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

Abstract

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 (μ_{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.

Revised version, accepted for publication in ApJ. 14 pages, 9 figures

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Cited by in corpus (63)

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• The jet feedback mechanism (jfm) in stars, galaxies and clusters
• The Galactic Center GeV Excess from a Series of Leptonic Cosmic-Ray Outbursts
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• Fermi Bubbles Inflated by Winds Launched from the Hot Accretion Flow in Sgr A*
• The Interaction of the Fermi Bubbles with the Milky Way's Hot Gas Halo
• The Return of the Templates: Revisiting the Galactic Center Excess with Multi-Messenger Observations
• Planck Intermediate Results. IX. Detection of the Galactic haze with Planck
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• X-ray and Gamma-ray Observations of the Fermi Bubbles and NPS/Loop I Structures
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• The Spatially Uniform Spectrum of the Fermi Bubbles: the Leptonic AGN Jet Scenario
• Unveiling the Origin of the Fermi Bubbles
• Simulating the Fermi Bubbles as Forward Shocks Driven by AGN Jets
• Search for Very High Energy Gamma Rays from the Northern $\text{Fermi}$ Bubble Region with HAWC
• The Fermi Bubbles as a Scaled-up Version of Supernova Remnants
• Reversing cooling flows with AGN jets: shock waves, rarefaction waves, and trailing outflows
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• The Fermi Bubble as a Source of Cosmic Rays in the Energy Range > 10E15 eV
• The Fermi Bubbles Revisited
• The Role of the Parker Instability in Structuring the Interstellar Medium
• The Shape of X-ray Cavities in Galaxy Clusters: Probing Jet Properties and Viscosity
• The Parker Instability with Cosmic Ray Streaming
• Detection Potential of the KM3NeT Detector for High-Energy Neutrinos from the Fermi Bubbles
• The Transport of Cosmic Rays Across Magnetic Fieldlines
• Possible connection between the asymmetry of North Polar Spur and Loop I with Fermi Bubbles
• On the Importance of Very-light Internally-subsonic AGN Jets in Radio-mode AGN Feedback
• Particle Diffusion and Acceleration in Magnetorotational Instability Turbulence
• Kinematics of the Magellanic Stream and Implications for its Ionization
• Suzaku X-ray Observations of the Fermi Bubbles: Northernmost Cap and Southeast Claw Discovered with MAXI-SSC
• Identifying the Radio Bubble Nature of the Microwave Haze
• Formation of large-scale magnetic structures associated with the Fermi bubbles
• Analyzing the Gamma-ray Sky with Wavelets
• Multi-wavelength Emission from the Fermi Bubble I. Stochastic acceleration from Background Plasma
• A Latitude-Dependent Analysis of the Leptonic Hypothesis for the Fermi Bubbles
• IceCube and HAWC constraints on very-high-energy emission from the Fermi bubbles
• Cosmic Rays in a Galactic Breeze
• Metal-rich trailing outflows uplifted by AGN bubbles in galaxy clusters
• On the origin of the central 1" hole in the stellar disk of Sgr A* and the Fermi gamma-ray bubbles
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• Analysis of WMAP 7-year Temperature Data: Astrophysics of the Galactic Haze
• Fermi Bubbles and Bubble-like emission from the Galactic Plane
• On the Galactic Center Being the Main Source of Galactic Cosmic Rays as Evidenced by Recent Cosmic Ray and Gamma Ray Observations
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• Dynamics Inside the Radio and X-ray Cluster Cavities of Cygnus A and Similar FRII Sources
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• The Bending Feature of the Fermi Bubbles: A Presumed Horizontal Galactic Wind and Its Implication on the Bubbles' Age
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• Probing the Halo Gas Distribution in the Inner Galaxy with Fermi Bubble Observations
• QUIJOTE scientific results -- VI. The Haze as seen by QUIJOTE
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• The AGN Jet Model of the Fermi Bubbles
• Galactic Cosmic-Rays in a Breeze
• Fermi bubbles as a source of cosmic rays above 10^{15} eV
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• Multi-wavelength emission from leptonic processes in ageing galaxy bubbles
• Fermi Bubbles: an Elephant in the Gamma-ray Sky
• Origin of Nonthermal Emission from the Fermi Bubbles and Mechanisms of Particle Acceleration There
• On the high-energy protons regular acceleration in the Fermi Bubbles