Cosmic-Ray Driven Galactic Winds from the Warm Interstellar Medium

TL;DR

This study models cosmic-ray driven galactic winds from the warm interstellar medium, revealing multiphase structures, wind dynamics, and potential observational signatures, with implications for galaxy evolution and feedback processes.

Contribution

It introduces a detailed simulation framework including radiative cooling and CR heating, uncovering new wind behaviors and critical point structures not previously characterized.

Findings

Winds exhibit multiphase, thermally unstable structures.

Mass outflow rates are larger than star formation rates in low-mass galaxies.

Wind speeds reach about 40% of the escape velocity.

Abstract

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 $T \approx 10^4$ 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 $\dot M$ somewhat larger than the observed star formation rate in low mass galaxies, and an approximately "energy-like" scaling $\dot M \propto v_{\rm esc}^{-2}$. The winds accelerate slowly and reach asymptotic wind speeds of only $\sim 0.4 v_{\rm esc}$. The total wind power is $\sim 1\%$ of the power from supernovae, suggesting inefficient preventive CR feedback for the physical conditions modeled 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.