Properties of the circumgalactic medium in cosmic ray-dominated galaxy haloes

We investigate the impact of cosmic rays (CRs) on the circumgalactic medium (CGM) in FIRE-2 simulations, for ultra-faint dwarf through Milky Way (MW)-mass haloes hosting star-forming (SF) galaxies. Our CR treatment includes injection by supernovae, anisotropic streaming and diffusion along magnetic field lines, and collisional and streaming losses, with constant parallel diffusivity kappa similar to 3 x 10(29) cm(2) s(-1) chosen to match gamma-ray observations. With this, CRs become more important at larger halo masses and lower redshifts, and dominate the pressure in the CGM in MW-mass haloes at z less than or similar to 1-2. The gas in these 'CR-dominated' haloes differs significantly from runs without CRs: the gas is primarily cool (a few similar to 10(4) K), and the cool phase is volume-filling and has a thermal pressure below that needed for virial or local thermal pressure balance. Ionization of the 'low' and 'mid' ions in this diffuse cool gas is dominated by photoionization, with OVI columns similar to 10(14.5) cm(-2) at distances greater than or similar to 150 kpc. CR and thermal gas pressure are locally anticorrelated, maintaining total pressure balance, and the CGM gas density profile is determined by the balance of CR pressure gradients and gravity. Neglecting CRs, the same haloes are primarily warm/hot (T greater than or similar to 10(5) K) with thermal pressure balancing gravity, collisional ionization dominates, OVI columns are lower and Ne VIII higher, and the cool phase is confined to dense filaments in local thermal pressure equilibrium with the hot phase.