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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)-masshalos hosting star-forming (SF) galaxies. Our CR treatment includes injectionby supernovae, anisotropic streaming and diffusion along magnetic field lines,collisional and streaming losses, with constant parallel diffusivity$\kappa\sim3\times10^{29}\,\mathrm{cm^2\ s^{-1}}$ chosen to match $\gamma$-rayobservations. With this, CRs become more important at larger halo masses andlower redshifts, and dominate the pressure in the CGM in MW-mass halos at$z\lesssim 1-2$. The gas in these "CR-dominated" halos differs significantlyfrom runs without CRs: the gas is primarily cool (a few $\sim10^{4}\,$K), andthe cool phase is volume-filling and has a thermal pressure below that neededfor virial or local thermal pressure balance. Ionization of the "low" and "mid"ions in this diffuse cool gas is dominated by photo-ionization, with O VIcolumns $\gtrsim 10^{14.5}\,\mathrm{cm^{-2}}$ at distances $\gtrsim150\,\mathrm{kpc}$. CR and thermal gas pressure are locally anti-correlated,maintaining total pressure balance, and the CGM gas density profile isdetermined by the balance of CR pressure gradients and gravity. Neglecting CRs,the same halos are primarily warm/hot ($T\gtrsim 10^{5}\,$K) with thermalpressure balancing gravity, collisional ionization dominates, O VI columns arelower and Ne VIII higher, and the cool phase is confined to dense filaments inlocal thermal pressure equilibrium with the hot phase.

Properties of the Circumgalactic Medium in Cosmic Ray-Dominated Galaxy Halos