Astronomy and Astrophysics – Astrophysics – High Energy Astrophysical Phenomena
Scientific paper
2011-02-28
Astronomy and Astrophysics
Astrophysics
High Energy Astrophysical Phenomena
Significantly expanded discussions; main conclusions unchanged. 16 pages, 9 figures
Scientific paper
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 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 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 Galactic center, 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, successfully explaining the ROSAT X-ray features surrounding the bubbles. Fermi bubbles provide strong evidence for a recent powerful AGN jet activity in our Galaxy, shedding new insights into the origin of the halo CR population and the potential role of active galactic nucleus feedback in regulating the co-evolution of massive black holes and their host bulges.
Guo Fulai
Mathews William G.
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