Cosmic-Ray Driven Galactic Winds from the Warm Interstellar Medium

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 $\dot{M}\propto v_{\mathrm{esc}}^{-2}$. The winds accelerate slowly and reach asymptotic wind speeds of only $\sim 0.4v_{\mathrm{esc}}$. 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.

arXiv commentUpdated to accepted version: 19 pages, 14 figures. Main simulation results in Table 1, Figure 3, and Figure 7. Comments still welcome!

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