The impact of cosmic rays on dynamical balance and disk-halo interaction
  in Lstar disk galaxies
  Cosmic rays (CRs) are an important component in the interstellar medium
(ISM), but their effect on the dynamics of the disk-halo interface (< 10 kpc
from the disk) is still unclear. We study the influence of CRs on the gas above
the disk with high-resolution FIRE-2 cosmological simulations of late-type
Lstar galaxies at redshift around zero. We compare runs with and without CR
feedback (with constant anisotropic diffusion around 3e29 cm^2/s and
streaming). Our simulations capture the relevant disk halo interactions,
including outflows, inflows, and galactic fountains. Extra-planar gas in all of
the runs satisfies dynamical balance, where total pressure balances the weight
of the overlying gas. While the kinetic pressure from non-uniform motion
(>1-kpc scale) dominates in the midplane, thermal and bulk pressures (or CR
pressure if included) take over at large heights. We find that with CR
feedback, (1) the warm (1e4K) gas is slowly accelerated by CRs; (2) the hot (>
5e5K) gas scale height is suppressed; (3) the warm-hot (2e4-5e5K) medium
becomes the most volume-filling phase in the disk-halo interface. We develop a
novel conceptual model of the near-disk gas dynamics in low-redshift Lstar
galaxies: With CRs, the disk-halo interface is filled with CR-driven warm winds
and hot super-bubbles that are propagating into the CGM with a small fraction
falling back to the disk. Without CRs, most outflows from hot superbubbles are
trapped by the existing hot halo and gravity, so typically they form galactic
fountains.
