The Fermi Bubbles. II. The Potential Roles of Viscosity and Cosmic Ray
  Diffusion in Jet Models
  The origin of the Fermi bubbles recently detected by the Fermi Gamma-ray
Space Telescope in the inner Galaxy is mysterious. In the companion paper Guo &
Mathews (Paper I), we use hydrodynamic simulations to show that they could be
produced by a recent powerful AGN jet event. Here we further explore this
scenario to study the potential roles of shear viscosity and cosmic ray (CR)
diffusion on the morphology and CR distribution of the bubbles. We show that
even a relatively low level of viscosity (\mu_{visc} >~ 3 g cm^{-1} s^{-1}, or
~0.1% - 1% of Braginskii viscosity in this context) could effectively suppress
the development of Kelvin-Helmholtz instabilities at the bubble surface,
resulting in smooth bubble edges as observed. Furthermore, viscosity reduces
circulating motions within the bubbles, which would otherwise mix the
CR-carrying jet backflow near bubble edges with the bubble interior. Thus
viscosity naturally produces an edge-favored CR distribution, an important
ingredient to produce the observed flat gamma-ray surface brightness
distribution. Generically, such a CR distribution often produces a
limb-brightened gamma-ray intensity distribution. However, we show that by
incorporating CR diffusion which is strongly suppressed across the bubble
surface (as inferred from sharp bubble edges) but is close to canonical values
in the bubble interior, we obtain a reasonably flat gamma-ray intensity
profile. The similarity of the resulting CR bubble with the observed Fermi
bubbles strengthens our previous result in Paper I that the Fermi bubbles were
produced by a recent AGN jet event. Studies of the nearby Fermi bubbles may
provide a unique opportunity to study the potential roles of plasma viscosity
and CR diffusion on the evolution of AGN jets and bubbles.
