Galactic Disk Winds Driven by Cosmic Ray Pressure

Abstract

Cosmic ray pressure gradients transfer energy and momentum to extraplanar gas in disk galaxies, potentially driving significant mass loss as galactic winds. This may be particularly important for launching high-velocity outflows of “cool” (T less than or similar to 10(4) K) gas. We study cosmic ray-driven disk winds using a simplified semi-analytic model assuming streamlines follow the large-scale gravitational potential gradient. We consider scaled Milky Way-like potentials including a disk, bulge, and halo with a range of halo velocities V-H = 50-300 km s(-1) and streamline footpoints with radii in the disk R-0 = 1-16 kpc at a height of 1 kpc. Our solutions cover a wide range of footpoint gas velocity u(0), magnetic-to-cosmic ray pressure ratio, gas-to-cosmic ray pressure ratio, and angular momentum. Cosmic ray streaming at the Alfven speed enables the effective sound speed C-eff to increase from the footpoint to a critical point where C-eff, c = u(c) similar to V-H; this differs from thermal winds, in which C-eff decreases outward. The critical point is typically at a height of 1-6 kpc from the disk, increasing with V-H, and the asymptotic wind velocity exceeds the escape speed of the halo. Mass-loss rates are insensitive to the footpoint values of the magnetic field and angular momentum. In addition to numerical parameter space exploration, we develop and compare to analytic scaling relations. We show that winds have mass-loss rates per unit area up to (Sigma) over dot similar to Pi V-0(H)-5/3 u(0)(2/3), where Pi(0) is the footpoint cosmic ray pressure and u(0) is set by the upwelling of galactic fountains. The predicted wind mass-loss rate exceeds the star formation rate for V-H less than or similar to 200 km s(-1) and u(0) = 50 km s(-1), a typical fountain velocity.