The Fermi Bubbles as Starburst Wind Termination Shocks

TL;DR

This paper proposes that the Fermi Bubbles are formed by starburst wind termination shocks that accelerate cosmic rays, producing gamma rays, and remain stable as long as the starburst activity persists.

Contribution

It introduces a model where the Fermi Bubbles are wind bubbles formed by starburst wind termination shocks, explaining their gamma-ray emission and stability.

Findings

Halo gas stops the wind a few kpc from the Galactic Centre.

The shock can accelerate PeV electrons and EeV protons.

The Bubbles can be inflated within 10 Myr and remain steady during starburst activity.

Abstract

The enhanced star formation in the inner 100 pc of the Galaxy launches a superwind at ~1600 km s$^{-1}$ for M82-like parameters. The ram pressure of the wind is very low compared to more powerful starburst winds. I show that halo gas stops the wind a few kpc from the Galactic Centre. I suggest that the termination shock accelerates cosmic rays, and that the resulting Inverse Compton gamma rays are visible as the Fermi Bubbles. The Bubbles are then wind bubbles, which the starburst can inflate within 10 Myr. They can remain in steady state as long as the starburst lasts. The shock may accelerate PeV electrons and EeV protons. The Bubbles may be analogues of galactic wind termination shocks in the intergalactic medium. I discuss the advantages and problems of this model. I note that any jets from Sgr A* must burrow through the starburst wind bubble before reaching the halo gas, which could affect the early evolution of such jets.