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

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.