Galactic Outflows by Alfv\'enic Poynting Flux: Application to Fermi Bubbles

TL;DR

This paper explores how magnetic activity and Alfvénic Poynting flux in the Galactic bulge can drive large-scale outflows, potentially explaining the physical properties of the Fermi bubbles through numerical simulations of wave propagation and shock formation.

Contribution

It demonstrates that magnetic buoyancy and Alfvénic waves can generate outflows with properties matching Fermi bubbles, using global and local simulations of magnetic and wave dynamics.

Findings

Alfvénic Poynting flux luminosity is $10^{40} - 10^{41}$ erg s$^{-1}$.

Upward propagating shocks reach temperatures >5 million K and velocities 400-500 km/s.

Wave damping and shock formation explain Fermi bubble characteristics.

Abstract

We investigate roles of magnetic activity in the Galactic bulge region in driving large-scale outflows of size $\sim 10$ kpc. Magnetic buoyancy and breakups of channel flows formed by magnetorotational instability excite Poynting flux by the magnetic tension force. A three-dimensional global numerical simulation shows that the average luminosity of such \Alfvenic Poynting flux is $10^{40} - 10^{41}$ erg s$^{-1}$. We examine the energy and momentum transfer from the Poynting flux to the gas by solving time-dependent hydrodynamical simulations with explicitly taking into account low-frequency \Alfvenic waves of period of 0.5 Myr in a one-dimensional vertical magnetic flux tube. The \Alfvenic waves propagate upward into the Galactic halo, and they are damped through the propagation along meandering magnetic field lines. If the turbulence is nearly trans-Alfv\'{e}nic, the wave damping is significant, which leads to the formation of an upward propagating shock wave. At the shock front, the temperature $\gtrsim 5\times 10^6$ K, the density $\approx 6\times 10^{-4}$ cm$^{-3}$, and the outflow velocity $\approx 400-500$ km s$^{-1}$ at a height $\approx 10$ kpc, which reasonably explain the basic physical properties of the thermal component of the Fermi bubbles.