The Fermi Bubbles. I. Possible Evidence for Recent AGN Jet Activity in
  the Galaxy
  The Fermi Gamma-ray Space Telescope reveals two large gamma-ray bubbles in
the Galaxy, which extend about 50 degrees (~ 10 kpc) above and below the
Galactic center (GC) and are symmetric about the Galactic plane. Using
axisymmetric hydrodynamic simulations with a self-consistent treatment of the
dynamical cosmic ray (CR) - gas interaction, we show that the bubbles can be
created with a recent active galactic nucleus (AGN) jet activity about 1 - 3
Myr ago, which was active for a duration of ~ 0.1 - 0.5 Myr. The bipolar jets
were ejected into the Galactic halo along the rotation axis of the Galaxy. Near
the GC, the jets must be moderately light with a typical density contrast 0.001
<~ \eta <~ 0.1 relative to the ambient hot gas. The jets are energetically
dominated by kinetic energy, and over-pressured with either CR or thermal
pressure which induces lateral jet expansion, creating fat CR bubbles as
observed. The sharp edges of the bubbles imply that CR diffusion across the
bubble surface is strongly suppressed. The jet activity induces a strong shock,
which heats and compresses the ambient gas in the Galactic halo, potentially
explaining the ROSAT X-ray shell features surrounding the bubbles. The Fermi
bubbles provide plausible evidence for a recent powerful AGN jet activity in
our Galaxy, shedding new insights into the origin of the halo CR population and
the channel through which massive black holes in disk galaxies release feedback
energy during their growth.
