The impact of cosmic rays on dynamical balance and disc–halo interaction in L⋆ disc galaxies

Abstract

Cosmic rays (CRs) are an important component in the interstellar medium, but their effect on the dynamics of the disc–halo interface (<10 kpc from the disc) is still unclear. We study the influence of CRs on the gas above the disc with high-resolution FIRE-2 cosmological simulations of late-type L⋆ galaxies at redshift z ∼ 0. We compare runs with and without CR feedback (with constant anisotropic diffusion κ_∥ ∼ 3 × 10²⁹ cm2 s⁻¹ and streaming). Our simulations capture the relevant disc–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 mid-plane, thermal and bulk pressures (or CR pressure if included) take over at large heights. We find that with CR feedback, (1) the warm (∼10⁴ K) gas is slowly accelerated by CRs; (2) the hot (>5 × 10⁵ K) gas scale height is suppressed; (3) the warm-hot (2 × 10⁴ - 5 × 10⁵ K) medium becomes the most volume-filling phase in the disc–halo interface. We develop a novel conceptual model of the near-disc gas dynamics in low-redshift L⋆ galaxies: with CRs, the disc–halo interface is filled with CR-driven warm winds and hot superbubbles that are propagating into the circumgalactic medium with a small fraction falling back to the disc. Without CRs, most outflows from hot superbubbles are trapped by the existing hot halo and gravity, so typically they form galactic fountains.