Cosmic ray driven Galactic winds

TL;DR

This paper investigates how cosmic rays can drive galactic winds by analyzing the non-linear interactions between cosmic ray pressure, wind dynamics, and wave excitation, affecting cosmic ray spectra and galaxy evolution.

Contribution

It presents a coupled model of cosmic ray transport and wind dynamics, revealing the non-linear feedback mechanisms involved in cosmic ray driven galactic winds.

Findings

Cosmic ray pressure gradients can launch galactic winds.

Wind properties depend on the cosmic ray spectrum and wave interactions.

The model predicts modifications to cosmic ray spectra due to wind effects.

Abstract

The escape of cosmic rays from the Galaxy leads to a gradient in the cosmic ray pressure that acts as a force on the background plasma, in the direction opposite to the gravitational pull. If this force is large enough to win against gravity, a wind can be launched that removes gas from the Galaxy, thereby regulating several physical processes, including star formation. The dynamics of these cosmic ray driven winds is intrinsically non-linear in that the spectrum of cosmic rays determines the characteristics of the wind (velocity, pressure, magnetic field) and in turn the wind dynamics affects the cosmic ray spectrum. Moreover, the gradient of the cosmic ray distribution function causes excitation of Alfven waves, that in turn determine the scattering properties of cosmic rays, namely their diffusive transport. These effects all feed into each other so that what we see at the Earth is the result of these non-linear effects. Here we investigate the launch and evolution of such winds, and we determine the implications for the spectrum of cosmic rays by solving together the hydrodynamical equations for the wind and the transport equation for cosmic rays under the action of self-generated diffusion and advection with the wind and the self-excited Alfven waves.