Cosmic-ray driven galactic winds from the warm interstellar medium

We study the properties of cosmic-ray (CR) driven galactic winds from the warm interstellar medium using idealized spherically symmetric time-dependent simulations. The key ingredients in the model are radiative cooling and CR-streaming-mediated heating of the gas. Cooling and CR heating balance near the base of the wind, but this equilibrium is thermally unstable, leading to a multiphase wind with large fluctuations in density and temperature. In most of our simulations, the heating eventually overwhelms cooling, leading to a rapid increase in temperature and a thermally driven wind; the exception to this is in galaxies with the shallowest potentials, which produce nearly isothermal T approximate to 10(4) K winds driven by CR pressure. Many of the time-averaged wind solutions found here have a remarkable critical point structure, with two critical points. Scaled to real galaxies, we find mass outflow rates (M)over dot somewhat larger than the observed star-formation rate in low-mass galaxies, and an approximately 'energy-like' scaling (M)over dot proportional to v(esc)(-2). The winds accelerate slowly and reach asymptotic wind speeds of only similar to 0.4v(esc). The total wind power is similar to 1 per cent of the power from supernovae, suggesting inefficient preventive CR feedback for the physical conditions modelled here. We predict significant spatially extended emission and absorption lines from 10(4)-10(5.5) K gas; this may correspond to extraplanar diffuse ionized gas seen in star-forming galaxies.

Automated summary

We have studied cosmic-ray driven galactic winds from the warm interstellar medium using simulations. The model includes radiative cooling and CR-streaming-mediated heating of the gas. The simulations show that a thermally unstable equilibrium leads to a multiphase wind with fluctuations in density and temperature. The results suggest that the preventive CR feedback is inefficient in the physical conditions modeled here.