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The Fermi Gamma-ray Space Telescope reveals two large bubbles in the Galaxy,which extend nearly symmetrically ~50 degrees above and below the Galacticcenter (GC). Using three-dimensional (3D) magnetohydrodynamic (MHD) simulationsthat self-consistently include the dynamical interaction between cosmic rays(CR) and thermal gas, and anisotropic CR diffusion along the magnetic fieldlines, we show that the key characteristics of the observed gamma-ray bubblesand the spatially-correlated X-ray features in ROSAT 1.5 keV map can besuccessfully reproduced by a recent jet activity from the central activegalactic nucleus (AGN). We find that after taking into account the projectionof the 3D bubbles onto the sky, the physical heights of the bubbles can be muchsmaller than previously thought, greatly reducing the formation time of thebubbles to about a Myr. This relatively small bubble age is needed to reconcilethe simulations with the upper limit of bubbles ages estimated from the coolingtime of high-energy electrons. No additional physical mechanisms are requiredto suppress large-scale hydrodynamic instabilities because the evolution timeis too short for them to develop. The simulated CR bubbles are edge-brightened,which is consistent with the observed projected flat surface brightnessdistribution. Furthermore, we demonstrate that the sharp edges of the observedbubbles can be due to anisotropic CR diffusion along magnetic field lines thatdrape around the bubbles during their supersonic expansion, with suppressedperpendicular diffusion across the bubble surface. Possible causes of theslight bends of the Fermi bubbles to the west are also discussed.

The Fermi Bubbles: Supersonic AGN Jets with Anisotropic Cosmic Ray Diffusion