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The origin of the Fermi bubbles recently detected by the Fermi Gamma-raySpace Telescope in the inner Galaxy is mysterious. In the companion paper Guo &Mathews (Paper I), we use hydrodynamic simulations to show that they could beproduced by a recent powerful AGN jet event. Here we further explore thisscenario to study the potential roles of shear viscosity and cosmic ray (CR)diffusion on the morphology and CR distribution of the bubbles. We show thateven 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 suppressthe development of Kelvin-Helmholtz instabilities at the bubble surface,resulting in smooth bubble edges as observed. Furthermore, viscosity reducescirculating motions within the bubbles, which would otherwise mix theCR-carrying jet backflow near bubble edges with the bubble interior. Thusviscosity naturally produces an edge-favored CR distribution, an importantingredient to produce the observed flat gamma-ray surface brightnessdistribution. Generically, such a CR distribution often produces alimb-brightened gamma-ray intensity distribution. However, we show that byincorporating CR diffusion which is strongly suppressed across the bubblesurface (as inferred from sharp bubble edges) but is close to canonical valuesin the bubble interior, we obtain a reasonably flat gamma-ray intensityprofile. The similarity of the resulting CR bubble with the observed Fermibubbles strengthens our previous result in Paper I that the Fermi bubbles wereproduced by a recent AGN jet event. Studies of the nearby Fermi bubbles mayprovide a unique opportunity to study the potential roles of plasma viscosityand CR diffusion on the evolution of AGN jets and bubbles.

The Fermi Bubbles. II. The Potential Roles of Viscosity and Cosmic Ray Diffusion in Jet Models