The Fermi Bubbles. I. Possible Evidence for Recent AGN Jet Activity in the Galaxy (1103.0055v3)

Abstract: The Fermi Gamma-ray Space Telescope reveals two large gamma-ray bubbles in the Galaxy, which extend about 50 degrees (~ 10 kpc) above and below the Galactic center (GC) and are symmetric about the Galactic plane. Using axisymmetric hydrodynamic simulations with a self-consistent treatment of the dynamical cosmic ray (CR) - gas interaction, we show that the bubbles can be created with a recent active galactic nucleus (AGN) jet activity about 1 - 3 Myr ago, which was active for a duration of ~ 0.1 - 0.5 Myr. The bipolar jets were ejected into the Galactic halo along the rotation axis of the Galaxy. Near the GC, the jets must be moderately light with a typical density contrast 0.001 <~ \eta <~ 0.1 relative to the ambient hot gas. The jets are energetically dominated by kinetic energy, and over-pressured with either CR or thermal pressure which induces lateral jet expansion, creating fat CR bubbles as observed. The sharp edges of the bubbles imply that CR diffusion across the bubble surface is strongly suppressed. The jet activity induces a strong shock, which heats and compresses the ambient gas in the Galactic halo, potentially explaining the ROSAT X-ray shell features surrounding the bubbles. The Fermi bubbles provide plausible evidence for a recent powerful AGN jet activity in our Galaxy, shedding new insights into the origin of the halo CR population and the channel through which massive black holes in disk galaxies release feedback energy during their growth.

Overview of AGN Jet Activity and Fermi Bubbles in the Galaxy

The paper by Guo and Mathews explores the enigmatic origin of the Fermi bubbles, which are two large gamma-ray emitting structures symmetrically extending from the Galactic center. Observed by the Fermi Gamma-ray Space Telescope, these bubbles span approximately 50 degrees above and below the Galactic center. The paper posits a theoretical exploration using simulations to suggest that these structures may have been formed by recent active galactic nucleus (AGN) jet activity.

Methodology and Findings

The research adopts axisymmetric hydrodynamic simulations to model the impact of AGN jets on the surrounding galactic environment. Specifically, these simulations incorporate a self-consistent treatment of dynamical cosmic ray (CR) interactions with gas. The primary hypothesis tested is that the Fermi bubbles were created by AGN jet activity around 1 to 3 million years ago, lasting for a relatively short duration of 0.1 to 0.5 million years.

Key parameters in the simulation include:

• Jet Characteristics: Energetically, the jets were dominated by kinetic energy and significantly over-pressured relative to the ambient halo gas. A typical density contrast of the jets, denoted by $\eta$, ranged from 0.001 to 0.1.
• Jet Dynamics: The research found that the jet-driven model can reproduce the observed morphology of the Fermi bubbles, provided that the jet density and kinetic energy adhere to specified constraints.

The paper makes several salient observations:

1. Bipolar Structure: The symmetry and alignment of the Fermi bubbles along the Galactic rotation axis suggest a compelling link to AGN jet activity.
2. Sharp Edges: The simulation results, consistent with the observational data, indicate that cosmic ray diffusion across the bubble surface is suppressed, leading to sharply defined boundaries.
3. X-ray Features: The paper correlates the simulated jets to observed X-ray features surrounding the bubbles, providing an explanation for the ROSAT X-ray shell indicating shock-heated gas.

Implications and Future Research

The hypothesis of Fermi bubbles being remnants of AGN jet activity provides new insights into the dynamics of our Galaxy. The implications extend to:

• Galactic CR Population: Fermi bubbles could significantly contribute to our understanding of cosmic ray populations in the Galactic halo.
• AGN Feedback Mechanisms: The paper underlines the potential of AGN jets in shaping galactic structures and affecting interstellar medium conditions.

The paper suggests that future studies could focus on more sophisticated simulations incorporating magnetic fields and investigate the gamma-ray emission mechanisms more thoroughly. Additionally, constraints on jet parameters and ambient gas densities need refinement through multi-wavelength observational data to precisely deduce the energetics and composition of the bubbles.

In summary, this research provides a robust framework for understanding the formation of Fermi bubbles and opens inquiries into broader implications of AGN activity on galactic evolution, urging further investigation into both theoretical modeling and observational astronomy.