Fermi bubbles: the collimated outburst needed to explain forward-shock edges

Abstract

The bipolar, nonthermal, high-latitude lobes known as the Fermi bubbles (FBs) are thought to originate from a massive energy release near the Galactic centre (GC). We constrain the FB engine and the circumgalactic medium (CGM) by analytically and numerically modeling the FB edges as strong forward shocks, as inferred from recent observations. A non-directed energy release produces shocks too spherical to account for observations even for a maximally massive Galactic disc, critical CGM rotation, or injection effectively offset from the GC. In contrast, collimated injection nearly perpendicular to the disc can account for observations in both ballistic (free expansion) and slowdown regimes, as we show using a simple stratified evolution model verified by hydrodynamic simulations. FBs still in their ballistic regime require injection (at $z\simeq 100$ pc heights in our model) with a half-opening angle $\theta \simeq 4^{\circ }$, a normalized velocity ${\beta }_{-2}\equiv v/(0.01c)\gtrsim 0.4$, and an energy $E\gtrsim 2{\beta }_{-2}^2\times 10^{55}$ erg, launched $\mathbb{T}\simeq 3.3{\beta }_{-2}^{-1}$ Myr ago, showing a distinctive low-pressure region behind the bubble head. Slowing-down (mass accumulated) FBs require a faster injection, a thinner jet, a smaller $E/({\beta }_{-2}\theta {)}^2$, and a comparable $\mathbb{T}$, and follow a ballistic stage that must reach a height $z_s\gtrsim 5$ kpc.

18 pages, 17 figures, 3 tables; comments welcome

References in corpus (19)

• PLUTO: a Numerical Code for Computational Astrophysics
• The mass distribution and gravitational potential of the Milky Way
• Giant Gamma-ray Bubbles from Fermi-LAT: AGN Activity or Bipolar Galactic Wind?
• Constraining the Milky Way's Hot Gas Halo with OVII and OVII Emission Lines
• Detection of large-scale X-ray bubbles in the Milky Way halo
• Alignment of Magnetized Accretion Disks and Relativistic Jets with Spinning Black Holes
• Extended Anomalous Foreground Emission in the WMAP 3-Year Data
• Giant Magnetized Outflows from the Centre of the Milky Way
• The Galactic Centre Chimney
• Probing the Fermi Bubbles in Ultraviolet Absorption: A Spectroscopic Signature of the Milky Way's Biconical Nuclear Outflow
• Fermi Bubbles Inflated by Winds Launched from the Hot Accretion Flow in Sgr A*
• Nuclear Outflow of the Milky Way: Studying the Kinematics and Spatial Extent of the Northern Fermi Bubble
• The Interaction of the Fermi Bubbles with the Milky Way's Hot Gas Halo
• The large-scale ionization cones in the Galaxy
• Long way to go: how outflows from large galaxies propagate through the hot halo gas
• Simulating the Fermi Bubbles as Forward Shocks Driven by AGN Jets
• Mapping Outflowing Gas in the Fermi Bubbles: a UV Absorption Survey of the Galactic Nuclear Wind
• Clues to the origin of Fermi Bubbles from OVIII/OVII line ratio
• Origin of Galactic Spurs: New Insight from Radio/X-ray All-sky Maps

Cited by in corpus (2)

• Misaligned jets from Sgr A${}^{\ast }$ and the origin of Fermi/eROSITA bubbles
• Evidence for powerful winds and the associated reverse shock as the origin of the Fermi bubbles