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

Ji et al., available on arXiv

Abstract: 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 halos hosting star-forming (SF) galaxies. Our CR treatment includes injection by supernovae, anisotropic streaming and diffusion along magnetic field lines, collisional and streaming losses, with constant parallel diffusivity kappa~3×10^29 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 halos at z<~1-2. The gas in these ``CR-dominated'' halos differs significantly from runs without CRs: the gas is primarily cool (a few ~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 photo-ionization, with O VI columns >~10^14.5 cm^-2 at distances >~150 kpc. CR and thermal gas pressure are locally anti-correlated, 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 halos are primarily warm/hot (T>~10^5 K) with thermal pressure balancing gravity, collisional ionization dominates, O VI 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.