The physics of galactic winds driven by cosmic rays - II. Isothermal streaming solutions

We use analytical calculations and time-dependent spherically symmetric simulations to study the properties of isothermal galactic winds driven by cosmic rays (CRs) streaming at the Alfven velocity. The simulations produce time-dependent flows permeated by strong shocks; we identify a new linear instability of sound waves that sources these shocks. The shocks substantially modify the wind dynamics, invalidating previous steady state models: the CR pressure p(c) has a staircase-like structure with dp(c)/dr similar or equal to 0 in most of the volume, and the time-averaged CR energetics are in many cases better approximated by p(c) proportional to rho(1/2), rather than the canonical p(c) proportional to rho(2/3). Accounting for this change in CR energetics, we analytically derive new expressions for the mass-loss rate, momentum flux, wind speed, and wind kinetic power in galactic winds driven by CR streaming. We show that streaming CRs are ineffective at directly driving cold gas out of galaxies, though CR-driven winds in hotter ISM phases may entrain cool gas. For the same physical conditions, diffusive CR transport (Paper I) yields mass-loss rates that are a few-100 times larger than streaming transport, and asymptotic wind powers that are a factor of similar or equal to 4 larger. We discuss the implications of our results for galactic wind theory and observations; strong shocks driven by CR-streaming-induced instabilities produce gas with a wide range of densities and temperatures, consistent with the multiphase nature of observed winds. We also quantify the applicability of the isothermal gas approximation for modelling streaming CRs and highlight the need for calculations with more realistic thermodynamics.

Automated summary

Analytical calculations and simulations on isothermal galactic winds reveal strong shocks driven by CR streaming, which leads to gas with a wide range of densities and temperatures, invalidating previous steady state models. Further analysis shows that diffusive CR transport yields higher mass-loss rates and wind powers compared to streaming transport, highlighting the need for more realistic thermodynamic calculations.