The Fermi bubbles as starburst wind termination shocks

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 γ-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.

A Unified Model of the Fermi Bubbles, Microwave Haze, and Polarized Radio Lobes: Reverse Shocks in the Galactic Center’s Giant Outflows

“…Note that there are other significant star-formation-related sources of mechanical energy, including stellar winds and protostellar outflows, that could, in a full accounting, double this estimate. However, we adopt Equation (2.3) as our fiducial value for the injected power (see Lacki 2014). With this power, a conservative minimum timescale to inflate the bubbles (derived assuming that an unrealistically high 100% of mechanical energy injected in the nucleus is transferred to the bubbles and neglecting cooling and gravitational losses, which we show below are both actually important) is t H Ė 8 10 FB GC SF 7 = yr.…”

“…Given the very hard spectrum, a total power in these particles that approaches 10 39 erg s −1 , and that the particles are delivered into the halo but not too far above the disk, these shocks are interesting candidates (see Parizot 2014) for the main accelerators of Galactic cosmic rays in this energy range (see Cheng et al 2012;Lacki 2014;Taylor et al 2014).…”

Multiwavelength features of Fermi bubbles as signatures of a Galactic wind

“…• axisymmetric (about R = 0 axis) disc-like injection region, with a radius R = 110 pc and midplane to edge height h = 42 pc (Lacki 2014). …”

“…The high energy electrons or protons can either be accelerated in situ by internal shocks and turbulence (Mertsch & Sarkar 2011), or advected from the disc. Outflows triggered by star formation in the Galactic centre (GC) region (Crocker 2012;Lacki 2014) and by the black hole at the GC (Guo & Mathews 2012;Yang et al 2012;Mou et al 2014) have been proposed for the dynamical origin of the FBs.…”

“…The dynamical modelling is limited to simple arguments invoking a steady wind, termination shock, thermal instabilities, etc. (Crocker et al 2014b;Lacki 2014). A realistic SNe-driven wind is expected to be affected by disc stratification and the presence of a CGM.…”