Galactic disk winds driven by cosmic ray pressure
  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 < 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, and streamline footpoints with radii in the disk R_0=1-16 kpc at height 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 Alfv\'en speed enables the
effective sound speed C_eff to increase from the footpoint to a critical point
where C_eff,c = u_c ~ 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 ~ Pi_0
V_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 < 200 km/s and u_0 = 50 km/s, a typical
fountain velocity.
