than the IC emission from
electrons in the Galactic disk, or the gamma-rays produced by decay of pions
from proton-ISM collisions. There is no significant spatial variation in the
spectrum or gamma-ray intensity within the bubbles, or between the north and
south bubbles. The bubbles are spatially correlated with the hard-spectrum
microwave excess known as the WMAP haze; the edges of the bubbles also line up
with features in the ROSAT X-ray maps at 1.5-2 keV. We argue that these
Galactic gamma-ray bubbles were most likely created by some large episode of
energy injection in the Galactic center, such as past accretion events onto the
central massive black hole, or a nuclear starburst in the last ~10 Myr. Dark
matter annihilation/decay seems unlikely to generate all the features of the
bubbles and the associated signals in WMAP and ROSAT; the bubbles must be
understood in order to use measurements of the diffuse gamma-ray emission in
the inner Galaxy as a probe of dark matter physics. Study of the origin and
evolution of the bubbles also has the potential to improve our understanding of
recent energetic events in the inner Galaxy and the high-latitude cosmic ray
population.
