The Fermi Bubbles: Supersonic AGN Jets with Anisotropic Cosmic Ray
  Diffusion
  The Fermi Gamma-ray Space Telescope reveals two large bubbles in the Galaxy,
which extend nearly symmetrically ~50 degrees above and below the Galactic
center (GC). Using three-dimensional (3D) magnetohydrodynamic (MHD) simulations
that self-consistently include the dynamical interaction between cosmic rays
(CR) and thermal gas, and anisotropic CR diffusion along the magnetic field
lines, we show that the key characteristics of the observed gamma-ray bubbles
and the spatially-correlated X-ray features in ROSAT 1.5 keV map can be
successfully reproduced by a recent jet activity from the central active
galactic nucleus (AGN). We find that after taking into account the projection
of the 3D bubbles onto the sky, the physical heights of the bubbles can be much
smaller than previously thought, greatly reducing the formation time of the
bubbles to about a Myr. This relatively small bubble age is needed to reconcile
the simulations with the upper limit of bubbles ages estimated from the cooling
time of high-energy electrons. No additional physical mechanisms are required
to suppress large-scale hydrodynamic instabilities because the evolution time
is too short for them to develop. The simulated CR bubbles are edge-brightened,
which is consistent with the observed projected flat surface brightness
distribution. Furthermore, we demonstrate that the sharp edges of the observed
bubbles can be due to anisotropic CR diffusion along magnetic field lines that
drape around the bubbles during their supersonic expansion, with suppressed
perpendicular diffusion across the bubble surface. Possible causes of the
slight bends of the Fermi bubbles to the west are also discussed.
